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Accelerated C++
Practical Programming by Example
by Andrew Koenig and Barbara E. Moo
Addison-Wesley, 2000
ISBN 0-201-70353-X
Pages 336
Second Printing
Table of Contents
Contents
Chapter 0 Getting started
0.1 Comments
0.2 #include
0.3 The main function
0.4 Curly braces
0.5 Using the standard library for output
0.6 The return statement
0.7 A slightly deeper look
0.8 Details
Chapter 1 Working with strings
1.1 Input
1.2 Framing a name
1.3 Details
Chapter 2 Looping and counting
2.1 The problem
2.2 Overall structure
2.3 Writing an unknown number of rows
2.4 Writing a row
2.5 The complete framing program
2.6 Counting
2.7 Details
Chapter 3 Working with batches of data
3.1 Computing student grades
3.2 Using medians instead of averages
3.3 Details
Chapter 4 Organizing programs and data
4.1 Organizing computations
4.2 Organizing data
4.3 Putting it all together
4.4 Partitioning the grading program
4.5 The revised grading program
4.6 Details
Chapter 5 Using sequential containers and analyzing strings
5.1 Separating students into categories
5.2 Iterators
5.3 Using iterators instead of indices
5.4 Rethinking our data structure for better performance
5.5 The list type
5.6 Taking strings apart
5.7 Testing our split function
5.8 Putting strings together
5.9 Details
Chapter 6 Using library algorithms
6.1 Analyzing strings
6.2 Comparing grading schemes
6.3 Classifying students, revisited
6.4 Algorithms, containers, and iterators
6.5 Details
Chapter 7 Using associative containers
7.1 Containers that support efficient look-up
7.2 Counting words
7.3 Generating a cross-reference table
7.4 Generating sentences
7.5 A note on performance
7.6 Details
Chapter 8 Writing generic functions
8.1 What is a generic function?
8.2 Data-structure independence
8.3 Input and output iterators
8.4 Using iterators for flexibility
8.5 Details
Chapter 9 Defining new types
9.1 Student_info revisited
9.2 Class types
9.3 Protection
9.4 The Student_info class
9.5 Constructors
9.6 Using the Student_info class
9.7 Details
Chapter 10 Managing memory and low-level data structures
10.1 Pointers and arrays
10.2 String literals revisited
10.3 Initializing arrays of character pointers
10.4 Arguments to main
10.5 Reading and writing files
10.6 Three kinds of memory management
10.7 Details
Chapter 11 Defining abstract data types
11.1 The Vec class
11.2 Implementing the Vec class
11.3 Copy control
11.4 Dynamic Vecs
11.5 Flexible memory management
11.6 Details
Chapter 12 Making class objects act like values
12.1 A simple string class
12.2 Automatic conversions
12.3 Str operations
12.4 Some conversions are hazardous
12.5 Conversion operators
12.6 Conversions and memory management
12.7 Details
Chapter 13 Using inheritance and dynamic binding
13.1 Inheritance
13.2 Polymorphism and virtual functions
13.3 Using inheritance to solve our problem
13.4 A simple handle class
13.5 Using the handle class
13.6 Subtleties
13.7 Details
Chapter 14 Managing memory (almost) automatically
14.1 Handles that copy their objects
14.2 Reference-counted handles
14.3 Handles that let you decide when to share data
14.4 An improvement on controllable handles
14.5 Details
Chapter 15 Revisiting character pictures
15.1 Design
15.2 Implementation
15.3 Details
Chapter 16 Where do we go from here?
16.1 Use the abstractions you have
16.2 Learn more
Appendix A Language details
A.1 Declarations
A.2 Types
A.3 Expressions
A.4 Statements
Appendix B Library summary
B.1 Input-output
B.2 Containers and iterators
B.3 Algorithms
Many of the designations used by manufacturers and sellers to distinguish their products
are claimed as trademarks. Where those designations appear in this book, and we were
aware of a trademark claim, the designations have been printed in initial capital letters
or in all capitals.
The authors and publisher have taken care in the preparation of this book, but make no
expressed or implied warranty of any kind and assume no responsibility for errors or
omissions. No liability is assumed for incidental or consequential damages in connection
with or arising out of the use of the information or programs contained herein.
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Library of Congress Cataloging-in-Publication Data
Koenig, Andrew
Accelerated C++ : practical programming by example / Andrew Koenig, Barbara E.
Moo.
p. cm.
Includes index. ISBN 0-201-70353-X
1. C++ (Computer program language) I. Moo, Barbara E. II. Title.
QA76.73.C153 K67 2000
005.13'3—dc21 00-
040172
Copyright © 2000 by AT&T, Inc., and Barbara E. Moo
Cover photo copyright © 1995, 2000 by Andrew Koenig
The authors typeset this book (pic | eqn | troff -mpm | dpost) in Palatino, Helvetica, and
Courier, with assorted Sun Sparcstations, Hewlett-Packard laser printers, and two three
helper cats.
All rights reserved. No part of this publication may be reproduced, stored in a retrieval
system, or transmitted, in any form, or by any means, electronic, mechanical,
photocopying, recording, or otherwise, without the prior consent of the publisher.
Printed in the United States of America. Published simultaneously in Canada.
ISBN 0-201-70353-X
Text printed on recycled paper
23456789 10—MA—O403020100
Second printing, November 2000
The C++ In-Depth Series
Bjarne Stroustrup, Editor
"I have made this letter longer than usual, because I lack the time to make it short. "
—Blaise Pascal
The advent of the ISO/ANSI C++ standard marked the beginning of a new era for C++
programmers. The standard offers many new facilities and opportunities, but how can a
real-world programmer find the time to discover the key nuggets of wisdom within this
mass of information? The C++ In-Depth Series minimizes learning time and confusion
by giving programmers concise, focused guides to specific topics.
Each book in this series presents a single topic, at a technical level appropriate to that
topic. The Series' practical approach is designed to lift professionals to their next level of
programming skills. Written by experts in the field, these short, in-depth monographs
can be read and referenced without the distraction of unrelated material. The books are
cross-referenced within the Series, and also reference The C++ Programming Language
by Bjarne Stroustrup.
As you develop your skills in C++, it becomes increasingly important to separate
essential information from hype and glitz, and to find the in-depth content you need in
order to grow. The C++ In-Depth Series provides the tools, concepts, techniques, and
new approaches to C++ that will give you a critical edge.
Titles in the Series
Accelerated C++: Practical Programming by Example, Andrew Koenig
and Barbara E. Moo
Essential C++, Stanley B. Lippman
Exceptional C++; 47 Engineering Puzzles, Programming Problems,
and Solutions, Herb Sutter
Modern C++ Design: Applied Generic Programming and Design Patterns,
Andrei Alexandrescu
For more information, check out the series Web site at
http://www.aw.com/cseng/series/indepth/
Preface
A new approach to C++ programming
We assume that you want to learn quickly how to write useful C++ programs. Therefore,
we start by explaining the most useful parts of C++. This strategy may seem obvious
when we put it that way, but it has the radical implication that we do not begin by
teaching C, even though C++ builds on C. Instead, we use high-level data structures
from the start, explaining only later the foundations on which those data structures rest.
This approach lets you to begin writing idiomatic C++ programs immediately.
Our approach is unusual in another way: We concentrate on solving problems, rather
than on exploring language and library features. We explain the features, of course, but
we do so in order to support the programs, rather than using the programs as an excuse
to demonstrate the features.
Because this book teaches C++ programming, not just features/it is particularly useful
for readers who already know some C++, and who want to use the language in a more
natural, effective style. Too often, people new to C++ learn the language mechanics
without learning how to apply the language to everyday problems.
Our approach works—for beginners and experienced
programmers
We used to teach a week-long intensive C++ course every summer at Stanford
University. We originally adopted a traditional approach to that course: Assuming that
the students already knew C, we started by showing them how to define classes, and
then moved systematically through the rest of the language. We found that our students
would be confused and frustrated for about two days—until they had learned enough
that they could start writing useful programs. Once they got to that point, they learned
quickly.
When we got our hands on a C++ implementation that supported enough of what was
then the brand-new standard library, we overhauled the course. The new course used
the library right from the beginning, concentrated on writing useful programs, and went
into details only after the students had learned enough to use those details productively.
The results were dramatic: After one day in the classroom, our students were able to
write programs that had taken them most of the week in the old course. Moreover, their
frustration vanished.
Abstraction
Our approach is possible only because C++, and our understanding of it, has had time
to mature. That maturity has let us ignore many of the low-level ideas that were the
mainstay of earlier C++ programs and programmers.
The ability to ignore details is characteristic of maturing technologies. For example, early
automobiles broke down so often that every driver had to be an amateur mechanic. It
would have been foolhardy to go for a drive without knowing how to get back home even
if something went wrong. Today's drivers don't need detailed engineering knowledge in
order to use a car for transportation. They may wish to learn the engineering details for
other reasons, but that's another story entirely.
We define abstraction as selective ignorance—concentrating on the ideas that are
relevant to the task at hand, and ignoring everything else—and we think that it is the
most important idea in modern programming. The key to writing a successful program is
knowing which parts of the problem to take into account, and which parts to ignore.
Every programming language offers tools for creating useful abstractions, and every
successful programmer knows how to use those tools.
We think, abstractions are so useful that we've filled this book with them. Of course, we
don't usually call them abstractions directly, because they come in so many forms.
Instead, we refer to functions, data structures, classes, and inheritance—all of which are
abstractions. Not only do we refer to them, but we use them throughout the book.
If abstractions are well designed and well chosen, we believe that we can use them even
if we don't understand all the details of how they work. We do not need to be
automotive engineers to drive a car, nor do we need to understand everything about
how C++ works before we can use it.
Coverage
If you are serious about C++ programming, you need to know everything in this book—
even though this book doesn't tell you everything you need to know.
This statement is not as paradoxical as it sounds. No book this size can contain
everything you'll ever need to know about C++, because different programmers and
applications require different knowledge. Therefore, any book that covers all of
C++—such as Stroustrup's The C++ Programming Language (Addison-Wesley,
2000)—will inevitably tell you a lot that you don't need to know. Someone else will need
it, even if you don't.
On the other hand, many parts of C++ are so universally important that it is hard to be
productive without understanding them. We have concentrated on those parts. It is
possible to write a wide variety of useful programs using only the information in this
book. Indeed, one of our reviewers, who is the lead programmer for a substantial
commercial system written in C++, told us that this book covers essentially all of the
facilities that he uses in his work.
Using these facilities, you can write true C++ programs—not C++ programs in the style
of C, or any other language. Once you have mastered the material in this book, you will
know enough to figure out what else you want to learn, and how to go about it. Amateur
telescope makers have a saying that it is easier to make a 3-inch mirror and then to
make a 6-inch mirror than to make a 6-inch mirror from scratch.
We cover only standard C++, and ignore proprietary extensions. This approach has the
advantage that the programs that we teach you to write will work just about anywhere.
However, it also implies that we do not talk about how to write programs that run in
windowing environments, because such programs are invariably tied to a specific
environment, and often to a specific vendor. If you want to write programs that will work
only in a particular environment, you will have to turn elsewhere to learn how to do so—
but don't put this book down quite yet! Because our approach is universal, you will be
able to use everything that you learn here in whatever environments you use in the
future. By all means, go ahead and read that book about GUI applications that you were
considering—but please read this one first.
A note to experienced C and C++ programmers
When you learn a new programming language, you may be tempted to write programs
in a style that is familiar from the languages that you already know. Our approach seeks
to avoid that temptation by using high-level abstractions from the C++ standard library
right from the start. If you are already an experienced C or C++ programmer, this
approach contains some good news and some bad news—and it's the same news.
The news is that you are likely to be surprised at how little of your knowledge will help
you understand C++ as we present it. You will have more to learn at first than you
might expect (which is bad), but you will learn more quickly than you might expect
(which is good). In particular, if you already know C++, you probably learned first how
to program in C, which means that your C++ programming style is built on a C
foundation. There is nothing wrong with that approach, but our approach is so different
that we think you'll see a side of C++ that you haven't seen before.
Of course, many of the syntactic details will be familiar, but they're just details. We treat
the important ideas in a completely different order from what you've probably
encountered. For example, we don't mention pointers or arrays until Chapter 10, and
we're not even going to discuss your old favorites, printf and malloc, at all. On the other
hand, we start talking about the standard-library string class in Chapter 1. When we say
we're adopting a new approach, we mean it!
Structure of this book
You may find it convenient to think of this book as being in two parts. The first part,
through Chapter 7, concentrates on programs that use standard-library abstractions.
The second part, starting with Chapter 8, talks about defining your own abstractions.
Presenting the library first is an unusual idea, but we think it's right. Much of the C++
language—especially the harder parts—exists mostly for the benefit of library authors.
Library users don't need to know those parts of the language at all. By ignoring those
parts of the language until the second part of the book, we make it possible to write
useful C++ programs much more quickly than if we had adopted a more conventional
approach.
Once you have understood how to use the library, you will be ready to learn about the
low-level facilities on which the library is built, and how to use those facilities to write
your own libraries. Moreover, you will have a feeling for how to make a library useful,
and when to avoid writing new library code altogether.
Although this book is smaller than many C++ books, we have tried to use every
important idea at least twice, and key ideas more than that. As a result, many parts of
the book refer to other parts. These references look like §39.4.3/857, which refers to
text on page 857 that is part of section 39.4.3—or at least it would do so if this book had
that many sections or pages. The first time we explain each idea, we mention it in bold
italic type to make it easy to find and to call your attention to it as an important point.
Every chapter (except the last) concludes with a section called Details. These sections
serve two purposes: They make it easy to remember the ideas that the chapter
introduced, and they cover additional, related material that we think you will need to
know eventually. We suggest that you skim these sections on first reading, and refer
back to them later as needed.
The two appendices summarize and elucidate the important parts of the language and
library at a level of detail that we hope will be useful when you are writing programs.
Getting the most out of this book
Every book about programming includes example programs, and this one is no different.
In order to understand how these programs work, there is no substitute for running
them on a computer. Such computers abound, and new ones appear constantly—which
means that anything we might say about them would be inaccurate by the time you
read these words. Therefore, if you do not yet know how to compile and execute a C++
program, please visit http://www.acceleratedcpp.com and see what we have to say
there. We will update that website from time to time with information and advice about
the mechanics of running C++ programs. The site also offers machine-readable versions
of some of the example programs, and other information that you might find interesting.
Acknowledgments
We would like to thank the people without whom this book would have been impossible.
It owes much of its form to our reviewers: Robert Berger, Dag Brück, Adam Buchsbaum,
Stephen Clamage, John Kalb, Jeffrey Oldham, David Slayton, Bjarne Stroustrup, Albert
Tenbusch, Bruce Tetelman, and Clovis Tondo. Many people from Addison-Wesley
participated in its publication; the ones we know about are Tyrrell Albaugh, Bunny Ames,
Mike Hendrickson, Deborah Lafferty, Cathy Ohala, and Simone Payment. Alexander
Tsiris checked the Greek etymology in §13.2.2/236. Finally, the idea of starting with
high-level programs grew over many years, stimulated by the hundreds of students who
have sat through our courses and the thousands of people who have attended our talks.
Andrew Koenig Gillette, New Jersey
Barbara E. Moo June 2000
To our students,
who taught us
how to teach.
0
Getting started
Let us begin by looking at a small C++ program:
// a small C++ program
#include
int main()
{
std::cout << "Hello, world!" << std::endl;
return 0;
}
Programmers often refer to such a program as a Hello, world! program. Despite its
small size, you should take the time to compile and run this program on your computer
before reading further. The program should write
Hello, world!
on the standard output, which will typically be a window on your display screen. If you
have trouble, find someone who already knows C++ and ask for help, or consult our
Website, http://www.acceleratedcpp.com, for advice.
This program is useful because it is so simple that if you have trouble, the most likely
reasons are obvious typographical errors or misconceptions about how to use the
implementation. Moreover, thoroughly understanding even such a small program can
teach a surprising amount about the fundamentals of C++. In order to gain this
understanding, we'll look in detail at each line of the program.
0.1 Comments
The first line of our program is
// a small C++ program
The // characters begin a comment, which extends to the end of the line. The compiler
ignores comments; their purpose is to explain the program to a human reader. In this
book, we shall put the text of each comment in italic type, to make it easier for you to
distinguish comments from other parts of the program.

0.2 #include
In C++, many fundamental facilities, such as input-output, are part of the standard
library, rather than being part of the core language. This distinction is important
because the core language is always available to all C++ programs, but you must
explicitly ask for the parts of the standard library that you wish to use.
Programs ask for standard-library facilities by using #include directives. Such
directives normally appear at the beginning of a program. The only part of the standard
library that our program uses is input-output, which we request by writing
#include
The name iostream suggests support for sequential, or stream, input-output, rather
than random-access or graphical input-output. Because the name iostream appears in
an #include directive and it is enclosed in angle brackets (< and >), it refers to a
part of the C++ library called a standard header.
The C++ standard does not tell us exactly what a standard header is, but it does define
each header's name and behavior. Including a standard header makes the associated
library facilities available to the program, but exactly how the implementation does so is
its concern, not ours.
0.3 The main function
A function is a piece of program that has a name, and that another part of the program
can call, or cause to run. Every C++ program must contain a function named main.
When we ask the C++ implementation to run a program, it does so by calling this
function.
The main function is required to yield an integer as its result, the purpose of which is to
tell the implementation whether the program ran successfully. A zero value indicates
success; any other value means there was a problem. Accordingly, we begin by writing
int main()
to say that we are defining a function named main that returns a value of type int.
Here, int is the name that the core language uses to describe integers. The
parentheses after main enclose the parameters that our function receives from the
implementation. In this particular example, there are no parameters, so there is nothing
between the parentheses. We'll see how to use main's parameters in §10.4/179.
0.4 Curly braces
We continue our definition of the main function by following the parentheses with a
sequence of statements enclosed in curly braces (often simply called braces):
int main()
{ // left brace
// the statements go here
} // right brace
In C++, braces tell the implementation to treat whatever appears between them as a
unit. In this example, the left brace marks the beginning of the statements in our main
function, and the right brace marks their end. In other words, the braces indicate that
all the statements between them are part of the same function.
When there are two or more statements within braces, as there are in this function, the
implementation executes them in the order in which they appear.
0.5 Using the standard library for output
The first statement inside the braces does our program's real work:
std::cout << "Hello, world!" << std::endl;
This statement uses the standard library's output operator, <<, to write Hello,
world! on the standard output, and then to write the value of std::endl.
Preceding a name by std:: indicates that the name is part of a namespace named
std. A namespace is a collection of related names; the standard library uses std to
contain all the names that it defines. So, for example, the iostream standard header
defines the names cout and endl, and we refer to these names as std::cout and
std::endl.
The name std::cout refers to the standard output stream, which is whatever facility
the C++ implementation uses for ordinary output from programs. In a typical C++
implementation under a windowing operating system, std::cout will denote the
window that the implementation associates with the program while it is running. Under
such a system, the output written to std::cout will appear in the associated window.
Writing the value of std::endl ends the current line of output, so that if this program
were to produce any more output, that output would appear on a new line.
0.6 The return statement
A return statement, such as
return 0;
ends execution of the function in which it appears, and passes the value that appears
between the return and the semicolon (0 in this example) back to the program that
called the function that is returning. The value that is returned must have a type that is
appropriate for the type that the function says it will return. In the case of main, the
return type is int and the program to which main returns is the C++ implementation
itself. Therefore, a return from main must include an integer-valued expression, which
is passed back to the implementation.
Of course, there may be more than one point at which it might make sense to terminate
a program; such a program may have more than one return statement. If the
definition of a function promises that the function returns a value of a particular type,
then every return statement in the function must return a value of an appropriate
type.
0.7 A slightly deeper look
This program uses two additional concepts that permeate C++: expressions and scope.
We will have much more to say about these concepts as this book progresses, but it is
worthwhile to begin with some of the basics here.
An expression asks the implementation to compute something. The computation yields
a result, and may also have side effects-that is, it may affect the state of the program
or the implementation in ways that are not directly part of the result. For example, 3+4
is an expression that yields 7 as its result, and has no side effects, and
std::cout << "Hello, world!" << std::endl
is an expression that, as its side effect, writes Hello, world! on the standard output
stream and ends the current line.
An expression contains operators and operands, both of which can take on many forms.
In our Hello, world! expression, the two << symbols are operators, and
std::cout, "Hello, world! " and std::endl are operands.
Every operand has a type. We shall have much more to say about types, but
essentially, a type denotes a data structure and the meanings of operations that make
sense for that data structure. The effect of an operator depends on the types of its
operands.
Types often have names. For example, the core language defines int as the name of a
type that represents integers, and the library defines std::ostream as the type that
provides stream-based output. In our program, std::cout has type std::ostream.
The << operator takes two operands, and yet we have written two << operators and
three operands. How can this be? The answer is that << is left-associative, which,
loosely speaking, means that when << appears twice or more in the same expression,
each << will use as much of the expression as it can for its left operand, and as little of
it as it can for its right operand. In our example, the first << operator has "Hello,
world! " as its right operand and std::cout as its left operand, and the second <<
operator has std::endl as its right operand and std::cout << "Hello, world!
" as its left operand. If we use parentheses to clarify the relationship between operands
and operators, we see that our output expression is equivalent to
(std::cout << "Hello, world!") << std::endl
Each << behaves in a way that depends on the types of its operands. The first << has
std::cout, which has type std::ostream, as its left operand. Its right operand is a
string literal, which has a mysterious type that we shall not even discuss until
§10.2/176. With those operand types, << writes its right operand's characters onto the
stream that its left operand denotes, and its result is its left operand.
The left operand of the second << is therefore an expression that yields std::cout,
which has type std::ostream; the right operand is std::endl, which is a
manipulator. The key property of manipulators is that writing a manipulator on a
stream manipulates the stream, by doing something other than just writing characters
to it. When the left operand of << has type std::ostream and the right operand is a
manipulator, << does whatever the manipulator says to do to the given stream, and
returns the stream as its result. In the case of std::endl, that action is to end the
current line of output.
The entire expression therefore yields std::cout as its value, and, as a side effect, it
writes Hello, world! on the standard output stream and ends the output line. When
we follow the expression by a semicolon, we are asking the implementation to discard
the value-which action is appropriate, because we are interested only in the side effects.
The scope of a name is the part of a program in which that name has its meaning. C++
has several different kinds of scopes, two of which we have seen in this program.
The first scope that we used is a namespace, which, as we've just seen, is a collection of
related names. The standard library defines all of its names in a namespace named std,
so that it can avoid conflicts with names that we might define for ourselves-as long as
we are not so foolish as to try to define std. When we use a name from the standard
library, we must specify that the name we want is the one from the library; for example,
std::cout means cout as defined in the namespace named std.
The name std::cout is a qualified name, which uses the :: operator. This operator
is also known as the scope operator. To the left of the :: is the (possibly qualified)
name of a scope, which in the case of std::cout is the namespace named std. To the
right of the :: is a name that is defined in the scope named on the left. Thus,
std::cout means "the name cout that is in the (namespace) scope std."
Curly braces form another kind of scope. The body of main-and the body of every
function-is itself a scope. This fact is not too interesting in such a small program, but it
will be relevant to almost every other function we write.
0.8 Details
Although the program we've written is simple, we've covered a lot of ground in this
chapter. We intend to build on what we've introduced here, so it is important for you to
be sure that you understand this chapter fully before you continue.
To help you do so, this chapter-and every chapter except Chapter 16-ends with a section
called Details and a set of exercises. The Details sections summarize and occasionally
expand on the information in the text. It is worth looking at each Details section as a
reminder of the ideas that the chapter introduced.
Program structure: C++ programs are usually in free form, meaning that spaces are
required only when they keep adjacent symbols from running together. In particular,
newlines (i.e., the way in which the implementation represents the change from one line
of the program to the next) are just another kind of space, and usually have no
additional special meaning. Where you choose to put spaces in a program can make it
much easier-or harder-to read. Programs are normally indented to improve readability.
There are three entities that are not free-form:
string literals
characters enclosed in double quotes; may not span lines
#include name
must appear on a line by themselves (except for comments)
// comments
// followed by anything; ends at the end of the current line
A comment that begins with /* is free-form; it ends with the first subsequent */ and
can span multiple lines.
Types define data structures and operations on those data structures. C++ has two
kinds of types: those built into the core language, such as int, and those that are
defined outside the core language, such as std::ostream.
Namespaces are a mechanism for grouping related names. Names from the standard
library are defined in the namespace called std.
String literals begin and end with double quotes ("); each string literal must appear
entirely on one line of the program. Some characters in string literals have special
meaning when preceded by a backslash (\):
\n newline character
\t tab character
\b backspace character
\" treats this symbol as part of the string rather than as the string terminator
\' same meaning as ' in string literals, for consistency with character literals
(§1.2/14)
\\ includes a \ in the string, treating the next character as an ordinary character
We'll see more about string literals in §10.2/176 and §A.2.1.3/302.
Definitions and headers: Every name that a C++ program uses must have a
corresponding definition. The standard library defines its names in headers, which
programs access through #include. Names must be defined before they are used;
hence, a #include must precede the use of any name from that header. The
header defines the library's input-output facilities.
The main function: Every C++ program must define exactly one function, named
main, that returns an int. The implementation runs the program by calling main. A
zero return from main indicates success; a nonzero return indicates failure. In general,
functions must include at least one return statement and are not permitted to fall off
the end of the function. The main function is special: It may omit the return; if it does
so, the implementation will assume a zero return value. However, explicitly including a
return from main is good practice.
Braces and semicolons: These inconspicuous symbols are important in C++ programs.
They are easy to overlook because they are small, and they are important because
forgetting one typically evokes compiler diagnostic messages that may be hard to
understand.
A sequence of zero or more statements enclosed in braces is a statement, called a
block, which is a request to execute the constituent statements in the order in which
they appear. The body of a function must be enclosed in braces, even if it is only a
single statement. The statements between a pair of matching braces constitute a scope.
An expression followed by a semicolon is a statement, called an expression statement,
which is a request to execute the expression for its side effects and discard its result.
The expression is optional; omitting it results in a null statement, which has no effect.
Output: Evaluating std::cout << e writes the value of e on the standard-output
stream, and yields std::cout, which has type ostream, as its value in order to allow
chained output operations.
Exercises
0-0. Compile and run the Hello, world! program.
0-1. What does the following statement do?
3 + 4;
0-2. Write a program that, when run, writes
This (") is a quote, and this (\) is a backslash.
0-3. The string literal "\t" represents a tab character; different C++ implementations
display tabs in different ways. Experiment with your implementation to learn how it
treats tabs.
0-4. Write a program that, when run, writes the Hello, world! program as its
output.
0-5. Is this a valid program? Why or why not?
#include
int main() std::cout << "Hello, world!" << std::endl;
0-6. Is this a valid program? Why or why not?
#include
int main() {{{{{{ std::cout << "Hello, world!" << std::endl; }}}}}}
0-7. What about this one?
#include
int main()
{
/* This is a comment that extends over several lines
because it uses /* and */ as its starting and ending delimiters */
std::cout << "Does this work?" << std::endl;
return 0;
}
0-8. ...and this one?
#include
int main()
{
// This is a comment that extends over several lines
// by using // at the beginning of each line instead of using /*
// or */ to delimit comments.
std::cout << "Does this work?" << std::endl;
return 0;
}
0-9. What is the shortest valid program?
0-10. Rewrite the Hello, world! program so that a newline occurs everywhere that
whitespace is allowed in the program.
1
Working with strings
Chapter 0 looked closely at a tiny program, which we used to introduce surprisingly
many fundamental C++ ideas: comments, standard headers, scopes, namespaces,
expressions, statements, string literals, and output. This chapter continues our overview
of the fundamentals by writing similarly simple programs that use character strings. In
the process, we'll learn about declarations, variables, and initialization, as well as
something about input and the C++ string library. The programs in this chapter are
so simple that they do not even require any control structures, which we will cover in
Chapter 2.
1.1 Input
Once we can write text, the logical next step is to read it. For example, we can modify
the Hello, world! program to say hello to a specific person:
// ask for a person's name, and greet the person
#include
#include
int main()
{
// ask for the person's name
std::cout << "Please enter your first name: ";
// read the name
std::string name; // define name
std::cin >> name; // read into
// write a greeting
std::cout << "Hello, " << name << "!" << std::endl;
return 0;
}
When we execute this program, it will write
Please enter your first name:
on the standard output. If we respond, for example,
Vladimir
then the program will write
Hello, Vladimir!
Let's look at what's going on. In order to read input, we must have a place to put it.
Such a place is called a variable. A variable is an object that has a name. An object, in
turn, is a part of the computer's memory that has a type. The distinction between
objects and variables is important because, as we'll see in §3.2.2/45, §4.2.3/65, and
§10.6.1/183, it is possible to have objects that do not have names.
If we wish to use a variable, we must tell the implementation what name to give it and
what type we want it to have. The requirement to supply both a name and a type makes
it easier for the implementation to generate efficient machine code for our programs.
The requirement also lets the compiler detect misspelled variable names-unless the
misspelling happens to match one of the names that our program said it intended to
use.
In this example, our variable is named name, and its type is std::string. As we saw
in §0.5/3 and §0.7/5, the use of std:: implies that the name, string, that follows it is
part of the standard library, not part of the core language or of a nonstandard library. As
with every part of the standard library, std::string has an associated header, namely
, so we've added an appropriate #include directive to our program.
The first statement,
std::cout << "Please enter your first name: ";
should be familiar by now: It writes a message that asks for the user's name. An
important part of this statement is what isn't there, namely the std::endl
manipulator. Because we did not use std::endl, the output does not begin a new line
after the program has written its message. Instead, as soon as it has written the
prompt, the computer waits-on the same line-for input. The next statement,
std::string name; // define name
is a definition, which defines our variable named name that has type std::string.
Because this definition appears within a function body, name is a local variable, which
exists only while the part of the program within the braces is executing. As soon as the
computer reaches the }, it destroys the variable name, and returns any memory that
the variable occupied to the system for other uses. The limited lifetime of local variables
is one reason that it is important to distinguish between variables and other objects.
Implicit in the type of an object is its interface-the collection of operations that are
possible on an object of that type. By defining name as a variable (a named object) of
type string, we are implicitly saying that we want to be able to do with name whatever
the library says that we can do with strings.
One of those operations is to initialize the string. Defining a string variable
implicitly initializes it, because the standard library says that every string object starts
out with a value. We shall see shortly that we can supply a value of our own when we
create a string. If we do not do so, then the string starts out containing no
characters at all. We call such a string an empty or null string.
Once we have defined name, we execute
std::cin >> name; // read into name
which is a statement that reads from std::cin into name. Analogous with its use of the
<< operator and std::cout for output, the library uses the >> operator and
std::cin for input. In this example, >> reads a string from the standard input and
stores what it read in the object named name. When we ask the library to read a
string, it begins by discarding whitespace characters (space, tab, backspace, or the
end of the line) from the input, then reads characters into name until it encounters
another whitespace character or end-of-file. Therefore, the result of executing
std::cin >> name is to read a word from the standard input, storing in name the
characters that constitute the word.
The input operation has another side effect: It causes our prompt, which asks for the
user's name, to appear on the computer's output device. In general, the input-output
library saves its output in an internal data structure called a buffer, which it uses to
optimize output operations. Most systems take a significant amount of time to write
characters to an output device, regardless of how many characters there are to write. To
avoid the overhead of writing in response to each output request, the library uses the
buffer to accumulate the characters to be written, and flushes the buffer, by writing its
contents to the output device, only when necessary. By doing so, it can combine several
output operations into a single write.
There are three events that cause the system to flush the buffer. First, the buffer might
be full, in which case the library will flush it automatically. Second, the library might be
asked to read from the standard input stream. In that case, the library immediately
flushes the output buffer without waiting for the buffer to become full. The third
occasion for flushing the buffer is when we explicitly say to do so.
When our program writes its prompt to cout, that output goes into the buffer
associated with the standard output stream. Next, we attempt to read from cin. This
read flushes the cout buffer, so we are assured that our user will see the prompt.
Our next statement, which generates the output, explicitly instructs the library to flush
the buffer. That statement is only slightly more complicated than the one that wrote the
prompt. Here we write the string literal "Hello, " followed by the value of the string
variable name, and finally by std::endl. Writing the value of std::endl ends the line
of output, and then flushes the buffer, which forces the system to write to the output
stream immediately.
Flushing output buffers at opportune moments is an important habit when you are
writing programs that might take a long time to run. Otherwise, some of the program's
output might languish in the system's buffers for a long time between when your
program writes it and when you see it.

1.2 Framing a name
So far, our program has been restrained in its greetings. We'd like to change that by
writing a more elaborate greeting, so that the input and output look like this:
Please enter your first name: Estragon
********************
* *
* Hello, Estragon! *
* *
********************
Our program will produce five lines of output. The first line begins the frame. It is a
sequence of * characters as long as the person's name, plus some characters to match
the salutation ("Hello, "), plus a space and an * at each end. The line after that will be
an appropriate number of spaces with an * at each end. The third line is an *, a space,
the message, a space, and an *. The last two lines will be the same as the second and
first lines, respectively.
A sensible strategy is to build up the output a piece at a time. First we'll read the name,
then we'll use it to construct the greeting, and then we'll use the greeting to build each
line of the output. Here is a program that uses that strategy to solve our problem:
// ask for a person's name, and generate a framed greeting
#include
#include
int main()
{
std::cout << "Please enter your first name: ";
std::string name;
std::cin >> name;
// build the message that we intend to write
const std::string greeting = "Hello, " + name + "!";
// build the second and fourth lines of the output
const std::string spaces(greeting.size(), ' ');
const std::string second = "* " + spaces + " *";
// build the first and fifth lines of the output
const std::string first(second.size(), '*');
// write it all
std::cout << std::endl;
std::cout << first << std::endl;
std::cout << second << std::endl;
std::cout << "* " << greeting << " *" << std::endl;
std::cout << second << std::endl;
std::cout << first << std::endl;
return 0;
}
First, our program asks for the user's name, and reads that name into a variable named
name. Then, it defines a variable named greeting that contains the message that it
intends to write. Next, it defines a variable named spaces, which contains as many
spaces as the number of characters in greeting. It uses the spaces variable to define
a variable named second, which will contain the second line of the output, and then the
program constructs first as a variable that contains as many * characters as the
number of characters in second. Finally, it writes the output, a line at a time.
The #include directives and the first three statements in this program should be
familiar. The definition of greeting, on the other hand, introduces three new ideas.
One idea is that we can give a variable a value as we define it. We do so by placing,
between the variable's name and the semicolon that follows it, an = symbol followed by
the value that we wish the variable to have. If the variable and value have different
types-as §10.2/176 shows that strings and string literals do-the implementation will
convert the initial value to the type of the variable.
The second new idea is that we can use + to concatenate a string and a string
literal-or, for that matter, two strings (but not two string literals). We noted in passing
in Chapter 0 that 3 + 4 is 7. Here we have an example in which + means something
completely different. In each case, we can determine what the + operator does by
examining the types of its operands. When an operator has different meanings for
operands of different types, we say that the operator is overloaded.
The third idea is that of saying const as part of a variable's definition. Doing so
promises that we are not going to change the value of the variable for the rest of its
lifetime. Strictly speaking, this program gains nothing by using const. However,
pointing out which variables will not change can make a program much easier to
understand.
Note that if we say that a variable is const, we must initialize it then and there,
because we won't have the opportunity later. Note also that the value that we use to
initialize the const variable need not itself be a constant. In this example, we won't
know the value of greeting until after we have read a value into name, which
obviously can't happen until we run the program. For this reason, we cannot say that
name is const, because we change its value by reading into it.
One property of an operator that never changes is its associativity. We learned in
Chapter 0 that << is left-associative, so that std::cout << s << t means the same
as (std::cout << s) << t. Similarly, the + operator (and, for that matter, the >>
operator) is also left-associative. Accordingly, the value of "Hello, " + name + "!"
is the result of concatenating "Hello, " with name, and concatenating the result of
that concatenation with "!". So, for example, if the variable name contains Estragon,
then the value of "Hello, " + name + "!" is Hello, Estragon!
At this point, we have figured out what we are going to say, and saved that information
in the variable named greeting. Our next job is to build the frame that will enclose our
greeting. In order to do so, we introduce three more ideas in a single statement:
std::string spaces(greeting.size(), ' ');
When we defined greeting, we used an = symbol to initialize it. Here, we are following
spaces by two expressions, which are separated by a comma and enclosed in
parentheses. When we use the = symbol, we are saying explicitly what value we would
like the variable to have. By using parentheses in a definition, as we do here, we tell the
implementation to construct the variable-in this case, spaces-from the expressions, in
a way that depends on the type of the variable. In other words, in order to understand
this definition, we must understand what it means to construct a string from two
expressions.
How a variable is constructed depends entirely on its type. In this particular case, we
are constructing a string from-well, from what? Both expressions are of forms that we
haven't seen before. What do they mean?
The first expression, greeting.size(), is an example of calling a member function.
In effect, the object named greeting has a component named size, which turns out
to be a function, and which we can therefore call to obtain a value. The variable
greeting has type std::string, which is defined so that evaluating
greeting.size() yields an integer that represents the number of characters in
greeting.
The second expression, ' ', is a character literal. Character literals are completely
distinct from string literals. A character literal is always enclosed in single quotes; a
string literal is always enclosed in double quotes. The type of a character literal is the
built-in type char; the type of a string literal is much more complicated, and we shall
not explain it until §10.2/176. A character literal represents a single character. The
characters that have special meaning inside a string literal have the same special
meaning in a character literal. Thus, if we want ' or \, we must precede it by \. For
that matter, '\n', '\t', '\"', and related forms work analogously to the way we
saw in Chapter 0 that they work for string literals.
To complete our understanding of spaces, we need to know that when we construct a
string from an integer value and a char value, the result has as many copies of the
char value as the value of the integer. So, for example, if we were to define
std::string stars(10, '*');
then stars.size() would be 10, and stars itself would contain **********.
Thus, spaces contains the same number of characters as greeting, but all of those
characters are blanks.
Understanding the definition of second requires no new knowledge: We concatenate "
* ", our string of spaces, and " *" to obtain the second line of our framed message.
The definition of first requires no new knowledge either; it gives first a value that
contains as many * characters as the number of characters in second.
The rest of the program should be familiar; all it does is write strings in the same way
we did in §1.1/9.
1.3 Details
Types:
char
Built-in type that holds ordinary characters as defined by the implementation.
wchar_t
Built-in type intended to hold "wide characters," which are big enough to
hold characters for languages such as Japanese.
The string type is defined in the standard header string contains a sequence of zero or more characters. If n is an integer, c is a char,
is is an input stream, and os is an output stream, then the string operations include
std::string s;
Defines s as a variable of type std::string that is initially empty.
std::string t = s;
Defines t as a variable of type std::string that initially contains a copy
of the characters in s , where s can be either a string or a string literal.
std::string z(n, c);
Defines z as a variable of type std::string that initially contains n copies
of the character c . Here, c must be a char , not a string or a string literal.
os << s
Writes the characters contained in s , without any formatting changes, on the
output stream denoted by os . The result of the expression is os .
is >> s
Reads and discards characters from the stream denoted by is until encountering
a character that is not whitespace. Then reads successive characters
from is into s , overwriting whatever value s might have had, until the next
character read would be whitespace. The result is is .
s + t
The result of this expression is an std::string that contains a copy of the
characters in s followed by a copy of the characters in t . Either s or t , but
not both, may be a string literal or a value of type char .
s.size()
The number of characters in s .
Variables can be defined in one of three ways:
std::string hello = "Hello"; // define the variable with an explicit initial value
std::string stars(100, '*'); // construct the variable
// according to its type and the given expressions
std::string name; // define the variable with an implicit initialization,
// which depends on its type
Variables defined inside a pair of curly braces are local variables/which exist only while
executing the part of the program within the braces. When the implementation reaches
the } , it destroys the variables, and returns any memory that they occupied to the
system. Defining a variable as const promises that the variable's value, will not change
during its lifetime. Such a variable must be initialized as part of its definition, because
there is no way to do so later.
Input: Executing std::cin >> v discards any whitespace characters in the standard
input stream, then reads from the standard input into variable v . It returns std::cin ,
which has type istream , in order to allow chained input operations.
Exercises
1-0. Compile, execute, and test the programs in this chapter.
1-1. Are the following definitions valid? Why or why not?
const std::string hello = "Hello";
const std::string message = hello + ", world" + "!";
1-2. Are the following definitions valid? Why or why not?
const std::string exclam = "!";
const std::string message = "Hello" + ", world" + exclam;
1-3. Is the following program valid? If so, what does it do? If not, why not?
#include
#include
int main()
{
{ const std::string s = "a string";
std::cout << s << std::endl; }
{ const std::string s = "another string";
std::cout << s << std::endl; }
return 0;
}
1-4. What about this one? What if we change }} to };} in the third line from the end?
#include
#include
int main()
{
{ const std::string s = "a string";
std::cout << s << std::endl;
{ const std::string s = "another string";
std::cout << s << std::endl; }}
return 0;
}
1-5. Is this program valid? If so, what does it do? If not, say why not, and rewrite it to
be valid.
#include
#include
int main()
{
{ std::string s = "a string";
{ std::string x = s + ", really";
std::cout << s << std::endl; }
std::cout << x << std::endl;
}
return 0;
}
1-6. What does the following program do if, when it asks you for input, you type two
names (for example, Samuel Beckett)? Predict the behavior before running the program,
then try it.
#include
#include
int main()
{
std::cout << "What is your name? ";
std::string name;
std::cin >> name;
std::cout << "Hello, " << name
<< std::endl << "And what is yours? ";
std::cin >> name;
std::cout << "Hello, " << name
<< "; nice to meet you too!" << std::endl;
return 0;
}
2
Looping and counting
In §1.2/11, we developed a program that writes a formatted frame around a greeting.
In this chapter, we're going to make the program more flexible so that we can change
the size of the frame without rewriting the program.
Along the way, we'll start learning about arithmetic in C++, and how C++ supports
loops and conditions, and we'll explore the related idea of loop invariants.
2.1 The problem
The program in §1.2/12 wrote a greeting with a frame around it. For example, if our
user gave us the name Estragon, our program would write
********************
* *
* Hello, Estragon! *
* *
********************
The program built up the output a line at a time. It defined variables named first and
second to contain the first and second lines of the output, and wrote the greeting itself,
surrounded by some characters, as the third line. We didn't need separate variables for
the fourth or fifth output lines, because those were the same as the second and first
lines respectively.
This approach has a major shortcoming: Each line of the output has a part of the
program-and a variable-that corresponds to it. Therefore, even a simple change to the
output format, such as removing the spaces between the greeting and the frame, would
require rewriting the program. We would like to produce a more flexible form of output
without having to store each line in a local variable.
We will approach this problem by generating each character of the output separately,
except for the greeting itself, which we already have available as a string. What we
shall discover is that there is no need to store the output characters in variables,
because once we have written a character, we don't need it any more.
2.2 Overall structure
We'll begin by reviewing the part of the program that we don't have to rewrite:
#include
#include
int main()
{
// ask for the person's name
std::cout << "Please enter your first name: ";
// read the name
std::string name;
std::cin >> name;
// build the message that we intend to write
const std::string greeting = "Hello, " + name + "!";
// we have to rewrite this part...
return 0;
}
As we rewrite the part of the program that the we have to rewrite this part... comment
represents, we shall already be in a context that defines name, greeting, and the
relevant names from the standard library. We will build up the new version of the
program a piece at a time, and then, in §2.5.4/29, we'll put all the pieces together.
2.3 Writing an unknown number of rows
We can think of our output as a rectangular array of characters, which we must write
one row at a time. Although we don't know how many rows it has, we do know how to
compute the number of rows.
The greeting takes up one row, as do the top and bottom rows of the frame. We've
accounted for three rows so far. If we know how many blank rows we intend to leave
between the greeting and the frame, we can double that number and add three to
obtain the total number of rows in the output:
// the number of blanks surrounding the greeting
const int pad = 1;
// total number of rows to write
const int rows = pad * 2 + 3;
We want to make it easy to find the part of our program that defines the number of
blanks, so we give that number a name. The variable called pad represents the amount
of padding around the frame. Having defined pad, we use it in computing rows, which
will control how many rows we write.
The built-in type int is the most natural type to use for integers, so we've chosen that
type for pad and rows. We also said that both variables are const, which we know
from §1.2/13 is a promise that we will not change the value of either pad or rows.
Looking ahead, we intend to use the same number of blanks on the left and right sides
as on the top and bottom, so one variable will serve for all four sides. If we are careful
to use this variable every time we want to refer to the number of blanks, changing the
size of the frame will require only changing the program to give the variable a different
value.
We have computed how many rows we need to write; our next problem is to do so:
// separate the output from the input
std::cout << std::endl;
// write rows rows of output
int r = 0;
// invariant: we have written r rows so far
while (r != rows) {
// write a row of output (as we will describe in §2.4/22)
std::cout << std::endl;
++r;
}
We start, as we did in §1.2/12, by writing a blank line, so that there will be some space
between the input and the output. The rest of this fragment contains so many new ideas
that we need to look at it closely. Once we've understood how it works, we'll think about
how to write each individual row.
2.3.1 The while statement
Our program controls how many rows of output it writes by using a while statement,
which repeatedly executes a given statement as long as a given condition is true. A
while statement has the form
while (condition)
statement
The statement is often called the while body.
The while statement begins by testing the value of the condition. If the condition is
false, it does not execute the body at all. Otherwise, it executes the body once, after
which it tests the condition again, and so on. The while alternates between testing the
condition and executing the body until the condition is false, at which point execution
continues after the end of the entire while statement.
Loosely speaking, we can think of the while statement in our example as saying, "As
long as the value of r is not equal to rows, do whatever is within the { }."
It is conventional to put the while body on a separate line and indent it, to make
programs easier to read. The implementation doesn't stop us from writing
while (condition) statement
but if we do so, we should think about whether we might be making life harder for other
people who might read our program.
Note that there is no semicolon after statement in this description. Either the statement
is indeed just a statement, or it is a block, which is a sequence of zero or more
statements enclosed in { }. If the statement is just an ordinary statement, it will end
with a semicolon of its own, so there's no need for another one. If it is a block, the
block's } marks the end of the statement, so again there's no need for a semicolon.
Because a block is a sequence of statements enclosed by braces, we know from §0.7/5
that a block is a scope.
The while begins by testing its condition, which is an expression that appears in a
context where a truth value is required. The expression r != rows is an example of a
condition. This example uses the inequality operator, !=, to compare r and rows.
Such an expression has type bool, which is a built-in type that represents truth values.
The two possible values of type bool are true and false, with the obvious meanings.
The other new facility in this program is the last statement in the while body, which is
++r;
The ++ is the increment operator, which has the effect of incrementing-adding 1 to-the
variable r. We could have written
r = r + 1;
instead, but incrementing an object is so common that a special notation for doing so is
useful. Moreover, as we shall see in §5.1.2/79, the idea of transforming a value into its
immediate successor, in contrast with computing an arbitrary value, is so fundamental
to abstract data structures that it deserves a special notation for that reason alone.
2.3.2 Designing a while statement
Determining exactly what condition to write in a while statement is sometimes difficult.
Similarly, it can be hard to understand precisely what a particular while statement
does. It is not too hard to see that the while statement in §2.3/19 will write a number
of output rows that depends on the value of rows, but how can we be confident that we
know exactly how many rows the program will write? For example, how do we know
whether the number will be rows, rows - 1, rows + 1, or something else entirely?
We could trace through the while by hand, noting the effect of each statement's
execution on the state of the program, but how do we know that we haven't made a
mistake along the way?
There is a useful technique for writing and understanding while statements that relies
on two key ideas-one about the definition of a while statement, and the other about
the behavior of programs in general.
The first idea is that when a while finishes, its condition must be false-otherwise the
while wouldn't have finished. So, for example, when the while in §2.3/19 finishes, we
know that r != rows is false and, therefore, that r is equal to rows.
The second idea is that of a loop invariant, which is a property that we assert will be
true about a while each time it is about to test its condition. We choose an invariant
that we can use to convince ourselves that the program behaves as we intend, and we
write the program so as to make the invariant true at the proper times. Although the
invariant is not part of the program text, it is a valuable intellectual tool for designing
programs. Every useful while statement that we can imagine has an invariant
associated with it. Stating the invariant in a comment can make a while much easier to
understand.
To make this discussion concrete, we will look again at the while statement in §2.3/19.
The comment immediately before the while says what the invariant is: We have written
r rows of output so far.
To determine that this invariant is correct for this program fragment, we must verify
that the invariant is true each time the while is about to test its condition. Doing so
requires us to verify that the invariant will be true at two specific points in the program.
The first point is just before the while tests its condition for the first time. It is easy to
verify the invariant at this point in our example: Because we have written no rows of
output so far, it is obvious that setting r to 0 makes the invariant true.
The second point is just before we reach the end of the while body. If the invariant is
true there, it will be true the next time the while tests the condition. Therefore, the
invariant will be true every time.
In exchange for writing our program so that it meets these two requirements-causing
the invariant to be true before the while starts, and again at the end of the while
body-we can be confident that the invariant is true not only each time the while tests
the condition, but also after the while finishes. Otherwise, the invariant would have
had to be true at the beginning of one of the iterations of the while body and false
afterward-and we have already arranged for that to be impossible.
Here is a summary of what we know about our program fragment:
// invariant: we have written r rows so far
int r = 0;
// setting r to 0 makes the invariant true
while (r != rows) {
// we can assume that the invariant is true here
// writing a row of output makes the invariant false
std::cout << std::endl;
// incrementing r makes the invariant true again
++r;
}
// we can conclude that the invariant is true here
The invariant for our while is that we have written r rows of output so far. When we
define r, we give it an initial value of 0. At this point, we haven't written anything at all.
Setting r to 0 obviously makes the invariant true, so we have met the first requirement.
To meet the second requirement, we must verify that whenever the invariant is true
when the while is about to test its condition, a trip through the condition and body will
leave the invariant true at the end of the body.
Writing a row of output causes the invariant to become false, because r is no longer the
number of rows we have written. However, incrementing r to account for the row that
was written will make the invariant true again. Doing so makes the invariant true at the
end of the body, so we have met the second requirement.
Because both requirements are true, we know that after the while finishes, we have
written r rows. Moreover, we have already seen that r == rows. Together, these two
facts imply that rows is the total number of rows that we have written.
The strategy that we used to understand this loop will come in handy in a variety of
contexts. The general idea is to find an invariant that states a relevant property of the
variables that the loop involves (we have written r rows), and to use the condition to
ensure that when the loop completes, those variables will have useful values (r ==
rows). The loop body's job is then to manipulate the relevant variables so as to arrange
for the condition to be false eventually, while maintaining the truth of the invariant.
2.4 Writing a row
Now that we understand how to write a given number of rows, we can turn our attention
to writing a single row. In other words, we can start filling in the part of the program
represented by the write a row of output comment in §2.3/19.
We begin by observing that all the output lines are the same length. If we think of the
output as a rectangular array, then that length is the number of columns in the array.
We can compute that number by adding twice the padding to the length of the greeting,
and then adding two for the asterisks at the ends:
const std::string::size_type cols = greeting.size() + pad * 2 + 2;
The easy part of reading this definition is to see that we've said that cols is const,
thereby promising that the value of cols will not change after we have defined it. The
harder part to understand is that we have defined cols using an unfamiliar type,
namely std::string::size_type. We know that the first :: is the scope operator
and that the qualified name std::string means the name string from the
namespace std. The second :: similarly says that we want the name size_type from
the class string. Like namespaces and blocks, classes define their own scopes. The
std::string type defines size_type to be the name of the appropriate type for
holding the number of characters in a string. Whenever we need a local variable to
contain the size of a string, we should use std::string::size_type as the type of
that variable.
The reason that we have given cols a type of std::string::size_type is to ensure
that cols is capable of containing the number of characters in greeting, no matter
how large that number might be. We could simply have said that cols has type int,
and indeed, doing so would probably work. However, the value of cols depends on the
size of the input to our program, and we have no control over how long that input might
be. It is conceivable that someone might give our program a string so long that an int
is insufficient to contain its length.
The int type is sufficient for rows because the number of rows depends only on the
value of pad, which we control. Every C++ implementation is required to allow every
int variable to take on values up to at least 32767, which is plenty. Nevertheless,
whenever we define a variable that contains the size of a particular data structure, it is a
good habit to use the type that the library defines as being appropriate for that specific
purpose.
It is impossible for a string to contain a negative number of characters. Accordingly,
std::string::size_type is an unsigned type-objects of that type are incapable of
containing negative values. This property does not affect the programs in this chapter,
but we shall see later on in §8.1.3/142 that it can be critically important.
Having figured out how many characters to write, we can use another while statement
to write them:
std::string::size_type c = 0;
// invariant: we have written c characters so far in the current row
while (c != cols) {
// write one or more characters
// adjust the value of c to maintain the invariant
}
This while behaves analogously to the one in §2.3/19, except for one difference in the
body: This time we have said write one or more characters instead of writing exactly
one row as we did in §2.3/19. There is no reason that we have to write only a single
character each time through the body. As long as we write at least one character, we
will ensure progress. All we have to do is ensure that the total number of characters we
write on this row is exactly cols.
2.4.1 Writing border characters
Our remaining problem is to figure out what characters to write. We can solve part of
this problem by noting that if we are on the first or last row, or on the first or last
column, then we know that we should write an asterisk. Moreover, we can use our
knowledge of the loop invariants to determine whether it is time to write an asterisk.
For example, if r is zero, we know from the invariant that we have not yet written any
rows, which means that we are writing part of the first row. Similarly, if r is equal to
rows - 1, we know that we have written rows - 1 rows already, so we must now be
writing part of the last row. We can use analogous reasoning to conclude that if c is
zero, then we are writing part of the first column, and if c is equal to cols - 1, we are
writing part of the last column. Using this knowledge, we can fill in more of our
program:
// invariant: we have written c characters so far in the current row
while (c != cols) {
if (r == 0 || r == rows - 1 || c == 0 || c == cols - 1) {
std::cout << "*";
} else {
// write one or more nonborder characters
// adjust the value of c to maintain the invariant
}
This statement introduces so many new ideas that it requires detailed explanation.
2.4.1.1 if statements
The while body consists of a block (§2.3.1/19) that contains an if statement, which
we use to determine whether it is time to write an asterisk. An if statement can take
either of two forms:
if (condition)
statement
or, as used here,
if (condition)
statement1
else
statement2
As with the while statement, the condition is an expression that yields a truth value. If
the condition is true, then the program executes the statement that follows the if. In
the second form of the if statement, if the condition is false, then the program
executes the statement that follows the else.
It is worth noting, just as with our description of the form of the while statement, that
the formatting that we use to illustrate the if statement is merely conventional.
However, readers will find it much easier if code follows formatting conventions such as
the ones that we've used in the examples in this book.
2.4.1.2 Logical operators
What about the condition itself?
r == 0 || r == rows - 1 || c == 0 || c == cols - 1
This condition is true if r is 0 or rows - 1, or if c is 0 or cols - 1. The condition
uses two new operators, the == operator and the || operator. C++ programs test for
equality by using the == symbol, to distinguish it from the assignment operator =.
Thus, r == 0 yields a bool that indicates whether the value of r is equal to 0. The
logical-or operator, written as ||, yields true if either of its operands is true.
The relational operators have lower precedence than the arithmetic operators. In
expressions that contain more than one operator, precedence defines how the operands
group. For example,
r == rows - 1
means
r == (rows - 1)
rather than
(r == rows) - 1
because the arithmetic operator - has higher precedence than the relational operator
==. In other words, we are subtracting 1 from rows and comparing the result with r,
which, in this program, is what we wanted. We can override precedence by enclosing in
parentheses a subexpression that we want to use as a single operand. For example, if
we really wanted to execute (r == rows) - 1, we could do so by including the
parentheses as shown. This expression would compare r with rows and subtract 1 from
the result, yielding either 0 or -1 depending on whether r was equal to rows.
The logical-or operator tests whether either of its operands is true. Its form is
condition1 || condition2
where, as usual, condition1 and condition2 are conditions-expressions that yield truth
values. The || expression yields a bool, which is true if either of the conditions is
true.
The || operator has lower precedence than the relational operators, and, like most C++
binary operators, is left-associative. Moreover, it has a property that most other C++
operators do not share: If a program finds that the left operand of || is true, it does not
evaluate the right operand at all. This property is often called short-circuit evaluation,
and as we shall see in §5.6/89, it can have a crucial effect on how we write our
programs.
Because || is left-associative, and because of the relative precedence of ||,== ,and -,
r == 0 || r == rows - 1 || c == 0 || c == cols - 1
means the same as it would if we were to place all of its subexpressions in parentheses:
((r == 0 || r == (rows - 1)) || c == 0) || c == (cols - 1)
In order to evaluate this latter expression using the short-circuit strategy, the program
first evaluates the left operand of the outermost ||, which is
(r == 0 || r == (rows - 1)) || c == 0
To do so, it must first evaluate the left operand of this inner ||, which is
r == 0 || r == (rows - 1)
which, in turn, means evaluating
r == 0
If r is equal to 0, then each of the expressions
r == 0 || r == (rows - 1)
(r == 0 || r == (rows - 1)) || c == 0
((r == 0 || r == (rows - 1)) || c == 0) || c == (cols - 1)
must be true. If r is nonzero, the next step is to compare r with rows - 1. If that test
fails, then the program will compare c with zero, and if that fails, it will compare c with
cols - 1 to determine the final result.
In other words, when we write a series of conditions separated by || operators, we are
asking the program to test each of these conditions in turn. If any of the inner
conditions is true, then the whole condition is true; otherwise, the whole condition is
false. Each || operator stops as soon as it can determine its result, so if any of the
inner conditions is true, the subsequent conditions go untested. If we step back from
the details, we should be able to see that these four equality tests are checking whether
we are in the first row, the last row, the first column, or the last column, and, therefore,
that the if statement writes an asterisk if we're in the top or bottom row, or if we're in
the first or last column. Otherwise, it does something else, which we must now define.
2.4.2 Writing nonborder characters
It is now time to write the statements that correspond to the comments that say
// write one or more nonborder characters
// adjust the value of c to maintain the invariant
in the program fragment in §2.4.1/23. These statements must deal with the characters
that are not part of the border. It should be easy to see that each of these characters is
either a space or part of the greeting. The only problem is figuring out which one it is,
and what to do about it.
We begin by testing whether we are about to write the first character of the greeting,
which we do by finding if we're in the correct row and on the correct column within that
row. The row we seek is the one after we've written the initial row of asterisks, followed
by pad additional rows. The appropriate column comes after we have written the initial
asterisk on this row, followed by pad spaces. Our knowledge of the invariants tells us
that we're on the right row when r is equal to pad + 1, and be at the appropriate
column when c is equal to pad + 1.
In other words, to determine whether we are about to write the first character of the
greeting, we must check whether r and c are both equal to pad + 1. If we've reached
the right place to write the greeting, we'll do so; otherwise, we'll write a space instead.
In both cases, we have to remember to update c appropriately:
if (r == pad + 1 && c == pad + 1) {
std::cout << greeting;
c += greeting.size();
} else {
std::cout << " ";
}
The condition inside the if statement uses the logical-and operator. As with the ||
operator, the && operator tests two conditions and yields a truth value. It is leftassociative
and uses a short-circuit evaluation strategy. Unlike the || operator, the &&
operator yields true only if both conditions are true. If either condition is false, the
result of && is false. The second condition will be tested if and only if the first
condition is true.
If the test succeeds, then it's time to write the greeting. In doing so, we falsify our
invariant, because c is no longer equal to the number of characters we have written on
this row. We make our invariant true again by adjusting the value of c to account for the
characters that we have written. The expression that updates c uses another new
operator, called the compound-assignment operator, to adjust c to account for the
number of characters in the name when we wrote it. Such a compound assignment is a
shorthand way of adding the right- and left-hand sides together and storing the result in
the left-hand side. In other words, if we write c += greeting.size(), that
statement has the same effect as if we had written c = c + greeting.size().
The remaining possibility is that we're not on the border, and we're not about to write
the greeting. In that case, we need to write a space and increment c to make the
invariant true again, which we do in the else branch of the if statement.
2.5 The complete framing program
At this point, we have revised the entire program, but the code is scattered enough to
be hard to find. Therefore, we shall show the whole program again. However, before we
do so, we want to shorten the program in three ways.
The first abbreviation will be a kind of declaration that lets us say once and for all that a
given name comes from the standard library. Doing so will allow us to avoid saying
std:: in so many places. The second abbreviation is a shorthand way of writing a
particularly common kind of while statement. Finally, we can shorten the program
slightly by incrementing c in one place instead of two.
2.5.1 Abbreviating repeated uses of std::
By now, you are probably tired of seeing-and writing-std:: in front of every name from
the standard library. Saying std:: explicitly was a good way of reminding you which
names came from the standard library, but you should have a pretty good idea of what
they are at this point.
C++ offers a way of saying that a particular name should always be interpreted as
coming from a particular namespace. For example, by writing
using std::cout;
we can say that we intend to use the name cout to mean std::cout exclusively, and
that we do not intend to define anything named cout ourselves. Once we have done so,
we can say cout instead of std::cout.
Logically enough, such a declaration is called a using-declaration. The name that it
mentions behaves similarly to other names. For example, if a using-declaration
appears within braces, the name that it defines has its given meaning from where it is
defined to the closing brace.
From now on, we'll write using-declarations to shorten our programs.
2.5.2 Using for statements for compactness
Let's look again at the control structures that we used in the program in §2.3/19. If we
look only at the program's outermost structure, we see
int r = 0;
while (r != rows) {
// stuff that doesn't change the value of r
++r;
}
This particular form of while appears frequently. Before it starts, we define and
initialize a local variable, which we test in the condition. The while body adjusts the
value of the variable so that eventually the condition will fail. Because this kind of
control structure is so common, the language provides a shorthand way of writing it:
for (int r = 0; r != rows; ++r) {
// stuff that doesn't change the value of r
}
Either of these examples will cause r to take on a sequence of values, the first of which
is 0, and the last of which is rows - 1. We can think of 0 as being the beginning of a
range, and rows as being the off-the-end value for the range. Such a range is called a
half-open range, and is often written as [begin, off-the-end). The deliberately
unbalanced brackets [ ) remind the reader that the range is asymmetric. So, for
example, the range [1, 4) contains 1, 2, and 3, but not 4. Similarly, we say that r
takes on the values in [0, rows). A for statement has the following form:
for (init-statement condition; expression)
statement
The first line is often known as the for header. It controls the statement that follows,
which is often called the for body. The init-statement must be either a definition
(§1.1/10) or an expression statement (§0.8/6). Because each of these kinds of
statement ends with its own semicolon, there is no additional semicolon between the
init-statement and the condition.
A for statement begins by executing the init-statement part of the for header, which it
does only once, at the beginning of the for. Typically, the init-statement defines and
initializes the loop control variable, which will be tested as part of the condition. If a
variable is defined in the init-statement, it is destroyed on exit from the for, so it is
inaccessible to code that follows the for statement.
On every trip through the loop, including the first, the program evaluates the condition.
If the condition yields true, then it executes the for body. Having done so, it executes
the expression. It then repeats the test, continuing to execute the for body followed by
the expression in the for header until the test condition fails.
More generally, the meaning of a for statement is
{
init-statement
while (condition) {
statement
expression;
}
where we have been careful to enclose the init-statement and the while in extra
braces, thereby limiting the lifetime of any variables declared in the init-statement. Note
particularly the presence and absence of semicolons. We do not write a semicolon after
the init-statement or statement because they are statements, with their own semicolons
if they need them. We do include a semicolon after expression in order to turn it into a
statement.
2.5.3 Collapsing tests
We can divide the code associated with the write one or more characters comment in
§2.4/23 into three cases: We are writing a single asterisk, a space, or the entire
greeting. As our program stands, we adjust c to maintain our invariant after we write an
asterisk, and we adjust it again after we write a space. There's nothing wrong with doing
so, but it is often possible to change the order of tests in a program so as to make it
possible to merge two or more identical statements into one.
Because our three cases are mutually exclusive, we can test them in any order. If we
begin by first testing whether we are about to write the greeting, then we know that in
the other two cases, incrementing c suffices to maintain the invariant, so we can
collapse the two increments into one:
if (we are about to write the greeting) {
cout << greeting;
c += greeting.size();
} else {
if (we are in the border)
cout << "*";
else
cout << " ";
++c;
}
After collapsing the increments, we also find that two of our blocks are just single
statements, so we can drop two pairs of braces. Notice how the different indentation of
++c; draws attention to the fact that it is executed regardless of whether we are in the
border.
2.5.4 The complete framing program
If we put all the pieces together and use these three abbreviation techniques, we get
the following program:
#include
#include
// say what standard-library names we use
using std::cin; using std::endl;
using std::cout; using std::string;
int main()
{
// ask for the person's name
cout << "Please enter your first name: ";
// read the name
string name;
cin >> name;
// build the message that we intend to write
const string greeting = "Hello, " + name + "!";
// the number of blanks surrounding the greeting
const int pad = 1;
// the number of rows and columns to write
const int rows = pad * 2 + 3;
const string::size_type cols = greeting.size() + pad * 2 + 2;
// write a blank line to separate the output from the input
cout << endl;
// write rows rows of output
// invariant: we have written r rows so far
for (int r = 0; r != rows; ++r) {
string::size_type c = 0;
// invariant: we have written c characters so far in the current row
while (c != cols) {
// is it time to write the greeting?
if (r == pad + 1 && c == pad + 1) {
cout << greeting;
c += greeting.size();
} else {
// are we on the border?
if (r == 0 || r == rows - 1 ||
c == 0 || c == cols - 1)
cout << "*";
else
cout << " ";
++c;
}
}
cout << endl;
}
return 0;
}
2.6 Counting
Most experienced C++ programmers have a habit that may seem weird at first: Their
programs invariably begin counting from 0 rather than from 1. For example, if we
reduce the outer for loop of the program above to its essentials, we get
for (int r = 0; r != rows; ++r) {
// write a row
}
We could have written this loop as
for (int r = 1; r <= rows; ++r) {
// write a row
}
One version counts from 0 and uses != as its comparison; the other counts from 1 and
uses <= as its comparison. The number of iterations is the same in each case. Is there
any reason to prefer one form over the other?
One reason to count from 0 is that doing so encourages us to use asymmetric ranges to
express intervals. For example, it is natural to use the range [0, rows) to describe the
first for statement, as it is to use the range [1, rows] to describe the second one.
Asymmetric ranges are usually easier to use than symmetric ones because of an
important property: A range of the form [m, n) has n - m elements, and a range of
the form [m, n] has n - m + 1 elements. So, for example, the number of elements
in [0, rows) is obvious (i.e., rows - 0, or rows) but the number in [1, rows] is
less so.
This behavioral difference between asymmetric and symmetric ranges is particularly
evident in the case of empty ranges: If we use asymmetric ranges, we can express an
empty range as [n, n), in contrast to [n, n-1] for symmetric ranges. The possibility
that the end of a range could ever be less than the beginning can cause no end of
trouble in designing programs.
Another reason to count from 0 is that doing so makes loop invariants easier to express.
In our example, counting from 0 makes the invariant straightforward: We have written
r rows of output so far. What would be the invariant if we counted from 1?
One would be tempted to say that the invariant is that we are about to write the rth
row, but that statement does not qualify as an invariant. The reason is that the last time
the while tests its condition, r is equal to rows + 1, and we intend to write only rows
rows. Therefore, we are not about to write the rth row, so the invariant is not true!
Our invariant could be that we have written r - 1 rows so far. However, if that's our
invariant, why not simplify it by starting r at 0?
Another reason to count from 0 is that we have the option of using != as our
comparison instead of <=. This distinction may seem trivial, but it affects what we know
about the state of the program when a loop finishes. For example, if the condition is r
!= rows, then when the loop finishes, we know that r == rows. Because the invariant
says that we have written r rows of output, we know that we have written exactly rows
rows all told. On the other hand, if the condition is r <= rows, then all we can prove is
that we have written at least rows rows of output. For all we know, we might have
written more.
If we count from 0, then we can use r != rows as a condition when we want to ensure
that there are exactly rows iterations, or we can use r < rows if we care only that the
number of iterations is rows or more. If we count from 1, we can use r <= rows if we
want at least rows iterations-but what if we want to ensure that rows is the exact
number? Then we must test a more complicated condition, such as r == rows + 1.
This extra complexity offers no compensating advantage.
2.7 Details
Expressions: C++ inherits a rich set of operators from C, several of which we have
already used. In addition, as we've already seen with the input and output operators,
C++ programs can extend the core language by defining what it means to apply built-in
operators to objects of class type. Correctly understanding complicated expressions is a
fundamental prerequisite to effective programming in C++. Understanding such
expressions requires understanding
How the operands group, which is controlled by the precedence and associativity of
the operators used in the expression
How the operands will be converted to other types, if at all
The order in which the operands are evaluated
Different operators have different precedence. Most of the operators are left-associative,
although the assignment operators and the operators taking a single argument are
right-associative. We list the most common operators here-regardless of whether we've
used them in this chapter. We've ordered them by precedence from highest to lowest,
with a double line separating groupings with the same precedence.
x.y
The member y of object x
x[y]
The element in object x indexed by y
x++
Increments x , returning the original value of x
x--
Decrements x , returning the original value of x
++x
Increments x , returning the incremented value
--x
Decrements x , returning the decremented value
!x
Logical negation. If x is true then !x is false .
x * y
Product of x and y
x / y
Quotient of x and y . If both operands are integers,
the implementation chooses whether to round toward zero or - 8
x % y
Remainder of x divided by y , equivalent to x - ((x / y) * y)
x + y
Sum of x and y
x - y
Result of subtracting y from x
x >> y
For integral x and y, x shifted right by y bits; y must be non-negative.
If x is an istream , reads from x into y
x << y
For integral x and y, x shifted left by y bits; y must be non-negative.
If x is an ostream , writes y onto x .
x relop y
Relational operators yield a bool indicating the truth of the relation.
The operators (<, >, <= , and >= ) have their obvious meanings.
x == y
Yields a bool indicating whether x equals y
x != y
Yields a bool indicating whether x is not equal to y
x && y
Yields a bool indicating whether both x and y are true .
Evaluates y only if x is true .
x || y
Yields a bool indicating whether either x or y is true .
Evaluates y only if x is false .
x = y
Assign the value of y to x , yielding x as its result.
x op= y
Compound assignment operators; equivalent to x = x op y ,
where op is an arithmetic or shift operator.
x ? y1 : y2
Yields y1 if x is true ; y2 otherwise.
Evaluates only one of y1 and y2 .
There is usually no guarantee as to the order in which an expression's operands are
evaluated. Because the order of evaluation is not fixed, it is important to avoid writing a
single expression in which one operand depends on the value of another operand. We'll
see an example in §4.1.5/60.
Operands will be converted to the appropriate type when possible. Numeric operands in
expressions or relational expressions are converted by the usual arithmetic
conversions described in detail in §A.2.4.4/304. Basically, the usual arithmetic
conversions attempt to preserve precision. Smaller types are converted to larger types,
and signed types are converted to unsigned. Arithmetic values may be converted to
bool : A value of 0 is considered false ; any other value is true . Operands of class
type are converted as specified by the type. We'll see in Chapter 12 how to control such
conversions.
Types:
bool
Built-in type representing truth values; may be either true or false
unsigned
Integral type that contains only non-negative values
short
Integral type that must hold at least 16 bits
long
Integral type that must hold at least 32 bits
size_t
Unsigned integral type (from ) that can hold any object's size
string::size_type
Unsigned integral type that can hold the size of any string
Half-open ranges include one but not both of their endpoints. For example, [1, 3)
includes 1 and 2 , but not 3 .
Condition: An expression that yields a truth value. Arithmetic values used in conditions
are converted to bool : Nonzero values convert to true ; zero values convert to false
.
Statements:
using namespace-name::name ;
Defines name as a synonym for namespace-name::name .
type-name name;
Defines name with type type-name .
type-name name = value;
Defines name with type type-name initialized as a copy of value.
type-name name(args) ;
Defines name with type type-name constructed as appropriate for the given
arguments in args .
expression ;
Executes expression for its side effects.
{ statement(s) }
Called a block. Executes the sequence of zero or more statement(s) in order. May
be used wherever a statement is expected. Variables defined inside the braces have
scope limited to the block.
while (condition) statement
If condition is false , do nothing; otherwise, execute statement and then repeat
the
entire while .
for(init-statement condition; expression) statement
Equivalent to { init-statement while (condition ) {statement
expression ; } } .
if (condition) statement
Executes statement if condition is true .
if (condition) statement else statement2
Executes statement if condition is true , otherwise executes statement2 .
Each else is associated with the nearest matching if .
return val ;
Exits the function and returns val to its caller.
Exercises
2-0. Compile and run the program presented in this chapter.
2-1 . Change the framing program so that it writes its greeting with no separation from
the frame.
2-2 . Change the framing program so that it uses a different amount of space to
separate the sides from the greeting than it uses to separate the top and bottom
borders from the greeting.
2-3. Rewrite the framing program to ask the user to supply the amount of spacing to
leave between the frame and the greeting.
2-4. The framing program writes the mostly blank lines that separate the borders from
the greeting one character at a time. Change the program so that it writes all the spaces
needed in a single output expression.
2-5. Write a set of "*" characters so that they form a square, a rectangle, and a
triangle.
2-6. What does the following code do?
int i = 0;
while (i < 10) {
i += 1;
std::cout << i << std::endl;
}
2-7. Write a program to count down from 10 to -5 .
2-8. Write a program to generate the product of the numbers in the range [1, 10) .
2-9. Write a program that asks the user to enter two numbers and tells the user which
number is larger than the other.
2-10. Explain each of the uses of std:: in the following program:
int main() {
int k = 0;
while (k != n) { // invariant: we have written k asterisks so far
using std::cout;
cout << "*";
++k;
}
std::cout << std::endl; // std:: is required here
return 0;
}
3
Working with batches of data
The programs that we explored in Chapters 1 and 2 did little more than read a single
string and write it again, sometimes with decoration. Most problems are more
complicated than such simple programs can solve. Among the most common sources of
complexity in programs is the need to handle multiple pieces of similar data.
Our programs have already started doing so, in the sense that a string comprises
multiple characters. Indeed, it is exactly the ability to put an unknown number of
characters into a single object-a string-that makes these programs easy to write.
In this chapter, we'll look at more ways of dealing with batches of data, by writing a
program that reads a student's exam and homework grades and computes a final grade.
Along the way, we'll learn how to store all the grades, even if we don't know in advance
how many grades there are.
3.1 Computing student grades
Imagine a course in which each student's final exam counts for 40% of the final grade,
the midterm exam counts for 20%, and the average homework grade makes up the
remaining 40%. Here is our first try at a program that helps students compute their final
grades:
#include
#include
#include
#include
using std::cin; using std::setprecision;
using std::cout; using std::string;
using std::endl; using std::streamsize;
int main()
{
// ask for and read the student's name
cout << "Please enter your first name: ";
string name;
cin >> name;
cout << "Hello, " << name << "!" << endl;
// ask for and read the midterm and final grades
cout << "Please enter your midterm and final exam grades: ";
double midterm, final;
cin >> midterm >> final;
// ask for the homework grades
cout << "Enter all your homework grades, "
"followed by end-of-file: ";
// the number and sum of grades read so far
int count = 0;
double sum = 0;
// a variable into which to read
double x;
// invariant:
// we have read count grades so far, and
// sum is the sum of the first count grades
while (cin >> x) {
++count;
sum += x;
}
// write the result
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * sum / count
<< setprecision(prec) << endl;
return 0;
}
As usual, we begin with #include directives and using-declarations for the library
facilities that we intend to use. These facilities include and , which we
have not yet seen. The header defines streamsize, which is the type that the
input-output library uses to represent sizes. The header defines the
manipulator setprecision, which lets us say how many significant digits we want our
output to contain.
When we used endl, which is also a manipulator, we did not have to include the
header. The endl manipulator is used so often that its definition appears in
, rather than in .
The program begins by asking for and reading the student's name, and the midterm and
final grades. Next, it asks for the student's homework grades, which it continues to read
until it encounters an end-of-file signal. Different C++ implementations offer their users
different ways of sending such a signal to a program, the most common way being to
begin a new line of input, hold down the control key, and press z (for computers running
Microsoft Windows) or d (for computers running the Unix or Linux systems).
While reading the grades, the program uses count to keep track of how many grades
were entered, and stores in sum a running total of the grades. Once the program has
read all the grades, it writes a greeting message, and reports the student's final grade.
In doing so, it uses count and sum to compute the average homework grade.
Much of this program should already be familiar, but there are several new usages,
which we will explain.
The first new idea occurs in the section that reads the student's exam grades:
cout << "Enter your midterm and final exam grades: ";
double midterm, final;
cin >> midterm >> final;
The first of these statements should be familiar: It writes a message, which, in this
case, tells the student what to do next. The next statement defines midterm and
final as having type double, which is the built-in type for double-precision floatingpoint
numbers. There is also a single-precision floating-point type, called float. Even
though it might seem that float is the appropriate type, it is almost always right to use
double for floating-point computations.
These types' names date back to when memory was much more expensive than it is
today. The shorter floating-point type, called float, is permitted to offer as little
precision as six significant (decimal) digits or so, which is not even enough to represent
the price of a house to the nearest penny. The double type is guaranteed to offer at
least ten significant digits, and we know of no implementation that does not offer at
least 15 significant digits. On modern computers, double is usually much more
accurate than float, and not much slower. Sometimes, double is even faster.
Now that we have defined the midterm and final variables, we read values into them.
Like the output operator (§0.7/4), the input operator returns its left operand as its
result. So, we can chain input operations just as we chain output operations, so
cin >> midterm >> final;
has the same effect as
cin >> midterm;
cin >> final;
Either form reads a number from the standard input into midterm, and the next
number into final.
The next statement asks the student to enter homework grades:
cout << "Enter all your homework grades, "
"followed by end-of-file: ";
A careful reading will reveal that this statement contains only a single <<, even though it
seems to be writing two string literals. We can get away with this because two or more
string literals in a program, separated only by whitespace, are automatically
concatenated. Therefore, this statement has exactly the same effect as
cout << "Enter all your homework grades, followed by end-of-file: ";
By breaking the string literal in two, we avoid lines in our programs that are too long to
read conveniently.
The next section of the code defines the variables that we'll use to hold the information
that we intend to read. Of these, the interesting part is
int count = 0;
double sum = 0;
Note that we give the initial value 0 to both sum and count. The value 0 has type int,
which means that the implementation must convert it to type double in order to use it
to initialize sum. We could have avoided this conversion by initializing sum to 0.0
instead of 0, but it makes no practical difference in this context: Any competent
implementation will do the conversion during compilation, so there is no run-time
overhead, and the result will be exactly the same.
In this case, what's more important than the conversion is that we give these variables
an initial value at all. When we do not specify an initial value for a variable we are
implicitly relying on default-initialization. The initialization that happens by default
depends on the type of the variable. For objects of class type, the class itself says what
initializer to use if there is not one specified. For example, we noted in §1.1/10 that if
we do not explicitly initialize a string, then the string is implicitly initialized to be
empty. No such implicit initialization happens for local variables of built-in type.
Local variables of built-in type that are not explicitly initialized are undefined, which
means that the variable's value consists of whatever random garbage already happens
to occupy the memory in which the variable is created. It is illegal to do anything to an
undefined value except to overwrite it with a valid value. Many implementations do not
detect the violations of this rule, and allow access to undefined values. The result is
almost always a crash or a wrong result, because whatever is in memory by
happenstance is almost never a correct value; often it is a value that is invalid for the
type.
Had we not given either sum or count an initial value, our program most likely would
have failed. The reason is that the first thing the program does with these variables is to
use their values: The program reads count in order to increment it, and it reads sum in
order to add its value to the one we just read. By the same token, we do not bother to
give an initial value to x, because the first thing we do with it is read into it, thereby
obliterating any value we might have given it.
The only new aspect of the while statement is the form of its condition:
// invariant:
// we have read count grades so far, and
// sum is the sum of the first count grades
while (cin >> x) {
++count;
sum += x;
}
We already know that the while loop executes so long as the condition cin >> x
succeeds. We'll explore the details of what it means to treat cin >> x as a condition in
§3.1.1/39, but for now, what's important to know is that this condition succeeds if the
most recent input request (i.e., cin >> x) succeeded.
Inside the while, we use the increment and compound-assignment operators, both of
which we used in Chapter 2. From the discussion there we know that ++count adds 1
to count, and that sum += x adds x to sum.
All that is left to explain is how the program does its output:
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * sum / count
<< setprecision(prec) << endl;
Our goal is to write the final grade with three significant digits, which we do by using
setprecision. Like endl, setprecision is a manipulator. It manipulates the stream
by causing subsequent output on that stream to appear with the given number of
significant digits. By writing setprecision(3), we ask the implementation to write
grades with three significant digits, generally two before the decimal point and one
after.
By using setprecision, we change the precision of any subsequent output that might
appear on cout. Because this statement is at the end of the program, we know that
there is no such output. Nevertheless, we believe that it is wise to reset cout's precision
to what it was before we changed it. We do so by calling a member function (§1.2/14) of
cout named precision. This function tells us the precision that a stream uses for
floating-point output. We use setprecision to set the precision to 3, write the final
grade, and then set the precision back to the value that precision gave us. The
expression that computes the grade uses several of the arithmetic operators: * for
multiplication, / for division, and + for addition, each of which has the obvious meaning.
We could have used the precision member function to set the precision, by writing
// set precision to 3, return previous value
streamsize prec = cout.precision(3);
cout << "Your final grade is "
<< 0.2 * midterm + 0.4 * final + 0.4 * sum / count << endl;
// reset precision to its original value
cout.precision(prec);
However, we prefer to use the setprecision manipulator, because by doing so, we
can minimize the part of the program in which the precision is set to an unusual value.
3.1.1 Testing for end of input
Conceptually, the only really new part of this program is the condition in the while
statement. That condition implicitly uses an istream as the subject of the while
condition:
while (cin >> x) {/*...*/}
The effect of this statement is to attempt to read from cin. If the read succeeds, x will
hold the value that we just read, and the while test also succeeds. If the read fails
(either because we have run out of input or because we encountered input that was
invalid for the type of x), then the while test fails, and we should not rely on the value
of x.
Understanding how this code works is a bit subtle. We can start by remembering that
the >> operator returns its left operand, so that asking for the value of cin >> x is
equivalent to executing cin >> x and then asking for the value of cin. For example,
we can read a single value into x, and test whether we were successful in doing so, by
executing
if (cin >> x) {/*...*/}
This statement has the same meaning as
cin >> x;
if (cin) { /* ... */ }
When we use cin >> x as a condition, we aren't just testing the condition; we are also
reading a value into x as a side effect. Now, all we need to do is figure out what it
means to use cin as a condition in a while statement.
Because cin has type istream, which is part of the standard library, we must look to
the definition of istream for the meaning of if (cin) or while (cin). The details
of that definition turn out to be complicated enough that we won't discuss it in detail
until §12.5/222. However, even without these details, we can already understand a
useful amount of what is happening.
The conditions that we used in Chapter 2 all involved relational operators that directly
yield values of type bool. In addition, we can use expressions that yield values of
arithmetic type as conditions. When used in a condition, the arithmetic value is
converted to a bool: Nonzero values convert to true; zero values convert to false.
For now, what we need to know is that similarly, the istream class provides a
conversion that can be used to convert cin into a value that can be used in a condition.
We don't yet know what type that value has, but we do know that the value can be
converted to bool. Accordingly, we know that the value can be used in a condition. The
value that this conversion yields depends on the internal state of the istream object,
which will remember whether the last attempt to read worked. Thus, using cin as a
condition is equivalent to testing whether the last attempt to read from cin was
successful.
There are several ways in which trying to read from a stream can be unsuccessful:
We might have reached the end of the input file.
We might have encountered input that is incompatible with the type of the variable
that we are trying to read, such as might happen if we try to read an int and find
something that isn't a number.
The system might have detected a hardware failure on the input device.
In any of these cases, the effect is the same: Using this input stream as a condition will
indicate that the condition is false. Moreover, once we have failed to read from a stream,
all further attempts to read from that stream will fail until we reset the stream, which
we'll learn how to do in §4.1.3/57.
3.1.2 The loop invariant
Understanding the invariant (§2.3.2/20) for this loop requires special care, because the
condition in the while has side effects. Those side effects affect the truth of the
invariant: Successfully executing cin >> x makes the first part of the invariant-the
part that says that we have read count grades-false. Accordingly, we must change our
analysis to account for the effect that the condition itself might have on the invariant.
We know that the invariant was true before evaluating the condition, so we know that
we have already read count grades. If cin >> x succeeds, then we have now read
count + 1 grades. We can make this part of the invariant true again by incrementing
count. However, doing so falsifies the second part of the invariant-the part that says
that sum is the sum of the first count grades-because after we have incremented
count, sum is now the sum of the first count - 1 grades, not the first count grades.
Fortunately, we can make the second part of the invariant true by executing sum += x;
so that the entire invariant will be true on subsequent trips through the while.
If the condition is false, it means that our attempt at input failed, so we didn't get any
more data, and so the invariant is still true. As a result, we do not have to account for
the condition's side effects after the whi1e finishes.
3.2 Using medians instead of averages
The program that we've written so far has a design shortcoming: It throws away each
homework grade as soon as it has read it. Doing so is fine for computing averages, but
what if we wanted to use the median homework grade instead of the average?
The most straightforward way to find the median of a collection of values is to sort the
values into increasing (or decreasing) order and pick the middle one—or, if the number
of values is even, take the average of the two values nearest the middle. Medians are
often more useful than averages, because although they still account correctly for
consistent performance, they won't cause a few lousy grades to blow the whole course.
A bit of thinking should convince us that to compute medians, we must change our
program fundamentally. In order to find the median of an unknown number of values,
we must store every value until we have read them all. To find the average, we were
able to store only the count and running total of the items we'd read. The average was
just the total divided by the count.
3.2.1 Storing a collection of data in a vector
To compute the median, we must read and store all the homework grades, then sort
them, and finally pick the middle one (or two). To do this computation conveniently and
efficiently, we need a way to
Store a number of values that we will read one at a time, without knowing in
advance how many values there are
Sort the values after we have read them all
Get at the middle value(s) efficiently
The standard library provides a type, named vector, that we can use to solve all these
problems easily. A vector holds a sequence of values of a given type, grows as needed
to accommodate additional values, and lets us get at each individual value efficiently.
Let's start rewriting our grading program by making it put the grades into a vector,
instead of computing the sum and throwing the grades away. The original version of
that code looked like
// original program (excerpt):
int count = 0;
double sum = 0;
double x;
// invariant:
// we have read count grades so far, and
// sum is the sum of the first count grades
while (cin >> x) {
++count;
sum += x;
}
This loop kept track of how many grades it read, and kept a running total of their value.
The need to keep both these variables in step with the values as we read them made
the loop invariant relatively complicated. In contrast, using a vector to store values as
we read them is much simpler:
// revised version of the excerpt:
double x;
vector homework;
// invariant: homework contains all the homework grades read so far
while (cin >> x)
homework.push_back(x);
We haven't changed the basic structure of our code: It still reads values one at a time
into x until it encounters end-of-file or invalid input. What's different is what we do with
those values.
Let's start with homework, which we define as having type vector. A
vector is a container that holds a collection of values. All of the values in an individual
vector are the same type, but different vectors can hold objects of different types.
Whenever we define a vector, we must specify the type of the values that the vector
will hold. Our definition of homework says that it is a vector, which will hold values of
type double.
The vector type is defined using a language feature called template classes. We'll see
how to define a template class in Chapter 11. For now, what's important is to realize
that we can separate what it means to be a vector from the particular type of the
objects that the vector holds. We specify the type of the objects inside angle brackets.
For example, objects of type vector are vectors that hold objects of type
double, objects of type vector hold strings, and so on.
The while loop operates by reading values from the standard input and storing them in
the vector. As before, we read into x until we hit end-of-file or encounter input that is
not a double. What is new is
homework.push_back(x);
As with greeting.size() in §1.2/14, we can see that push_back is a member
function, which is defined as part of the vector type, and that we are asking that
function to act on behalf of the object named homework. We call that function, passing
it x. What push_back does is append a new element to the end of the vector. It gives
that new element the value that we passed as the argument to push_back. Thus,
push_back pushes its argument onto the back of a vector. As a side effect, it
increases the size of the vector by one.
Because the push_back function is such a good match for what we're trying to do, it is
trivial to see that calling it will maintain our loop invariant. Therefore, it is clear that
when we drop out of the while, we will have read all the homework grades and stored
them in homework, which is what we wanted.
Next, we have to think about the output.
3.2.2 Generating the output
In the original version of the program from §3.1/35, we calculated the student's grade
within the output expression itself:
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * sum / count
<< setprecision(prec) << endl;
where final and midterm held the exam grades, and sum and count contained the
sum of all the homework grades and the count of how many grades were entered.
As we remarked in §3.2.1/41, the easiest way to calculate the median is to sort our data
and then find the middle value, or the average of the two middle values if we have an
even number of elements. We can make the computation easier to understand if we
separate the computation of the median from the code that writes the output.
In order to find the median, we begin by noting that we are going to need to know the
size of the homework vector at least twice: once to check whether the size is zero, and
again to compute the location of the middle element(s). To avoid having to ask for the
size twice, we will store the size in a local variable:
typedef vector::size_type vec_sz;
vec_sz size = homework.size();
The vector type defines a type named vector::size_type, and a
function named size. These members operate analogously to the ones in string: The
type defined by size_type is an unsigned type guaranteed sufficient to hold the size
of the largest possible vector, and size() returns a size_type value that
represents the number of elements in the vector.
Because we need to know the size in two places, we will remember the value in a local
variable. Different implementations use different types to represent sizes, so we cannot
write the appropriate type directly and remain implementation-independent. For that
reason, it is good programming practice to use the size_type that the library defines
to represent container sizes, which we do in naming the type of size.
In this example, that type is unwieldy to write—and to read. To simplify our program,
we have used a language facility that we haven't encountered before, called a typedef.
When we include the word typedef as part of a definition, we are saying that we want
the name that we define to be a synonym for the given type, rather than a variable of
that type. Thus, because our definition includes typedef, it defines the name vec_sz
as a synonym for vector::size_type. Names defined via typedef have
the same scope as any other names. That is, we can use the name vec_sz as a
synonym for the size_type until the end of the current scope.
Once we know how to name the type of the value that homework.size() returns, we
can store that value in a local variable, named size, of the same type. It is worth
noting that even though we are using the name size for two different purposes, there
is no conflict or ambiguity. The only way to ask for the size of a vector is by putting a
call to the size function on the right-hand side of a dot, with a vector on the left-hand
side. In other words, the size that is defined as a local variable is in a different scope
than the one that is an operation on vectors. Because these names are in different
scopes, the compiler (and the programmer) can know which size is intended.
Because it is meaningless to find the median of an empty dataset, our next job is to
verify that we have some data:
if (size == 0) {
cout << endl << "You must enter your grades. "
"Please try again." << endl;
return 1;
}
We can detect this state of affairs by checking whether size is zero. If it is, the most
sensible action is to complain and stop the program. We do so by returning 1 to indicate
failure. As discussed in Chapter 0, the system presumes that if main returns 0, the
program succeeded. Returning anything else has an implementation-defined meaning,
but most implementations treat any nonzero value as failure.
Now that we have verified that we have data, we can begin computing the median. The
first part of doing so is to sort the data, which we do by calling a library function:
sort(homework.begin(), homework.end());
The sort function, defined in the header, rearranges the values in a
container so that they are in nondecreasing order. We say nondecreasing instead of
increasing because some elements might be equal to one another.
The arguments to sort specify the range of elements to be sorted. The vector class
provides two member functions, begin and end, for this kind of usage. We'll have
much more to say about begin and end in §5.2.2/81, but what is important for now is
to know that homework.begin() denotes the first element in the vector named
homework, whereas homework.end() denotes (one past) the last element in
homework.
The sort function does its work in place: It moves the values of the container's
elements around rather than creating a new container to hold the results.
Once we have sorted homework, we need to find the middle element or elements:
vec_sz mid = size/2;
double median;
median = size % 2 == 0 ? (homework[mid] + homework[mid-1]) / 2
: homework[mid];
We begin by dividing size by 2 in order to locate the middle of the vector. If the
number of elements is even, this division is exact. If the number is odd, then the result
is the next lower integer.
Exactly how we compute the median depends on whether the number of elements is
even or odd. If it is even, the median is the average of the two elements closest to the
middle. Otherwise, there is an element in the middle, the value of which is the median.
The expression that assigns a value to median uses two new operators: the remainder
operator, %, and the conditional operator, often called the ? : operator.
The remainder operator, %, returns the remainder that results from dividing its left
operand by its right operand. If the remainder after dividing by 2 is 0, then the program
has read an even number of elements.
The conditional operator is shorthand for a simple if-then-else expression. First, it
evaluates the expression, size % 2 == 0, that precedes the ? part of the operator as
a condition to obtain a bool value. If the condition yields true, then the result is the
value of the expression between the ? and the : that follows; otherwise, the result is
the value of the expression after the :. So, if we read an even number of elements, we'll
set median to the average of the two middle elements. If we read an odd number, then
we'll set median to homework[mid]. Analogous to && and ||, the ? : operator first
evaluates its leftmost operand. Based on the resulting value, it then evaluates exactly
one of its other operands.
The references to homework[mid] and homework[mid-1] show one way to access an
element of a vector. Every element of every vector has an integer, called its index,
associated with it. So, for example, homework[mid] is the element of homework with
index mid. As you might suspect from §2.6/30, the first element of the vector named
homework is homework[0], and the last element is homework[size - 1].
Each element is itself an (unnamed) object of the type stored in the container. So,
homework[mid] is an object of type double, on which we can invoke any of the
operations that type double supports. In particular, we can add two elements, and we
can divide the resulting sum by 2 to get the average value of the two objects.
Once we know how to access the elements of homework, we can see how the median
computation works. Assume first that size is even, so that mid is size / 2. Then
there must be exactly mid elements of homework on each side of the middle:
Because we know that each half of homework has exactly mid elements, it should be
easy to see that the indices of the two elements nearest the middle are mid - 1 and
mid; the median is the average of these elements.
If the number of elements is odd, then mid is really (size-1) / 2, because of
truncation. In that case, we can think of our sorted homework vector as two segments
with mid elements each, separated by a single element in the middle. That element is
the median:
In either case, our median computation relies on the ability to access a vector element
knowing only its index.
Once we have computed the median, We need only compute and write the final grade:
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * median
<< setprecision(prec) << endl;
The final program isn't much more complicated than the program in §3,1/35, even
though it does much more work. In particular, even though our homework vector will
grow as needed to accommodate grades for as many homework assignments as our
students can tolerate, our program doesn't need to worry about obtaining the memory
to store all those grades. The standard library does all that work for us.
Here is the entire program. The only parts that we have not already mentioned are the
#include directives, the corresponding using-declarations, and a few more
comments:
#include
#include
#include
#include
#include
#include
using std::cin; using std::sort;
using std::cout; using std::streamsize;
using std::endl; using std::string;
using std::setprecision; using std::vector;
int main()
{
// ask for and read the student's name
cout << "Please enter your first name: ";
string name;
cin >> name;
cout << "Hello, " << name << "!" << endl;
// ask for and read the midterm and final grades
cout << "Please enter your midterm and final exam grades: ";
double midterm, final;
cin >> midterm >> final;
// ask for and read the homework grades
cout << "Enter all your homework grades, "
"followed by end-of-file: ";
vector homework;
double x;
// invariant: homework contains all the homework grades read so far
while (cin >> x)
homework.push_back(x);
// check that the student entered some homework grades
typedef vector::size_type vec_sz;
vec_sz size = homework.size();
if (size == 0) {
cout << endl << "You must enter your grades. "
"Please try again." << endl;
return 1;
}
// sort the grades
sort(homework.begin(), homework.end());
// compute the median homework grade
vec_sz mid = size/2;
double median;
median = size % 2 == 0 ? (homework[mid] + homework[mid-1]) / 2
: homework[mid];
// compute and write the final grade
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * median
<< setprecision(prec) << endl;
return 0;
}
3.2.3 Some additional observations
This code contains some points that deserve particular attention. First, there's a bit
more to know about why we exit the program if homework is empty. Logically, taking
the median of an empty collection of values is undefined—we have no idea what it might
mean. Therefore, exiting does the right thing: If we don't know what to do, we might as
well quit. But it is important to know what would happen if we had continued execution.
If the input were empty, and we neglected to test that we had read at least one number,
the code to compute the median would fail. Why?
If we had read no elements, then homework.size(), and therefore size, would be 0.
Likewise, mid would be 0. When we executed homework[mid], we would be looking at
the first element (the one indexed by 0) in homework. But there are no elements in
homework! When we execute homework[0], all bets are off as to what we get.
vectors do not check whether the index is in range. Such checking is up to the user.
The next important observation is that vector::size_type, like all
standard-library size types, is an unsigned integral type. Such types are incapable of
storing negative values at all; instead, they store values modulo 2n, where n depends on
the implementation. So, for example, there would never be any point in checking
whether homework.size() < 0, because that comparison would always yield false.
Moreover, whenever ordinary integers and unsigned integers combine in an expression,
the ordinary integer is converted to unsigned. In consequence, expressions such as
homework.size() - 100 yield unsigned results, which means that they, too, cannot
be less than zero—even if homework.size() < 100.
Finally, it is also worth noting that the execution performance of our program is actually
quite good, even though the vector object grows as needed to
accommodate its input, rather than being allocated with the right size immediately.
We can be confident about the program's performance because the C++ standard
imposes performance requirements on the library's implementation. Not only must the
library meet the specifications for behavior, but it must also achieve well-specified
performance goals. Every standard-conforming C++ implementation must
Implement vector so that appending a large number of elements to a vector is
no worse than proportional to the number of elements
Implement sort to be no slower than nlog(n), where n is the number of elements
being sorted
The whole program is therefore guaranteed to run in nlog(n) time or better on any
standard-conforming implementation. In fact, the standard library was designed with a
ruthless attention to performance. C++ is designed for use in performance-critical
applications, and the emphasis on speed pervades the library as well.
3.3 Details
Local variables are default-initialized if they are defined without an explicit initializer.
Default-initialization of a built-in type means that the value is undefined. Undefined
values may be used only as the left-hand side of an assignment.
Type definitions:
typedef type name; Defines name as a synonym for type.
The vector type, defined in , is a library type that is a container that holds a
sequence of values of a specified type, vectors grow dynamically. Some important
operations are:
vector::size_type
A type guaranteed to be able to hold the number of elements in the
largest possible vector.
v.begin()
Returns a value that denotes the first element in v.
v.end()
Returns a value that denotes (one past) the last element in v.
vector v;
Creates an empty vector that can hold elements of type T.
v.push_back(e)
Grows the vector by one element initialized to e.
v[i]
Returns the value stored in position i.
v.size()
Returns the number of elements in v.
Other library facilities
sort(b, e)
Rearranges the elements defined by the range [b, e) into nondecreasing order.
Defined in .
max(el, e2)
Returns the larger of the expressions e1 and e2; e1 and e2 must have exactly the
same type. Defined in .
while (cin >> x)
Reads a value of an appropriate type into x and tests the state of the stream. If the
stream is in an error state, the test fails; otherwise, the test succeeds, and the body of
the while is executed.
s.precision(n)
Sets the precision of stream s to n for future output (or leaves it unchanged if n is
omitted). Returns the previous precision.
setprecision(n)
Returns a value that, when written on an output stream s, has the effect of calling
s.precision(n). Defined in .
streamsize
The type of the value expected by setprecision and returned by precision.
Defined in .
Exercises
3-0. Compile, execute, and test the programs in this chapter.
3-1. Suppose we wish to find the median of a collection of values. Assume that we have
read some of the values so far, and that we have no idea how many values remain to be
read. Prove that we cannot afford to discard any of the values that we have read. Hint:
One proof strategy is to assume that we can discard a value, and then find values for
the unread—and therefore unknown—part of our collection that would cause the median
to be the value that we discarded.
3-2. Write a program to compute and print the quartiles (that is, the quarter of the
numbers with the largest values, the next highest quarter, and so on) of a set of
integers.
3-3. Write a program to count how many times each distinct word appears in its input.
3-4. Write a program to report the length of the longest and shortest string in its
input.
3-5. Write a program that will keep track of grades for several students at once. The
program could keep two vectors in sync: The first should hold the student's names,
and the second the final grades that can be computed as input is read. For now, you
should assume a fixed number of homework grades. We'll see in §4.1.3/56 how to
handle a variable number of grades intermixed with student names.
3-6. The average-grade computation in §3.1/36 might divide by zero if the student
didn't enter any grades. Division by zero is undefined in C++, which means that the
implementation is permitted to do anything it likes. What does your C++
implementation do in this case? Rewrite the program so that its behavior does not
depend on how the implementation treats division by zero.
4
Organizing programs and data
Although the program in §3.2.2/46 is larger than we would like, it would have been
larger still without vector, string, and sort . These library facilities, like others
that we have used, share several qualities: Each one
Solves a particular kind of problem
Is independent of most of the others
Has a name
Our own programs have the first of these qualities, but lack the others. This lack is fine
for small programs, but as we set out to solve larger problems, we will find that our
solutions will become unmanageable unless we break them into independent, named
parts.
Like most programming languages, C++ offers two fundamental ways of organizing
large programs: functions (sometimes called subroutines) and data structures. In
addition, C++ lets programmers combine functions and data structures into a single
notion called a class, which we'll explore starting in Chapter 9.
Once we have learned how to use functions and data structures to organize our
computations, we also need the ability to divide our programs into files that we can
compile separately and combine after compilation. The last part of this chapter will show
how C++ supports separate compilation.
4.1 Organizing computations
We shall begin by writing a function to calculate a student's final grade from the
midterm and final exam grades and overall homework grade. We'll assume that we've
already calculated the overall homework grade from the individual homework grades,
which we have been computing as the average or median. Aside from that assumption,
this function will use the same policy as the one that we've been using all along:
Homework and the final exam contribute 40% each to the total, and the midterm makes
up the remaining 20%.
Whenever we do—or might do—a computation in several places, we should think about
putting it in a function. An obvious reason for doing so is that then we can use the
function instead of redoing the computation explicitly. Not only does using functions
reduce our total programming effort, but doing so also makes it easier for us to change
the computation if we wish. For example, assume we wanted to change our grading
policy. If we had to hunt through every program we had ever written, looking for the
parts that dealt with grading, we would probably become discouraged quickly.
There is a more subtle advantage to using functions for such computations: A function
has a name. If we name a computation, we can think about it more abstractly—we can
think more about what it does and less about how it works. If we can identify important
parts of our problems, and create named pieces of our programs that correspond to
those parts, then our programs will be easier to understand and the problems easier to
solve.
Here is a function that computes grades according to our policy:
// compute a student's overall grade from midterm and final exam grades and homework grade
double grade(double midterm, double final, double homework)
{
return 0.2 * midterm + 0.4 * final + 0.4 * homework;
}
Until now, all the functions that we've defined have been named main . We define most
other functions similarly, by specifying the return type, followed first by the function
name, next by a parameter list enclosed in ( ), and, finally, by the function body, which
is enclosed in { } . The rules are more complicated for functions that return values that
denote other functions; see §A.1.2/297 for the full story.
In this example, the parameters are midterm, final , and homework , each of which
has type double . They behave like variables that are local to the function, which
means that calling the function creates them and returning from the function destroys
them.
As with any other variables, we must define the parameters before using them. Unlike
other variables, defining them does not create them immediately; only calling the
function creates them. Therefore, whenever we call the function, we must supply
corresponding arguments , which are used to initialize the parameters when the
function begins execution. For example, in §3.1/36 we computed a grade by writing
cout << "Your final grade is " << setprecision(3)
<< 0.2 * midterm + 0.4 * final + 0.4 * sum / count
<< setprecision(prec) << endl;
If we had the grade function available, we could have written
cout << "Your final grade is " << setprecision(3)
<< grade(midterm, final, sum / count)
<< setprecision(prec) << endl;
Not only must we supply arguments that correspond to the parameters of the functions
that we call, but we must supply them in the same order. Accordingly, when we call the
grade function, the first argument must be the midterm grade, the second must be the
final exam grade, and the third must be the homework grade.
Arguments can be expressions, such as sum / count , not just variables. In general,
each argument is used to initialize the. corresponding parameter, after which the
parameters behave like ordinary local variables inside the function. So, for example,
when we call grade(midterm, final, sum / count) , the grade function's
parameters are initialized to copies of the arguments' values, and do not refer directly to
the arguments themselves. This behavior is often called call by value, because the
parameter takes on a copy of the value of the argument.
4.1.1 Finding medians
Another problem that we solved in §3.2.2/46, and that we can imagine wanting to solve
in other contexts, is finding the median of a vector . We'll see in §8.1.1/140 how to
define a function that is so general that it works with a vector of any type of value. For
now, we'll limit our attention to vector .
To write our function, we'll start with the part of the program in §3.2.2/47 that
computes medians, and make a few changes:
// compute the median of a vector
// note that calling this function copies the entire argument vector
double median(vector vec)
{
typedef vector::size_type vec_sz;
vec_sz size = vec.size();
if (size == 0)
throw domain_error("median of an empty vector");
sort(vec.begin(), vec.end());
vec_sz mid = size / 2;
return size % 2 == 0 ? (vec[mid] + vec[mid-1]) / 2 : vec[mid];
}
One change is that we named our vector vec , rather than homework . After all, our
function can compute the median of anything, not just homework grades. We also
eliminated the variable median , because we no longer need it: We can return the
median as soon as we've calculated it. We are still using size and mid as variables, but
now they are local to the median function and, therefore, inaccessible (and irrelevant)
elsewhere. Calling the median function will create these variables, and returning from
the function will destroy them. We define vec_sz as a local type name, because we
don't want to conflict with anyone who might want to use that name for another
purpose.
The most significant change is what we do if the vector is empty. In §3.2.2/46, we
knew that we would have to complain to whoever was running our program, and we also
knew who that person would be and what kind of complaint would make sense. In this
revised version, on the other hand, we don't know who is going to be using it, or for
what purpose, so we need a more general way of complaining. That more general way is
to throw an exception if the vector is empty.
When a program throws an exception, execution stops in the part of the program in
which the throw appears, and passes to another part of the program, along with an
exception object , which contains information that the caller can use to act on the
exception.
The most important part of the information that is passed is usually the mere fact that
an exception was thrown. That fact, along with the type of the exception object, is
usually enough to let the caller figure out what to do. In this particular example, the
exception that we throw is domain_error , which is a type that the standard library
defines in header for use in reporting that a function's argument is
outside the set of values that the function can accept. When we create a
domain_error object to throw, we can give it a string that describes what went
wrong. The program that catches the exception can use this string in a diagnostic
message, as we shall see in §4.2.3/65.
There is one more detail about how functions behave that is important to understand.
When we call a function, we can think of the parameters as local variables whose initial
values are the arguments. If we do so, we can see that calling a function involves
copying the arguments into the parameters. In particular, when we call median , the
vector that we use as an argument will be copied into vec .
In the case of the median function, it is useful to copy the argument to the parameter,
even if doing so takes significant time, because the median function changes the value
of its parameter by calling sort . Copying the argument prevents changes made by
sort from propagating back to the caller. This behavior makes sense, because taking
the median of a vector should not change the vector itself.
4.1.2 Reimplementing our grading policy
The grade function in §4.1/52 assumes that we already know the student's overall
homework grade, and not just the individual homework assignments' grades. How we
obtain that grade is part of our policy: We used the average in §3.1/36 and the median
in §3.2.2/47. Accordingly, we might wish to express this part of our grading policy in a
function, along the same lines as we did in §4.1/52:
// compute a student's overall grade from midterm and final exam grades
// and vector of homework grades.
// this function does not copy its argument, because median does so for us.
double grade(double midterm, double final, const vector& hw)
{
if (hw.size() == 0)
throw domain_error("student has done no homework");
return grade(midterm, final, median(hw));
}
This function has three points of particular interest.
The first point is the type, const vector& , that we specify for the third
argument. This type is often called "reference to vector of const double ." Saying
that a name is a reference to an object says that the name is another name for the
object. So, for example, if we write
vector homework;
vector& hw = homework; // hw is a synonym for homework
we are saying that hw is another name for homework . From that point on, anything we
do to hw is equivalent to doing the same thing to homework , and vice versa. Adding a
const , as in
// chw is a read-only synonym for homework
const vector& chw = homework;
still says that chw is another name for homework , but the const promises that we will
not do anything to chw that might change its value.
Because a reference is another name for the original, there is no such thing as a
reference to a reference. Defining a reference to a reference has the same effect as
defining a reference to the original object. For example, if we write
// hw1 and chw1 are synonyms for homework; chw1 is read-only
vector& hw1 = hw;
const vector& chw1 = chw;
then hw1 is another name for homework , just as hw is, and chw1 , like chw , is another
name for homework that does not allow write access.
If we define a nonconst reference—a reference that allows writing—we cannot make it
refer to a const object or reference, because doing so would require permission that
the const denies. Therefore, we cannot write
vector& hw2 = chw; // error: requests write access to chw
because we promised not to modify chw .
Analogously, when we say that a parameter has type const vector& , we
are asking the implementation to give us direct access to the associated argument,
without copying it, and also promising that we won't change the parameter's value
(which would otherwise change the argument too). Because the parameter is a
reference to const , we can call this function on behalf of both const and nonconst
vector s. Because the parameter is a reference, we avoid the overhead of copying the
argument.
The second point of particular interest about the grade function is that like the one in
§4.1/52, it is named grade —even though it calls the other grade function. The notion
that we can have several functions with the same name is called overloading , and
figures prominently in many C++ programs. Even though we have two functions with
the same name, there is no ambiguity, because whenever we call grade , we will supply
an argument list, and the implementation will be able to tell from the type of the third
argument which grade function we mean.
The third point is that we check whether homework.size() is zero, even though we
know that median will do so for us. The reason is that if median discovers that we are
asking for the median of an empty vector , it throws an exception that includes the
message median of an empty vector . This message is not directly useful to
someone who is computing student grades. Therefore, we throw our own exception,
which we hope will give the user more of a clue as to what has gone wrong.
4.1.3 Reading homework grades
Another problem that we have had to solve in several contexts is reading homework
grades into a vector .
There is a problem in designing the behavior of such a function: It needs to return two
values at once. One value is, of course, the homework grades that it read. The other is
an indication of whether the attempted input was successful.
There is no direct way to return more than one value from a function. One indirect way
to do so is to give the function a parameter that is a reference to an object in which it is
to place one of its results. This strategy is common for functions that read input, so we'll
use it. Doing so will make our function look like this:
// read homework grades from an input stream into a vector
istream& read_hw(istream& in, vector& hw) {
// we must fill in this part
return in;
}
In §4.1.2/54, we saw a program with a const vector& parameter; now
we're dropping the const . A reference parameter without a const usually signals an
intent to modify the object that is the function's argument. For example, when we
execute
vector homework;
read_hw(cin, homework);
the fact that read_hw 's second parameter is a reference should lead us to expect that
calling read_hw will change the value of homework .
Because we expect the function to modify its argument, we cannot call the function with
just any expression. Instead, we must pass an lvalue argument to a reference
parameter. An lvalue is a value that denotes a nontemporary object. For example, a
variable is an lvalue, as is a reference, or the result of calling a function that returns a
reference. An expression that generates an arithmetic value, such as sum / count ,
is not an lvalue.
Both of the parameters to read_hw are references, because we expect the function to
change the state of both arguments. We don't know the details of how cin works, but
presumably the library defines it as a data structure that stores everything the library
needs to know about the state of our input file. Reading input from the standard input
file changes the state of the file, so it should logically change the value of cin as well.
Notice that read_hw returns in . Moreover, it does so as a reference. In effect, we are
saying that we were given an object, which we are not going to copy, and we will return
that same object, again without copying it. Returning the stream allows our caller to
write
if (read_hw(cin, homework)){/*...*/}
as an abbreviation for
read_hw(cin, homework);
if (cin) {/*...*/}
We can now think about how to read the homework grades. Obviously, we want to read
as many grades as exist, so it would seem as if we could just write
// first try not quite right
double x;
while (in >> x)
hw.push_back(x);
This strategy doesn't quite work, for two reasons. The first reason is that we haven't
defined hw —our caller defined it for us. Because we didn't define it, we don't know what
data might be there already. For all we know, our caller might be using our function to
process homework for lots of students, in which case hw might contain the previous
student's grades. We can solve this problem by calling hw.clear() before we begin our
work.
The second reason that our strategy fails is that we don't quite know when to stop. We
can keep reading grades until we can no longer do so, but at that point we have a
problem. There are two reasons why we might no longer be able to read a grade: We
might have reached end-of-file, or we might have encountered something that is not a
grade.
In the first case, our caller will think that we have reached end-of-file. This thought will
be true but misleading, because the end-of-file indication will have occurred only after
we have successfully read all the data. Normally, an end-of-file indication means that an
input attempt failed.
In the second case, when we have encountered something that isn't a grade, the library
will mark the input stream as being in failure state , which means that future input
requests will fail, just as if we had reached end-of file. Therefore, our caller will think
that something is wrong with the input data, when the only problem was that the last
homework grade was followed by something that was not a homework grade.
In either case, then, we would like to pretend that we never saw whatever followed the
last homework grade. Such pretense turns out to be easy: If we reached end-of-file,
there was no additional input to read; if we encountered something that wasn't a grade,
the library will have left it unread for the next input attempt. Therefore, all we must do
is tell the library to disregard whatever condition caused the input attempt to fail, be it
end-of-file or invalid input. We do so by calling in.clear() to reset the error state
inside in , which tells the library that input can continue despite the failure.
There is one more detail to consider: Perhaps we have already run out of input, or
encountered an error condition, before even trying to read the first homework grade. In
that case, we must leave the input stream strictly alone, lest we inadvertently seduce
our caller into trying to read nonexistent input at some point in the future.
Here is the complete read_hw function:
// read homework grades from an input stream into a vector
istream& read_hw(istream& in, vector& hw)
{
if (in) {
// get rid of previous contents
hw.clear() ;
// read homework grades
double x;
while (in >> x)
hw.push_back(x);
// clear the stream so that input will work for the next student
in.clear();
}
return in;
}
Note that the clear member behaves completely differently for istream objects than
it does for vector objects. For istream objects, it resets any error indications so that
input can continue; for vector objects, it discards any contents that the vector might
have had, leaving us with an empty vector again.
4.1.4 Three kinds of function parameters
We would like to pause at this point for an important observation: We have defined
three functions—median, grade, and read_hw —that operate on homework vectors.
Each of these functions treats the corresponding parameter in a fundamentally different
way from the others, and each treatment has a purpose.
The median function (§4.1.1/53) has a parameter of type vector .
Therefore, calling that function causes the argument to be copied, even though that
argument might be a huge vector . Despite the inefficiency, vector is the
right parameter type for median , because this type ensures that taking the median of a
vector doesn't change the vector . The median function sort s its parameter. If it
did not copy its argument, then calling median(homework) would change the value of
homework .
The grade function that takes a homework vector (§4.1.2/54) has a parameter of
type const vector& . In this type, the & asks the implementation not to
copy the argument, and the const promises that the program will not change the
parameter. Such parameters are an important technique for making programs more
efficient. They are a good idea whenever the function will not change the parameter's
value, and the parameter is of a type, such as vector or string , with values that
might be time-consuming to copy. It is usually not worth the bother to use const
references for parameters of simple built-in types, such as int or double . Such small
objects are usually fast enough to copy that there's little, if any, overhead in passing
them by value.
The read_hw function has a parameter of type vector& , without the const
. Again, the & asks the implementation to bind the parameter directly to the argument,
thus avoiding having to copy the argument. But here, the reason to avoid the copy is
that the function intends to change the argument's value.
Arguments that correspond to nonconst reference parameters must be lvalues—that is,
they must be nontemporary objects. Arguments that are passed by value or bound to a
const reference can be any value. For example, suppose we have a function that
returns an empty vector :
vector emptyvec()
{
vector v; // no elements
return v;
}
We could call this function, and use the result as an argument to our second grade
function from §4.1.2/54:
grade(midterm, final, emptyvec());
When run, the grade function would throw an exception immediately, because its
argument is empty. However, calling grade this way would be syntactically legal.
When we call read_hw , both of its arguments must be lvalues, because both
parameters are nonconst references. If we give read_hw a vector that is not an lvalue
read_hw(cin, emptyvec()); // error: emptyvec() is not an lvalue
the compiler will complain, because the unnamed vector that we create in the call to
emptyvec will disappear as soon as read_hw returns. If we were allowed to make this
call, the effect would be to store input in an object that we couldn't access!
4.1.5 Using functions to calculate a student's grade
The whole point of writing these functions is to use them in solving problems. For
example, we can use them to reimplement our grading program from §3.2.2/46:
// include directives and using-declarations for library facilities
// code for median function from §4.1.1/53
// code for grade(double, double, double) function from §4.1/52
// code for grade(double, double, const vector&) function from §4.1.2/54
// code for read_hw(istream&, vector&) function from §4.1.3/57
int main()
{
// ask for and read the student's name
cout << "Please enter your first name: ";
string name;
cin >> name;
cout << "Hello, " << name << "!" << endl;
// ask for and read the midterm and final grades
cout << "Please enter your midterm and final exam grades: ";
double midterm, final;
cin >> midterm >> final;
// ask for the homework grades
cout << "Enter all your homework grades, "
"followed by end-of-file: ";
vector homework;
// read the homework grades
read_hw(cin, homework);
// compute and generate the final grade, if possible
try {
double final_grade = grade(midterm, final, homework);
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< final_grade << setprecision(prec) << endl;
} catch (domain_error) {
cout << endl << "You must enter your grades. "
"Please try again." << endl;
return 1;
}
return 0;
}
The changes from the earlier version are in how we read the homework grades, and in
how we calculate and write the result.
After asking for our user's homework grades, we call our read_hw function to read the
data. The while statement inside read_hw repeatedly reads homework grades until we
hit end-of-file or encounter a data value that is not valid as a double.
The most important new idea in this example is the try statement. It tries to execute
the statements in the { } that follow the try keyword. If a domain_error exception
occurs anywhere in these statements, then it stops executing them and continues with
the other set of { } -enclosed statements. These statements are part of a catch clause
, which begins with the word catch , and indicates the type of exception it is catching.
If the statements between try and catch complete without throwing an exception,
then the program skips the catch clause entirely and continues with the next
statement, which is return 0 ; in this example.
Whenever we write a try statement, we must think carefully about side effects and
when they occur. We must assume that anything between try and catch might throw
an exception. If it does so, then any computation that would have been executed after
the exception is skipped. What is important to realize is that a computation that might
have followed an exception in time does not necessarily follow it in the program text.
For example, suppose that we had written the output block more succinctly as
// this example doesn't work
try {
streamsize prec = cout.precision();
cout << "Your final grade is " << setprecision(3)
<< grade(midterm, final, homework) << setprecision(prec);
} ...
The problem with this rewrite is that although the implementation is required to execute
the << operators from left to right, it is not required to evaluate the operands in any
specific order. In particular, it might call grade after it writes Your final grade is .
If grade throws an exception, then the output might contain that spurious phrase.
Moreover, the first call to setprecision might set the output stream's precision to 3
without giving the second call the opportunity to reset the precision to its previous
value. Alternatively, the implementation might call grade before writing any output;
whether it does so depends entirely on the implementation.
This analysis explains why we separated the output block into two statements: The first
statement ensures that the call to grade happens before any output is generated.
A good rule of thumb is to avoid more than one side effect in a single statement.
Throwing an exception is a side effect, so a statement that might throw an exception
should not cause any other side effects, particularly including input and output.
Of course, we cannot run our main function as written. We need the include directives
and using -declarations for the library facilities that the program uses. We also use the
names read_hw and the grade function that takes a const vector& third
argument. The definitions of these functions, in turn, use the median function and the
grade function that takes three doubles.
In order to execute this program, we have to ensure that those functions are defined (in
the proper order) before our main function. Doing so yields an inconveniently large
program. Rather than write it out directly here, we'll see in §4.3/65 how we can partition
such programs more succinctly into files. Before we do so, let's look at better ways to
structure our data.
4.2 Organizing data
Computing one student's grades may be useful to that student, but the computation is
simple enough that a pocket calculator could handle it almost as well as our program.
On the other hand, if we are teaching a course, we will want to compute grades for a
class full of students. Let's revise our program to make it useful for an instructor.
Instead of interactively reporting one student's grade, we'll assume that we are given a
file that contains many students' names and grades. Each name is followed by a
midterm grade and a final exam grade, and then by one or more homework assignment
grades. Such a file might look like
Smith 93 91 47 90 92 73 100 87
Carpenter 75 90 87 92 93 60 0 98
...
Our program should calculate each student's overall grade using medians: The median
homework grade counts 40%; the final, 40%; and the midterm, 20%. For this input, the
output would be
Carpenter 86.8
Smith 90.4
In the output, we want the report to be organized alphabetically by student, and we
want the final grades to line up vertically so that they are easier to read. These
requirements imply that we'll need a place to store the records for all the students, so
that we can alphabetize them. We'll also need to find the length of the longest name, so
that we know how many spaces to put between each name and its corresponding grade.
Assuming that we have a place to store the data about a single student, we can use a
vector to hold all the student data. Once the vector contains data for all the
students, we can sort it, and then calculate and write each student's grades. We'll start
by creating a data structure to hold the student data, and by writing some auxiliary
functions to read and process those data. After we have developed these abstractions,
we'll use them to solve the overall problem.
4.2.1 Keeping all of a student's data together
We know that we need to read each student's data and then arrange the students in
alphabetical order. When we do so, we want to keep the students' names and grades
together. Therefore, we need a way to store in one place all the information that
pertains to one student. That place should be a data structure that holds the student's
name, midterm and final exam grades, and all the homework grades.
In C++, we define such a data structure as follows:
struct Student_info {
string name;
double midterm, final;
vector homework;
}; // note the semicolon it's required
This struct definition says that Student_info is a type, which has four data members.
Because Student_info is a type, we can define objects of that type, each of which will
contain an instance of these four data members.
The first member, named name , is of type string ; the second and third are double s
named midterm and final ; and the last is a vector of double s named homework .
Each object of Student_info type holds information about one student. Because
Student_info is a type, we can use a vector object to hold
information about an arbitrary number of students, just as we used a vector
object to hold an arbitrary number of homework grades.
4.2.2 Managing the student records
If we break our problem into manageable components, we'll see that there are three
separable steps, which we can represent by separate functions: We need to read data
into a Student_info object, we need to generate the overall grade for a
Student_info object, and we need to be able to sort a vector of Student_info
objects.
The function that reads one of our records is a lot like the read_hw function that we
wrote in §4.1.3/57. In fact, we can use that function to read the homework grades. In
addition, we'll need to read the student's name and exam grades:
istream& read(istream& is, Student_info& s)
{
// read and store the student's name and midterm and final exam grades
is >> s.name >> s.midterm >> s.final;
read_hw(is, s.homework); // read and store all the student's homework grades
return is;
}
There is no ambiguity in naming this function read , because the type of its second
parameter will tell us what we're reading. Overloading will distinguish it from any other
function called read that might read into another kind of structure. Like read_hw , this
function takes two references: one to the istream from which to read, and another to
the object in which to store what it reads. When we use the parameter s inside the
function, we will affect the state of the argument that we were passed.
This function works by reading values into the name, midterm , and final members
of the object s , and then calling read_hw to read the homework grades. We might
reach end-of-file, or encounter input failure, at any point during this process. If so, the
subsequent input attempts will do nothing, and when we return, is will be in the
appropriate error state. Note that this behavior relies on the fact that the read_hw
function (§4.1.3/57) carefully leaves the input stream in an error state if it was already
in such a state when we called read_hw .
The other function that we need computes a final grade for a Student_info object. We
already solved most of this problem when we defined the grade function in §4.1.2/54.
We will continue that work just a little further by overloading the grade function with a
version that determines the overall grade for a Student_info object:
double grade(const Student_info& s)
{
return grade(s.midterm, s.final, s.homework);
}
This function operates on an object of type Student_info , and returns a double that
represents the overall grade. Note that the parameter has type const
Student_info& , rather than just plain Student_info , so that when we call it, we
do not incur the overhead of copying an entire Student_info object.
Note also that this function does not protect against an exception being thrown by the
grade function that it calls. The reason is that there isn't anything that our grade
function can do to handle the exception beyond what the grade function that it calls has
already done. Because our grade function doesn't catch the exception, any exception
that occurs will be passed back to our caller, which presumably will be in a better
position than we are to decide what to do about students who did no homework.
Our last task, before writing the whole program, is to decide how we will sort our
vector of Student_info objects. In the median function (§4.1.1/53), we sorted a
vector parameter, named vec , by using the library sort function:
sort(vec.begin(), vec.end());
However, assuming our data is in a vector called students , we can't just say
sort(students.begin(), students.end()); // not quite right
Why not? We'll have much more to say about sort and other library algorithms in
Chapter 6, but it is worth thinking a bit abstractly about how the sort function might
operate. In particular, how does sort know how to arrange the values in the vector ?
The sort function must compare elements of the vector in order to put them in
sequence. It does so by using the < operator for the element type of whatever vector
it is asked to sort. We can call sort on a vector , because the < operator
will compare two double s and give us an appropriate result. What will happen when
sort tries to compare the values of type Student_info ? The < operator does not
have an obvious meaning when applied to Student_info objects. Indeed, when sort
tries to compare two such objects, the compiler will complain.
Fortunately, the sort function takes an optional third argument that is a predicate . A
predicate is a function that yields a truth value, typically of type bool . If this third
argument is present, the sort function will use it to compare elements instead of using
the < operator. Therefore, we need to define a function that takes two Student_info
s, and that says whether the first is less than the second. Because we want to order the
students alphabetically by name, we'll write our comparison function to compare only
the names:
bool compare(const Student_info& x, const Student_info& y)
{
return x.name < y.name;
}
This function simply delegates the work of comparing Student_info s to the string
class, which provides a < operator for comparing string s. That operator compares
string s by applying normal dictionary ordering. That is, it considers the left-hand
operand to be less than the right-hand operand if it is alphabetically ahead of the righthand
operand. This behavior is exactly what we need.
Having defined compare , we can sort the vector by passing the compare function as
a third argument to the sort library function:
sort(students.begin(), students.end(), compare);
When sort compares elements, it will call our compare function to do so.
4.2.3 Generating the report
Now that we have functions to process student records, we can generate our report:
int main()
{
vector students;
Student_info record;
string::size_type maxlen = 0;
// read and store all the records, and find the length of the longest name
while (read(cin, record)) {
maxlen = max(maxlen, record.name.size());
students.push_back(record);
}
// alphabetize the records
sort(students.begin(), students.end(), compare);
for (vector::size_type i = 0;
i != students.size(); ++i) {
// write the name, padded on the right to maxlen + 1 characters
cout << students[i].name
<< string(maxlen + 1 - students[i].name.size(), ' ');
// compute and write the grade
try {
double final_grade = grade(students[i]);
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec);
} catch (domain_error e) {
cout << e.what();
}
cout << endl;
}
return 0;
}
We have already seen most of this code, but a couple of points are new.
The first novelty is the call to the library function max , which is defined in the header
. On the surface, max 's behavior is obvious. However, one aspect of its
behavior is not obvious: Its arguments must both have the same type, for complicated
reasons that we shall explore in §8.1.3/142. This requirement makes it essential for us
to define maxlen to be a variable of type string::size_type ; it won't do merely to
define it as an int .
The second novelty is the expression
string(maxlen + 1 - students[i].name.size(), ' ')
This expression constructs a nameless object (§1.1/10) of type string . The object
contains maxlen + 1 - students[i].name.size() characters, all of which are
blank. This expression is similar to the definition of spaces in §1.2/13, but it omits the
name of the variable. This omission effectively turns the definition into an expression.
Writing students[i].name followed by this expression yields output that contains the
characters in students[i].name , padded on the right to exactly maxlen + 1
characters.
The for statement uses the index i to walk through students one element at a time.
We get the name to write by indexing into students to get the current Student_info
element. We then write the name member from that object, using an appropriately
constructed string of blanks to pad the output.
Next we write the final grade for each student. If the student didn't do any homework,
the grade computation will throw an exception. In that case, we catch the exception,
and instead of writing a numeric grade, we write the message that was thrown as part
of the exception object. Every one of the standard-library exceptions, such as
domain_error , remembers the (optional) argument used to describe the problem that
caused the exception to be thrown. Each of these types makes a copy of the contents of
that argument available through a member function named what . The catch in this
program gives a name to the exception object that it gets from grade (§4.1.2/54), so
that it can write as output the message that it obtains from what() . In this case, that
message will tell the user that the student has done no homework . If there is no
exception, we use the setprecision manipulator to specify that we'll write three
significant digits, and then write the result from grade .
4.3 Putting it all together
So far we have defined a number of abstractions (functions and a data structure) that
are useful in solving our various grading problems. The only way we have seen of using
these abstractions is to put all their definitions in a single file and compile that file.
Obviously, this approach becomes complicated very quickly. To reduce that complexity,
C++, like many languages, supports the notion of separate compilation, which allows
us to put our program into separate files and to compile each of these files
independently.
We'll start by understanding how to package the median function so that others can use
it. We begin by putting the definition of the median function into a file by itself so that
we can compile it separately. That file must include declarations for all the names that
the median function uses. From the library, median uses the vector type, the sort
function, and the domain_error exception, so we will have to include the appropriate
headers for these facilities:
// source file for the median function
#include > // to get the declaration of sort
#include // to get the declaration of domain_error
#include // to get the declaration of vector
using std::domain_error; using std::sort; using std::vector;
// compute the median of a vector
// note that calling this function copies the entire argument vector
double median(vector vec)
{
// function body as defined in §4.1.1/53
}
As with any file, we must give our source file a name. The C++ standard does not tell us
how to name source files, but in general, a source file's name should suggest its
contents. Moreover, most implementations constrain source-file names, usually
requiring the last few characters of the name to have a specific form. Implementations
use these file suffixes to determine whether a file is a C++ source file. Most
implementations require the names of C++ source files to end in .cpp, .C, or .c, so
we might put the median function in a file named median.cpp, median.C, or
median.c, depending on the implementation.
The next step is to make the median function available to other users. Analogous with
the standard library, which puts the names it defines into headers, we can write our own
header file that will allow users to access names that we define. For example, we could
note in a file named median.h that our median function exists. If we did so, users
could use it by writing:
// a much better way to use median
#include "median.h"
#include
int main(){/*...*/}
When we use a #include directive with double quotes rather than angle brackets,
surrounding the header name, we are saying that we want the compiler to copy the
entire contents of the header file that corresponds to that name into our program in
place of the #include directive. Each implementation decides where to look for header
files, and what the relationship is between the string between the quotes and the name
of the file. We shall talk about "the header file median.h" as shorthand for "the file that
the implementation decides is the one that corresponds to the name median.h."
It is worth noting that although we refer to our own headers as header files, we refer to
the implementation-supplied headers as standard headers rather than standard header
files. The reason is that header files are genuine files in every C++ implementation, but
system headers need not be implemented as files. Even though the #include directive
is used to access both header files and system headers, there is no requirement that
they be implemented in the same way.
Now that we know we must supply a header file, the obvious question is what should be
in it. The simple answer is that we must write a declaration for the median function,
which we do by replacing the function body with a semicolon. We can also eliminate the
names of the parameters, because they are irrelevant without the function body:
double median(vector);
Our median.h header cannot contain just this declaration; we must also include any
names that the declaration itself uses. This declaration uses vector, so we must make
sure that that name is available before the compiler sees our declaration:
// median.h
#include
double median(std::vector);
We include the vector header so that we can use the name std::vector in declaring
the argument to median. The reason that we mention std::vector explicitly, rather
than writing a using-declaration, is more subtle.
In general, header files should declare only the names that are necessary. By restricting
the names contained in a header file, we can preserve maximum flexibility for our users.
For example, we use the qualified name for std::vector because we have no way of
knowing how the user of our median function wants to refer to std::vector. Users of
our code might not want a using-declaration for vector. If we write one in our header,
then all programs that include our header get a using std::vector declaration,
regardless of whether they wanted it. Header files should use fully qualified names
rather than using-declarations.
There is one last detail to cover: Header files should ensure that it is safe to include the
file more than once as part of compiling the program. As it happens, our header file is
already safe as it stands, because it contains only declarations. However, we consider it
good practice to cater to multiple inclusion in every header file, not just the ones that
need it. We do so by adding some preprocessor magic to the file:
#ifndef GUARD_median_h
#define GUARD_median_h
// median.h—final version
#include
double median(std::vector);
#endif
The #ifndef directive checks whether GUARD_median_h is defined. This is the name of
a preprocessor variable, which is one of a variety of ways of controlling how a
program is compiled. A full discussion of the preprocessor is beyond the scope of this
book.
In this context, the #ifndef directive asks the preprocessor to process everything
between it and the next matching #endif if the given name is not defined. We must
choose a name that is unique in the entire program, so we make one from the name of
our file and a string, such as GUARD_, that we hope will not be duplicated elsewhere.
The first time median.h is included in a program, GUARD_median_h will be undefined,
so the preprocessor will look at the rest of the file. The first thing it does is to define
GUARD_median_h, so that subsequent attempts to include median.h will have no
effect.
The only other subtlety is that it is a good idea for the #ifndef to be the very first line
of the file, without even a comment before it:
#ifndef variable
...
#endif
The reason is that some C++ implementations detect files that have this form and, if
the variable is defined, do not even bother to read the file the second time around.
4.4 Partitioning the grading program
Now that we know how to arrange to compile the median function separately, the next
step is to package our Student_info structure and associated functions:
#ifndef GUARD_Student_info
#define GUARD_Student_info
// Student_info.h header file
#include
#include
#include
struct Student_info {
std::string name;
double midterm, final;
std::vector homework;
};
bool compare(const Student_info&, const Student_info&);
std::istream& read(std::istream&, Student_info&);
std::istream& read_hw(std::istream&, std::vector&);
#endif
Notice that we explicitly use std:: to qualify names from the standard library, rather
than including using -declarations, and that Student_info.h declares the compare,
read , and read_hw functions, which are closely associated with the Student_info
structure. We will use these functions only if we are also using this structure, so it
makes sense to package these functions with the structure definition.
The functions should be defined in a source file that will look something like:
// source file for Student_info-related functions
#include "Student_info.h"
using std::istream; using std::vector;
bool compare(const Student_info& x, const Student_info& y)
{
return x.name < y.name;
}
istream& read(istream& is, Student_info& s)
{
// read and store the student's name and midterm and final exam grades
is >> s.name >> s.midterm >> s.final;
read_hw(is, s.homework); // read and store all the student's homework grades
return is;
}
// read homework grades from an input stream into a `vector'
istream& read_hw(istream& in, vector& hw)
{
if (in) {
// get rid of previous contents
hw.clear();
// read homework grades
double x;
while (in >> x)
hw.push_back(x);
// clear the stream so that input will work for the next student
in.clear();
}
return in;
}
Note that because we include the Student_info.h file, this file contains both
declarations and definitions of our functions. This redundancy is harmless, and is
actually a good idea. It gives the compiler the opportunity to check for consistency
between the declarations and the definitions. These checks are not exhaustive in most
implementations, because complete checking requires seeing the entire program, but
they are useful enough to make it worthwhile for source files to include the
corresponding header files.
The checking and its incompleteness stem from a common source: The language
requires function declarations and definitions to match exactly in their result type, and
in the number and types of parameters. This rule explains the implementation's ability
to check—but why the incompleteness? The reason is that if a declaration and definition
differ enough, the implementation can only assume that they describe two different
versions of an overloaded function, and that the missing definition will appear
elsewhere. For example, suppose we defined median as in §4.1.1/53, and we declared it
incorrectly as
int median (std::vector); // return type should be double
If the compiler sees this declaration when it compiles the definition, it will complain,
because it knows that the return type of median(vector) cannot
simultaneously be double and int . Suppose, however, that instead we had declared
double median (double); // argument type should be vector
Now the compiler can't complain, because median(double) could be defined
elsewhere. If we call the function, then the implementation must eventually look for its
definition. If it doesn't find the definition, it will complain at that point.
Note, too, that in the source file, there is no problem with using -declarations. Unlike a
header file, a source file has no effect on the programs that use these functions. Hence
reliance on using -declarations in a source file is purely a local decision.
What's left is to write a header file to declare the various overloaded grade functions:
#ifndef GUARD_grade_h
#define GUARD_grade_h
// grade.h
#include
#include "Student_info.h"
double grade(double, double, double);
double grade(double, double, const std::vector&);
double grade(const Student_info&);
#endif
Notice how bringing the declarations of these overloaded functions together makes it
easier to see all the alternatives. We will define all three functions in a single file,
because they are closely related. Again, the name of the file will depend on the
implementation, but will probably be grade.cpp, grade.C , or grade.c:
#include
#include
#include "grade.h"
#include "median.h"
#include "Student_info.h"
using std::domain_error; using std::vector;
// definitions for the grade functions from §4.1/52, §4.1.2/54 and §4.2.2/63
4.5 The revised grading program
Finally we can write our complete program:
#include
#include
#include
#include
#include
#include
#include
#include "grade.h"
#include "Student_info.h"
using std::cin; using std::setprecision;
using std::cout; using std::sort;
using std::domain_error; using std::streamsize;
using std::endl; using std::string;
using std::max; using std::vector;
int main()
{
vector students;
Student_info record;
string::size_type maxlen = 0; // the length of the longest name
// read and store all the students data.
// Invariant: students contains all the student records read so far
// maxlen contains the length of the longest name in students
while (read(cin, record)) {
// find length of longest name
maxlen = max(maxlen, record.name.size());
students.push_back(record);
}
// alphabetize the student records
sort(students.begin(), students.end(), compare);
// write the names and grades
for (vector::size_type i = 0;
i != students.size(); ++i) {
// write the name, padded on the right to maxlen + 1 characters
cout << students[i].name
<< string(maxlen + 1 - students[i].name.size(), ' ');
// compute and write the grade
try {
double final_grade = grade(students[i]);
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec);
} catch (domain_error e) {
cout << e.what();
}
cout << endl;
}
return 0;
}
This program should be fairly straightforward to understand. As usual, we start with the
necessary includes and using-declarations. Of course, we need mention only those
headers and declarations that we use in this source file. In this program, we have our
own headers to include as well as library headers. Those headers make available the
definition of the Student_info type, and declarations for the functions that we use to
manipulate Student_info objects and generate grades. The main function itself is the
same as the one that we presented in §4.2.3/64.
4.6 Details
Program structure:
#include
Angle brackets, < >, enclose system headers. System headers may or may not be
implemented as files.
#include "user-defined-header-file-name"
User-defined header files are included by enclosing the name in quotes. Typically,
user-defined headers have a suffix of .h.
Header files should be guarded against multiple inclusion by wrapping the file in an
#ifndef GUARD_header_name directive. Headers should avoid declaring names that
they do not use. In particular, they should not include using-declarations, but instead
should prefix standard-library names with std:: explicitly.
Types:
T&
Denotes a reference to the type T. Most commonly used to pass a parameter that a
function may change. Arguments to such parameters must be lvalues.
const T&
Denotes a reference to the type T that may not be used to change the value to which
the reference is bound. Usually used to avoid cost of copying a parameter to a
function.
Structures: A structure is a type that contains zero or more members. Each object of
the structure type contains its own instance of each of its members. Every structure
must have a corresponding definition:
struct type-name {
type-specifier member-name;
...
}; // note the semicolon
Like all definitions, a structure definition may appear only once per source file, so it
should normally appear in a properly guarded header file.
Functions: A function must be declared in every source file that uses it, and defined
only once. The declarations and definitions have similar forms:
ret-typefunction-name (parm-decls) // function declaration
[inline] ret-typefunction-name (parm-decls) { // function definition
// function body goes here
}
Here, ret-type is the type that the function returns, parm-decls is a commaseparated
list of the types for the parameters of the function. Functions must be
declared before they are called. Each argument's type must be compatible with the
corresponding parameter. A different syntax is necessary to declare or define functions
with sufficiently complicated return types; see §A.1.2/297 for the full story.
Function names may be overloaded: The same function-name may define multiple
functions so long as the functions differ in the number or types of the parameters. The
implementation can distinguish between a reference and a const reference to the same
type.
We can optionally qualify a function definition with inline, which asks the compiler to
expand calls to the function inline when appropriate—that is, to avoid function-call
overhead by replacing each call to the function by a copy of the function body, modified
as necessary. To do so, the compiler needs to be able to see the function definition, so
inlines are usually defined in header files, rather than in source files.
Exception handling:
try { // code
Initiates a block that might throw an exception.
} catch(t) { /* code* / }
Concludes the try block and handles exceptions that match the type t. The code
following the catch performs whatever action is appropriate to handle the exception
reported in t.
throw e;
Terminates the current function; throws the value e back to the caller.
Exception classes: The library defines several exception classes whose names suggest
the kinds of problems they might be used to report:
logic_error domain_error invalid_argument
length_error out_of_range runtime_error
range_error overflow_error underflow_error
e.what()
Returns a value that reports on what happened to cause the error.
Library facilities:
s1 < s2
Compares strings s1 and s2 by applying dictionary ordering.
s.width(n)
Sets the width of stream s to n for the next output operation (or leaves it unchanged if
n is omitted). The output is padded on the left to the given width. Returns the previous
width. The standard output operators use the existing width value and then call
width(0) to reset the width.
setw(n)
Returns a value of type streamsize (§3.1/36) that, when written on an output
stream s, has the effect of calling s.width(n).
Exercises
4-0. Compile, execute, and test the programs in this chapter.
4-1. We noted in §4.2.3/65 that it is essential that the argument types in a call to max
match exactly. Will the following code work? If there is a problem, how would you fix it?
int maxlen;
Student_info s;
max(s.name.size(), maxlen);
4-2. Write a program to calculate the squares of int values up to 100. The program
should write two columns: The first lists the value; the second contains the square of
that value. Use setw (described above) to manage the output so that the values line up
in columns.
4-3. What happens if we rewrite the previous program to allow values up to but not
including 1000 but neglect to change the arguments to setw? Rewrite the program to
be more robust in the face of changes that allow i to grow without adjusting the setw
arguments.
4-4. Now change your squares program to use double values instead of ints. Use
manipulators to manage the output so that the values line up in columns.
4-5. Write a function that reads words from an input stream and stores them in a
vector. Use that function both to write programs that count the number of words in
the input, and to count how many times each word occurred.
4-6. Rewrite the Student_info structure to calculate the grades immediately and
store only the final grade.
4-7. Write a program to calculate the average of the numbers stored in a
vector.
4-8. If the following code is legal, what can we infer about the return type of f?
double d = f()[n];
5
Using sequential containers and analyzing strings
We've made a fair start on the core C++ language, and we've learned something about
the string and vector classes as well. We can solve a lot of problems with just these
tools.
In this chapter, we'll expand our focus beyond these facilities, and start to understand in
more depth how to use the library. As we'll see, the facilities that the library provides
can help us solve messier problems than the ones that we've confronted so far.
Not only does the standard library provide useful data structures and functions, but it
also reflects a consistent architecture: Once we have learned how one kind of container
behaves, we are well on our way to understanding how to use all the library containers.
For example, as we'll see in the second half of this chapter, we can often use a string
as if it were a vector. Many useful operations on one library type are logically the same
as the operations on another. The library is constructed so that such equivalent
operations work the same way on different types.
5.1 Separating students into categories
Let's revisit our student-grading problem from §4.2/61, supposing this time that not
only do we wish to calculate all the students' grades, but we would also like to know
which students failed the course. The idea is that if we have a vector of
Student_info records, we would like to extract the ones for students who failed the
course, and store those records in another vector. We also want to remove the data
for failing students from the original vector, so that it will contain only records for
students who passed. We'll start by writing a simple function to test whether a student
failed:
// predicate to determine whether a student failed
bool fgrade(const Student_info& s)
{
return grade(s) < 60;
}
We use the grade function from §4.2.2/63 to calculate the grade, and we arbitrarily
define a failing grade as one that is less than 60.
The most straightforward approach to solving our overall problem is to examine each
student record and push it onto one of two vectors, one for students with passing
grades and the other for students with failing grades:
// separate passing and failing student records: first try
vector extract_fails(vector& students)
{
vector pass, fail;
for (vector::size_type i = 0;
i != students.size(); ++i)
if (fgrade(students[i]))
fail.push_back(students[i]);
else
pass.push_back(students[i]);
students = pass;
return fail;
}
Of course, before we can compile this code, we must add #include directives and
using-declarations for the names that we are using. In general, we will no longer show
these statements in code that we present. When we use a new header, though, we will
continue to mention it.
Like the read_hw and read functions from Chapter 4, this function effectively has two
outputs: One is the vector that we return, which contains the
records for students who failed; the other is created as a side effect of calling
extract_fails. The function's parameter is a reference, so changes to the parameter
are reflected in the argument. When the function finishes, the vector that was passed
as an argument will contain records only for students who passed the course.
The function works by creating two vectors, which hold the data for passing and failing
students respectively. The function looks at each record in students, and appends a
copy of that record to pass or fail depending on the student's grade.
After the for statement is finished, we copy the passing records back into students
and return the failing records.
5.1.1 Erasing elements in place
Our extract_fails function does what we want, and is reasonably efficient, but it has
a subtle flaw: It requires enough memory to hold two copies of each student record. The
reason is that as it builds up pass and fail, the original records are still around. When
the function is done with its for statement, and is ready to copy the results and return,
there are two copies of each student record.
We would like to avoid keeping multiple copies of data around any longer than
necessary. One way to do so is to eliminate pass entirely. Instead of creating two
vectors, we will create a single local variable, named fail, to hold the value that we
intend to return. For each record in students, we will compute the grade. If it is a
passing grade, we'll leave the record alone; if it's a failing grade, we'll append a copy of
it to fail and remove it from students.
To use this strategy, we need a way to remove an element from a vector. The good
news is that such a facility exists; the bad news is that removing elements from
vectors is slow enough to argue against using this approach for large amounts of input
data. If the data we process get really big, performance degrades to an astonishing
extent.
For example, if all of our students were to fail, the execution time of the function that
we are about to see would grow proportionally to the square of the number of students.
That means that for a class of 100 students, the program would take 10,000 times as
long to run as it would for one student. The problem is that our input records are stored
in a vector, which is optimized for fast random access. One price of that optimization
is that it can be expensive to insert or delete elements other than at the end of the
vector.
We shall see two ways to solve the performance problem: We can use a data structure
that is better suited to our algorithm, or we can use a smarter algorithm that avoids the
overhead of our initial design. From here through §5.5.2/87, we'll develop a solution
that uses a more appropriate data structure. We'll show an algorithmic solution in
§6.3/116.
Before we can understand why these solutions are improvements, we must have
something to improve. Therefore, we'll begin by looking at the slow but direct solution:
// second try: correct but potentially slow
vector extract_fails(vector& students)
{
vector fail;
vector::size_type i = 0;
// invariant:elements [0, i) of students represent passing grades
while (i != students.size()) {
if (fgrade(students[i])) {
fail.push_back(students[i]};
students.erase(students.begin() + i);
} else
++i;
}
return fail;
}
We begin this version by creating fail, which is the vector into which we'll copy the
records for students with failing grades. We next define i, which we'll use as an index
into students. We'll process each record, iterating through students until we've seen
all the entries in students.
For each record in students, we determine whether it represents a failing grade. If so,
then we need to copy that record into fail and remove it from students. The
push_back call to append a copy of students[i] to fail is nothing new. What is
new is the way we remove the element from students:
students.erase(students.begin() + i);
The vector type includes a member named erase, which removes an element from
the vector. The argument to erase indicates which element to remove. As it happens,
there is no version of the erase function that operates on indices, because, as we shall
see in §5.5/85, not all containers support indices, and it is more useful for the library to
offer a form of erase that will work the same way with all containers. Instead, the
erase function takes a type that we shall discuss in §5.2.1/80. What's important to
understand now is that we can indicate which element to erase by adding our index to
the value returned by students.begin(). Recall that students.begin() returns a
value that denotes the vector's initial element—the one with index 0 . If we add an
integer, such as i, to that value, the result denotes the element with index i. We can
now see that this call to erase removes the ith element from students.
Once we have removed an element from the vector, the vector now has one fewer
element than it did before:
In addition to changing the size of the vector, erase removes the element with index
i, thereby causing i to denote the next element in the sequence. Each element after
position i is copied to the preceding position. Thus, although i does not change, erase
has the effect of adjusting the index to denote the next element in the vector, which
means that we must not increment it for the next iteration.
If the record we're looking at does not contain a failing grade, then we want to leave it
in students. In that case, we must increment i, so that i will refer to the next record
on the next trip through the while.
We determine whether we have seen all the records in students by comparing i with
students.size(). When we erase an element from the vector, the vector has one
fewer element than it did before. Therefore, it is essential that we call students.size
on each trip through the condition. If, instead, we precomputed and stored the result of
size
// this code will fail because of misguided optimization
vector::size_type size = students.size();
while (i != size) {
if (fgrade(students[i])) {
fail.push_back(students[i]);
students.erase(students.begin() + i);
} else
++i;
}
our program would fail, because calling erase would have changed the number of
elements in students. If we precomputed the size and actually erased any records
for failing students, then we would make too many trips through students, and the
references to students[i] would be to nonexistent elements! Fortunately, calls to
size() are usually fast, so the expected overhead from calling size each time is
negligible.
5.1.2 Sequential versus random access
Both versions of our extract_fails function share a property with many programs
that work with containers, which property is not immediately obvious from the code:
Each of these functions accesses container elements only sequentially. That is, each
version of the function looks at each student record in turn, decides what to do with it,
and then proceeds to the next record.
The reason that this property is not obvious from the code is that the function uses an
integer, i, to access each element of students. It is possible to compute the value of
an integer in arbitrary ways, which means that in order for us to determine whether we
are accessing the container sequentially, we must look at every operation that might
affect the value of i, and determine that operation's effect. Another way to view the
problem is that when we write students[i] to access an element of students, we
are implicitly saying that we might access students's elements in any order, not just
sequentially.
The reason we care about the sequence in which we access container elements is that
different types of containers have different performance characteristics and support
different operations. If we know that our program uses only those operations that a
particular type of container supports efficiently, then we can make our program more
efficient by using that kind of container.
In other words, because our function requires only sequential access, we do not need to
use indices, which provide the ability to access any element randomly. Instead, we'd like
to rewrite the function so as to restrict access to the container elements to operations
that support only sequential access. To that end, the C++ library supplies an assortment
of types called iterators, which allow access to data structures in ways that the library
can control. This control lets the library ensure efficient implementation.
5.2 Iterators
To make our discussion more concrete, let us look at the container operations that
extract_fails actually uses.
The first such operation is using the index i to fetch values from the Student_info
structure. For example, fgrade(students[i]) fetches the ith element of the
vector named students, and passes that element to the fgrade function. We know
that we access the elements of students sequentially, because we access those
elements only by using i as an index, and the only operations we ever perform on i are
to read it in order to compare it with the size of the vector, and to increment it:
while (i != students.size()) {
// work gets done here; but doesn't change the value of i
i++;
}
From this usage, it is clear that we use i only sequentially.
Unfortunately, even though we know this fact, the library has no way to know it. By
using iterators instead of indices, we can make that knowledge available to the library.
An iterator is a value that
Identifies a container and an element in the container
Lets us examine the value stored in that element
Provides operations for moving between elements in the container
Restricts the available operations in ways that correspond to what the container can
handle efficiently
Because iterators behave analogously to indices, we can often rewrite programs that use
indices to make them use iterators instead. As an example, suppose that students is a
vector that contains records for some students. Let's look at how we
could write those students' names onto cout. One way uses an index for the iteration:
for (vector::size_type i = 0;
i != students.size(); ++i)
cout << students[i].name << endl;
Another way uses iterators:
for (vector::const_iterator iter = students.begin();
iter != students.end(); ++iter) {
cout << (*iter).name << endl;
}
There's quite a lot going on in this rewrite, so let's pick it apart a bit at a time.
5.2.1 Iterator types
Every standard container, such as vector, defines two associated iterator types:
container-type::const_iterator
container-type::iterator
where container-type is the container type, such as vector, that
includes the type of the container elements. When we want to use an iterator to
change the values stored in the container, we use the iterator type. If we need only read
access, then we use the const_iterator type.
Abstraction is selective ignorance. The details of what particular type an iterator has
may be complicated, but we don't need to understand these details. All we need to know
is how to refer to the iterator type, and what operations the iterator allows. We need to
know the type so that we can create variables that are iterators. We don't need to know
anything about that type's implementation. For example, our definition of iter
vector::const_iterator iter = students.begin();
says that iter is of type vector::const_iterator. We don't
know the actual type of iter—that's an implementation detail of vector—nor do we
need to know. All that we need to know is that vector has a member
named const_iterator that defines a type that we can use to obtain read-only
access to elements of the vector.
The other thing we need to know is that there is an automatic conversion from type
iterator to type const_iterator. As we're about to learn, students.begin()
returns an iterator, but we said that iter is a const_iterator. In order to
initialize iter with the value of students.begin(), the implementation converts the
iterator value into the corresponding const_iterator. This conversion is one way,
meaning that we can convert an iterator to a const_iterator but not vice versa.
5.2.2 Iterator operations
Having defined iter, we set it to the value of students.begin(). We have used the
begin and end functions before, so we should know what these functions do: They
return a value that denotes the beginning or (one past) the end of a container. We'll
explain our repeated emphasis on "(one past)" in §8.2.7/149. What's useful to know
now is that begin and end functions return a value of the iterator type for the
container. Thus, begin returns a vector::iterator positioned at
the initial element of the container, so iter initially refers to the first element in
students. The condition in the for statement,
iter != students.end()
checks whether we've reached the end of the container. Recall that end returns a value
that denotes (one past) the end of the container. As with begin, the type of this value
is vector::iterator. We can compare two iterators, const or
not, for inequality (or equality). If iter is equal to the value returned by
students.end(), then we're through.
The last expression in the for header, ++iter, increments the iterator so that it refers
to the next element in students on the next trip through the for. The expression
++iter uses the increment operator, overloaded for the iterator type. The increment
operator has the effect of advancing the iterator to the next element in the container.
We don't know, and shouldn't care, how the increment operator works. All that we need
to know is that afterward, the iterator denotes the next element in the container.
In the body of the for, iter is positioned on an element in students, which element
we need to write. We access that element by calling the dereference operator, *. When
applied to an iterator, the * operator returns an lvalue (§4.1.3/56) that is the element
to which the iterator refers. Therefore, the output operation
cout << (*iter).name
has the effect of writing the current element's name member on the standard output.
In order to execute correctly, this expression requires parentheses that override the
normal operator precedence. The expression *iter returns the value that the iterator
iter denotes. The precedence of . is higher than the precedence of *, which means
that if we want the * operation to apply only to the left operand of the ., we must
enclose *iter in parentheses to get (*iter). If we wrote *iter.name, the compiler
would treat it as *(iter.name), which would be a request to fetch the name member
from object iter, and apply the dereference operator to that object. The compiler would
complain because iter does not have a member named name. By writing
(*iter).name, we say that we want to refer to the name member of the *iter object.
5.2.3 Some syntactic sugar
In the code we just saw, we dereferenced an iterator, and then fetched an element from
the value returned. This combination of operations is so common that there is an
abbreviation for it: Instead of
(*iter).name
we can write
iter->name
We can use this syntactic sugar to rewrite the last example in §5.2/80:
for (vector::const_iterator iter = students.begin();
iter != students.end(); ++iter) {
cout << iter->name << endl;
}
5.2.4 The meaning of students.erase (students.begin () + i)
Now that we understand more about iterators, we can see the real point of
students.erase(students.begin() + i);
in the program in §5.1.1/77. We've already seen that students.begin() is an iterator
that refers to the initial element of students, and that students.begin() + i
refers to the ith element of students. What is important to realize is that this latter
expression gets its meaning from the definition of + on the types of
students.begin() and i. In other words, the iterator and index types determine the
meaning of + in this expression.
If students were a container that did not support random-access indexing, it is likely
that students.begin() would be of a type that did not have + defined—in which case
the expression students.begin() + i would not compile. In effect, such a container
would be able to shut off random access to its elements, while still allowing sequential
access through iterators.
5.3 Using iterators instead of indices
Using what we have learned about iterators, and one more new fact, we can
reimplement the extract_fails function in a way that does not use indexing at all:
// version 3: iterators but no indexing; still potentially slow
vector extract_fails(vector& students)
{
vector fail;
vector::iterator iter = students.begin();
while (iter != students.end()) {
if (fgrade(*iter)) {
fail.push_back(*iter);
iter = students.erase(iter);
} else
++iter;
}
return fail;
}
We start by defining fail as we did before. Next, we define the iterator, named iter,
that we'll use—in place of an index—to look at the elements in students. Note that we
give it type iterator instead of const_iterator:
vector::iterator iter = student.begin();
because we intend to use it to modify students, which we do in the call to erase. We
initialize iter to denote the first element in students.
We continue with a while statement that will look at every element of students.
Remember that iter is an iterator that denotes an element in the container, so *iter
is the value of that element. To decide whether a student passed or failed, we pass that
value to fgrade. Similarly, we changed the code that copies the failing records into
fail by writing
fail.push_back(*iter) ; // dereference the iterator to get the element
instead of
fail.push_back(students[i]); // index into the vector to get the element
The erase has gotten simpler, because we now have an iterator to pass directly:
iter = students.erase(iter);
We no longer have to calculate an iterator by adding the index i to
students.begin().
The new fact that we used here is easy to overlook, but crucially important: We now
assign to iter the value that erase returns. Why?
A bit of thinking should convince us that removing the element that iter denoted must
invalidate that iterator. After we have called students.erase(iter), we know that
iter can no longer refer to the same element because that element is gone. In fact,
calling erase on a vector invalidates all iterators that refer to elements after the one
that was just erased. If you look back at the diagram in §5.1.1/78, it should be obvious
that after we erase the element marked FAIL, that element is gone, and each of the
elements after it has moved. If the elements have moved, any iterators referring to
them must be meaningless as well.
Fortunately, erase returns an iterator that is positioned on the element that follows the
one that we just erased. Therefore, executing
iter = students.erase(iter);
makes iter refer to the element after the erasure, which is exactly what we need.
If we're dealing with an element that did not represent a failing grade, then we still need
to increment iter so that we'll be positioned on the next element for the next trip
through the loop. We do so by incrementing iter in the else branch.
Incidentally, as in §5.1.1/78, we might be tempted to optimize the loop by saving the
value of students.end() to avoid evaluating it each time through the while. In other
words, we might be tempted to change
while (iter != students.end())
to
// this code will fail because of misguided optimization
vector::iterator iter = students.begin(),
end_iter = students.end();
while (iter != end_iter) {
// . . .
}
This loop will almost surely fail at run time. Why?
The reason is that if we ever execute students.erase, doing so will invalidate every
iterator after the point erased, including end_iter! Therefore, it is essential that we
call students.end each time through the loop, just as it was essential in §5.1.1/78 to
call students.size each time through the loop.
5.4 Rethinking our data structure for better performance
For small inputs, our implementation works fine. However, as we said in §5.1.1/77, as
our input grows, the performance degrades substantially. Why?
Let's think again about using erase to remove an element from a vector. The library
optimizes the vector data structure for fast access to arbitrary elements. Moreover, we
saw in §3.2.3/48 that vectors perform well when growing a vector one element at a
time, as long as elements are added at the end of the vector.
Inserting or removing elements from the interior of a vector is another story. Doing so
requires that all elements after the one inserted or removed be moved in order to
preserve fast random access. Moving elements means that the run time of our new code
might be as slow as quadratic in the number of elements in the vector. For small
inputs, we might not notice, but each time the size of our input doubles, the execution
time can quadruple. If we ask our program to deal with all the students in a school
rather than just the students in a class, even a fast computer will take too long to
execute the program.
If we want to do better, we need a data structure that lets us insert and delete elements
efficiently anywhere in the container. Such a container is unlikely to support random
access through indices. Even if it did so, integer indices would be less than useful,
because inserting and deleting elements would have to change the indices of other
elements. Now that we know how to use iterators, we have a way of dealing with such a
data structure that does not provide index operations.
5.5 The list type
By rewriting the code to use iterators, we have removed our reliance on indices. We now
need to reimplement our program using a data structure that will let us delete elements
efficiently from within the container.
The need to insert or delete elements inside a data structure is pretty common. Not
surprisingly, the library provides a type, named list and defined in the
header, that is optimized for this kind of access.
Just as vectors are optimized for fast random access, lists are optimized for fast
insertion and deletion anywhere within the container. Because lists have to maintain a
more complicated structure, they are slower than vectors if the container is accessed
only sequentially. That is, if the container grows and shrinks only or primarily from the
end, a vector will outperform a list. However, if a program deletes many elements
from the middle of the container—as our program does—then lists will be faster for
large inputs, becoming much faster as the inputs grow.
Like a vector, a list is a container that can hold objects of most.any type. As we'll
see, lists and vectors share many operations. As a result, we can often translate
programs that operate on vectors into programs that operate on lists, and vice
versa. Often, all that changes is our variables' types.
One key operation that vectors support, but lists do not, is indexing. As we just saw,
we can write a version of extract_fails that uses vectors to extract records that
correspond to failing students, but that uses iterators instead of indices. It turns out that
we can transform that version of extract_fails to use lists instead of vectors
merely by changing the appropriate types:
// version 4: use list instead of vector
list extract_fails(list& students)
{
list fail;
list::iterator iter = students.begin();
while (iter != students.end()) {
if (fgrade(*iter)) {
fail.push_back(*iter);
iter = students.erase(iter);
} else
++iter;
}
return fail;
}
If we compare this code with the version from §5.3/82, we see that the only change is
to replace vector by list in the first four lines. So, for example, the return type and
the parameter to the function are now list, as is the local container
fail, into which we put the failing grades. Similarly, the type of the iterator is the one
defined by the list class. Hence, we define iter as the iterator type that is a
member of list. The list type is a template, so we must say what
kind of object the list holds by naming that type inside angle brackets, just as we do
when we define a vector.
There are no changes in the program's logic. Of course, our caller will now have to
provide us with a list, and will get a list in return. Moreover, the details of how the
library implements the operations are quite different, because this version operates on
lists and the other ones operate on vectors. When we execute ++iter, we are doing
whatever it means to advance the iterator to the next element in the list. Similarly,
iter = students.erase(iter);
calls the list version of erase and assigns the list iterator returned from erase
into iter. The implementations of the increment and erase operations will surely differ
from their vector counterparts.
5.5.1 Some important differences
One important way in which the operations on lists differ from those on vectors is
the effect of some of the operations on iterators. For example, when we erase an
element from a vector, all iterators that refer to the element erased, or subsequent
elements, are invalidated. Using push_back to append an element to a vector
invalidates all iterators referring to that vector. This behavior follows from the fact that
erasing an element moves the following elements, and appending to a vector might
reallocate the entire vector to make room for the new element. Loops that use these
operations must be especially careful to ensure that they have not saved copies of any
iterators that may be invalidated. Inappropriately saving the value of students.end()
is a particularly rich source of bugs.
For lists, on the other hand, the erase and push_back operations do not invalidate
iterators to other elements. Only iterators that refer to the element actually erased are
invalidated, because that element no longer exists.
We have already mentioned that list class iterators do not support full random- access
properties. We'll have much more to say about iterator properties in §8.2.1/144. For
now, what's important to know is that, because of this lack of support, we cannot use
the standard-library sort function to sort values that are stored in a list. Because of
this restriction, the list class provides its own sort member function, which uses an
algorithm that is optimized for sorting data stored in a list. Thus, to sort elements in a
list, we must call the sort member
list students;
students.sort(compare);
rather than the global sort function
vector students;
sort(students.begin(),students.end(), compare);
as we do for vectors. It is worth noting that because the compare function operates
on Student_info objects, we can use the same compare function to sort a list of
Student_info records that we used to sort a vector of them.
5.5.2 Why bother?
The code that extracts records for failing students is a good example of the effect of
data structure choices on performance. The code accesses elements sequentially, which
generally implies that a vector is the best choice. On the other hand, we also delete
elements from the interior of the container, thus favoring lists.
As with any performance-related question, the data structure that is "best" depends on
whether performance even matters. Performance is a tricky subject that is generally
outside the scope of this book, but it is worth noting that the choice of data structure
can have a profound effect on a program's performance. For small inputs, lists are
slower than vectors. For large inputs, a program that uses vectors in an
inappropriate way can run much more slowly than it would if it were to use lists. It
can be surprising how quickly performance degrades as the input grows.
To test our programs' performance, we used three files of student records. The first file
had 735 records. The second file was ten times as big, and the third, ten times bigger
than that, or 73,500 records. The following table records the time, in seconds, that it
took to execute the programs on each file size:
File size list vector
735 0.1 0.1
7,350 0.8 6.7
73,500 8.8 597.1
For the file with 73,500 records, the list version of the program took less than nine
seconds to run, whereas the vector version took nearly ten minutes. The discrepancy
would have been even greater had there been more failing students.
5.6 Taking strings apart
Now that we've seen some of what we can do with containers, we're going to turn our
attention back to string s. Until now, we've done only a few things with string s:
We've created them, read them, concatenated them, written them, and looked at their
size. In each of these uses, we have dealt with the string as a single entity. Often,
this kind of abstract usage is what we want: We want to ignore the detailed contents of
a string . Sometimes, though, we need to look at the specific characters in a string .
As it turns out, we can think of a string as a special kind of container: It contains only
characters, and it supports some, but not all, of the container operations. The operations
that it does support include indexing, and the string type provides an iterator that is
similar to a vector iterator. Thus, many of the techniques that we can apply to vector
s apply also to string s.
For example, we might want to break a line of input into words, separated from each
other by whitespace (space, tab, backspace, or the end of the line). If we can read the
input directly, we can get the words from the input trivially. After all, that's exactly how
the string input operator executes: It reads characters up to the whitespace
character.
However, there are times when we want to read an entire line of input and examine the
words within that line. We'll see examples in §7.3/126 and §7.4.2/131.
Because such an operation might be generally useful, we'll write a function to do it. The
function will take a string and return a vector , which will contain an entry
for each whitespace-separated word in that string . In order to understand this
function, you need to know that string s support indexing much the same way as
vector s do. So, for example, if s is a string that contains at least one character, the
first character of s is s[0] , and the last character of s is s[s.size() - 1] .
Our function will define two indices, i and j , that will delimit each word in turn. The
idea is that we will locate a word by computing values for i and j such that the word
will be the characters in the range [i, j) . For example,
Once we have these indices, we'll use the characters that they delimit to create a new
string , which we will copy into our vector . When we are done, we will return the
vector to our caller:
vector split(const string& s)
{
vector ret;
typedef string::size_type string_size;
string_size i = 0;
// invariant: we have processed characters [original value of i, i)
while (i != s.size()) {
// ignore leading blanks
// invariant: characters in range [original i, current i) are all spaces
while (i != s.size() && isspace(s[i]))
++i;
// find end of next word
string_size j = i;
// invariant: none of the characters in range [original j, current j)is a space
while (j != s.size() && !isspace(s[j]))
j++;
// if we found some nonwhitespace characters
if (i != j) {
// copy from s starting at i and taking j - i chars
ret.push_back(s.substr(i, j - i));
i = j;
}
}
return ret;
}
In addition to the system headers that we have already encountered, this code needs
the header, which defines isspace . More generally, this header defines
useful functions for processing individual characters. The c at the beginning of cctype
is a reminder that the ctype facility is part of C++'s inheritance from C.
The split function has a single parameter, which is a reference to a const string
that we'll name s . Because we will be copying words from s , split does not need to
change the string . As in §4.1.2/54, we can pass a const reference to avoid the cost
of copying the string , while still ensuring that split will not change its argument.
We start off by defining ret , which will hold the words from the input string . The
next two statements define and initialize our first index, i . As we saw in §2.4/22,
string::size_type is the name for the appropriate type to index a string .
Because we need to use this type more than once, we start by defining a shorter
synonym for this type, as we did in §3.2.2/43, to simplify the subsequent declarations.
We will use i as the index that finds the start of each word, advancing i through the
input string one word at a time.
The test in the outermost while ensures that once we've processed the last word in the
input, we'll stop.
Inside the while , we start by positioning our two indices. First, we find the first nonspace
character in s that is at or after the position currently indicated by i . Because
there might be multiple whitespace characters in the input, we increment i until it
denotes a character that is not whitespace.
There is a lot going on in this statement:
while (i != s.size() && isspace(s[i]))
++i;
The isspace function is a predicate that takes a char and returns a value that
indicates whether that char is whitespace. The && operator tests whether both its
operands are true , failing if either of them is false . In this expression, the operation
will succeed if i is not equal to the size of s (meaning that we have not reached the end
of the string ), and s[i] is a whitespace character. In that case, we will increment i
and check again.
As we described in §2.4.2.2/26, the logical && operation uses a short-circuit strategy for
evaluating its operands. Unlike our earlier examples, this one relies on the short-circuit
property of && . The binary logical operations (operators && and || ) execute by testing
their left-hand operands first. If that test suffices to determine the overall result, then
the right-hand operand is not evaluated. In the case of the && , the second condition is
evaluated if and only if the first condition is true . Thus, the condition in the while
executes by first checking whether i != s.size() . Only if this test succeeds does it
use i to look at a character in s . Of course, if i is equal to s.size() , then there are
no more characters left to examine, and so we drop out of the loop.
Once we fall out of this while , we know either that i denotes a character that is not
whitespace, or that we've run out of input without finding such a character.
Assuming that i is still a valid index, the next while will find the space that terminates
the current word in s . We start by creating our other index, j , and initializing it to the
value of i . The next while ,
while (j != s.size() && !isspace(s[j]))
++j;
executes similarly to the previous one, but this time the while stops when it
encounters a whitespace character. As before, we start by ensuring that j is still in
range. If so, we again call isspace on the character indexed by j . This time, we
negate the return from isspace using the logical negation operator, ! . In other
words, we want the condition to be true if isspace(s[j]) is not true .
Having completed the two inner while loops, we know that we have either found
another word or run out of input while looking for a word. If we have run out of input,
then both i and j will be equal to s.size() . Otherwise, we have found a word, which
we must push onto ret :
// if we found some nonwhitespace characters
if (i != j) {
// copy from s starting at i and taking j - i chars
ret.push_back(s.substr(i, j - i));
i = j;
}
The call to push_back uses a member of the string class, named substr , that we
have not previously seen. It takes an index and a length, and creates a new string
that contains a copy of characters from the initial string , starting at the index given
by the first argument, and copying as many characters as indicated by its second
argument. The substring that we extract starts at i , which is the first character in the
word that we just found. We copy characters from s starting with the one indexed by i ,
and continuing until we have copied the characters in the (half-open) range [i, j) .
Remembering from §2.6/31 that the number of elements in a half-open range is the
difference between the bounds, we see that we will copy exactly j - i characters.
5.7 Testing our split function
Having written our function, we'd like to test it. The easiest way to do so is to write a
program that reads a line of input and passes that line to the split function. We can
then write the contents of the vector that split returns. Such a test program will
make it easy to inspect the output, and to verify that the split function generates the
words that we expect.
More usefully, this test function should produce the same results as a program that just
reads words from the standard input and writes the words one per output line. We can
write this latter program, run it and our test program on the same input files, and verify
that our programs generate identical output. If so, we can be fairly confident in our
split function.
Let's start by writing the test program for split:
int main() {
string s;
// read and split each line of input
while (getline(cin, s)) {
vector v = split(s);
// write each word in v
for (vector::size_type i = 0; i != v.size(); ++i)
cout << v[i] << endl;
}
return 0;
}
This program needs to read the input an entire line at a time. Fortunately, the string
library provides what we need in the getline function, which reads input until it
reaches the end of the line. The getline function takes two arguments. The first is the
istream from which to read; the second is a reference to the string into which to
store what is read. As usual, the getline function returns a reference to the istream
from which we read, so that we can test that istream in a condition. If we hit end-offile
or encounter invalid input, then the return from getline will indicate failure and
we'll break out of the while.
As long as we can read a line of input, we store that line in s and pass it to split,
storing the return value from split in v. Next, we loop through v, writing each
string in that vector on a separate line.
Assuming that we added the proper #includes, including one for our own header that
contained a declaration for split, we could run this function and visually verify that it
and split work as expected. We can do even better, though, by comparing the output
of this program with a program that lets the library do all the work:
int main()
{
string s;
while (cin >> s)
cout << s << endl;
return 0;
}
This program and the previous one should generate identical output. Here, we let the
string input operator separate the input stream into a series of words, which we write
one to a line. By running both programs on the same, complex input, we can have a
good idea that our split function works.
5.8 Putting strings together
In §1.2/12 and §2.5.4/29, we wrote a program to write someone's name centered in a
box of asterisks. However, we never actually created a string to hold our program's
output. Instead, we wrote the various parts of our output, one at a time, and let the
output file combine those fragments into a picture.
We will now revisit this problem, with the aim of building a single data structure that
represents the entire framed string. This program is a simplified version of one of our
favorite examples, called character pictures. A character picture is a rectangular array of
characters that can be displayed. It is a simplification of what happens in a real
application—in this case, applications based on bitmap graphics. The simplifications are
to use characters instead of bits, and to write onto ordinary files instead of displaying on
graphical hardware. The problem builds on an exercise originally presented in the first
edition of Stroustrup's The C++ Programming Language (Addison-Wesley, 1986), and
that we explored in some depth in Ruminations on C++ (Addison-Wesley, 1997).
5.8.1 Framing a picture
The particular variation of the character-picture problem that we'd like to explore in this
section writes all the words stored in a vector, one to a line, and surrounds
these strings with a border. We'll line the strings up along the left-hand border, and
leave a single space between the edge of asterisks and the words we are writing.
Assume that p is a vector that contains the strings "this is an", "
example", "to", "illustrate", and "framing". Then we would like to have a
function named frame, which behaves in such a way that calling frame(p) yields a
value of type vector with elements that, when written, are
**************
* this is an *
* example *
* to *
* illustrate *
* framing *
**************
Note that the border is rectangular, not ragged, even though the strings themselves
are of different lengths. This fact implies that we'll need a function to find the length of
the longest string in the vector. Let's start there:
string::size_type width(const vector& v)
{
string::size_type maxlen = 0;
for (vector::size_type i = 0; i != v.size(); ++i)
maxlen = max(maxlen, v[i].size());
return maxlen;
}
This function will iterate through the vector, setting maxlen to the largest size that
we've seen so far. When we fall out of the loop, maxlen will hold the length of the
longest string in v.
The only tricky aspect of the frame function is its interface. We know that it will operate
on a vector, but what about the return type? It will be convenient if the
function creates a new picture rather than change the picture it was given:
vector frame(const vector& v) {
vector ret;
string::size_type maxlen = width(v);
string border(maxlen + 4, '*');
// write the top border
ret.push_back(border);
// write each interior row, bordered by an asterisk and a space
for (vector::size_type i = 0; i != v.size(); ++i) {
ret.push_back( "* " + v[i] +
string(maxlen - v[i].size(), ' ') + ." *")
}
// write the bottom border
ret.push_back(border);
return ret;
}
We said that the function will not change the picture that it is passed, so we declare the
parameter as a reference to const. The function will return a vector, which
we'll build in ret. We begin by figuring out how long each output string will be; then
we create a string with that many asterisks, which we'll use to create the top and
bottom border.
These borders are four characters longer than the longest string: one each for the
right- and left-hand borders, and another two for the spaces that separate the borders
from the strings. Taking a syntactic cue from the definition of spaces in §1.2/12 that
we explained in §1.2/13, we define border to be a string that contains maxlen + 4
asterisks. We call push__back to append a copy of border to ret, thereby forming the
top border.
Next, we copy the picture that we are framing. We define the index i, which we will use
to walk through v until we've copied each element. In the call to push_back, we use
the + operator from string, which, as we learned in §1.2/13, concatenates its
arguments.
To form the output line, we concatenate the right- and left-hand borders with the
string that we want to display, which is stored in v[i]. The third string in our
concatenation, string(maxlen - v[i].size(), ' '), constructs an unnamed,
temporary string that holds the right number of blanks. We construct this string in
the same way that we initialized border. We obtain the number of blanks by subtracting
the size of the current string from maxlen.
With this knowledge, we can see that the argument to push_back is a new string
that consists of an asterisk, followed by a space, followed by the current string,
followed by enough spaces to make the string as long as the longest string,
followed by another space and another asterisk.
All that's left is to append the bottom border and return.
5.8.2 Vertical concatenation
What makes character pictures a fun example is that, once we have them, we can do
things with them. We just saw one operation—framing a picture. Another operation is
concatenation, which we can do both vertically and horizontally. We'll look at vertical
concatenation here, and at horizontal concatenation in the next section.
Pictures are naturally organized by rows, in the sense that we represent a picture by a
vector, each element of which is a row. Therefore, concatenating two
pictures vertically is simple: We merely concatenate the vectors that represent them.
Doing so will cause the two pictures to line up along their left margins, which is a
reasonable way to define vertical concatenation.
The only problem is that although there is a string concatenation operation, there is
no vector concatenation operation. As a result, we have to do the work ourselves:
vector vcat(const vector& top,
const vector& bottom)
{
// copy the top picture
vector ret = top;
// copy entire bottom picture
for (vector::const_iterator it = bottom.begin();
it != bottom.end(); ++it)
ret.push_back(*it);
return ret;
}
This function uses only facilities that we have already seen: We define ret as a copy of
top, append each element of bottom to ret, and return ret as its result.
The loop in this function implements one form of a common idea, namely, that of
inserting a copy of elements from one container into another. In this particular case, we
are appending the elements, which we can think of as inserting them at the end.
Because this operation is so common, the library offers a way of doing it without writing
a loop. Instead of
for (vector::const_iterator it = bottom.begin();
it != bottom.end(); ++it)
ret.push_back(*it);
we can write
ret.insert(ret.end(), bottom.begin(), bottom.end());
with the same effect.
5.8.3 Horizontal concatenation
By horizontal concatenation, we mean taking two pictures, and making a new picture in
which one of the input pictures forms the left part of the new picture, and the other
forms the right part. Before we start, we need to think about what we want to do when
the pictures to concatenate are different sizes. We'll arbitrarily decide that we'll align
them along their top edges. Thus, each row of the output picture will be the result of
concatenating the corresponding rows of the two input pictures. We'll have to pad the
left-hand picture's rows to make them take up the right amount of space in the output
picture.
In addition to padding the left-hand picture, we also have to worry about what to do
when the pictures have a different number of rows. For example, if p holds our initial
picture, we might want to concatenate the original value of p horizontally with the result
of framing p. That is, we'd like hcat(p, frame(p)) to produce
this is an **************
example * this is an *
to * to *
illustrate * illustrate *
framing * framing *
**************
Note that the left-hand picture has fewer rows than the right-hand picture. This fact
implies that we will have to pad the output on the left-hand side to account for these
missing rows. If the left-hand picture is longer, we'll just copy the strings from it into
the new picture; we won't bother to pad the (empty) right side with blanks. With this
analysis complete, we can write our function:
vector
hcat(const vector& left, const vector& right)
{
vector ret;
// add 1 to leave a space between pictures
string::size_type width1 = width(left) + 1;
// indices to look at elements from left and right respectively
vector::size_type i = 0, j = 0;
// continue until we've seen all rows from both pictures
while (i != left.size() || j != right.size()) {
// construct new string to hold characters from both pictures
string s;
// copy a row from the left-hand side, if there is one
if (i != left.size())
s = left[i++];
// pad to full width
s += string(width1 - s.size(), ' ');
// copy a row from the right-hand side, if there is one
if (j != right.size())
s += right[j++];
// add s to the picture we're creating
ret.push_back(s);
}
return ret;
}
We start, as we did for frame and vcat, by defining the picture that we'll return. Our
next step is to compute the width to which we must pad the left-hand picture. That
width will be one more than the width of the picture itself, to leave a space between the
pictures when we concatenate them. Next, we iterate through both pictures, copying an
element from the first, padded as necessary, followed by an element from the second.
The only tricky part is taking care of what to do if we run out of elements in one picture
before we run out of elements in the other. Our iteration continues until we have copied
all the elements for each input vector. Hence, the while loop continues until both
indices reach the end of their respective pictures.
If we have not yet exhausted left, we copy its current element into s. Regardless of
whether we copied anything from left, we next call the string compound assignment
operator, +=, to pad the output to the appropriate width. The compound assignment
operator defined by the string library operates as you might expect: It adds the righthand
operand to its left-hand operand and stores the result in the left-hand side. Of
course, "add" here means string concatenation.
We determine how much to pad by subtracting s.size() from width1. We know that
either s.size() is the size of the string that we copied from left, or it is zero
because there was no entry to copy. In the first case, s.size() will be greater than
zero and less than width1, because we added one to the length of the longest string
to account for a space between the two pictures. Thus, in this case, we'll append one or
more blanks to s. If s.size() is zero, then we'll pad the entire output line.
Having copied and padded the string for the left-hand picture, we need only append
the string from the right-hand picture, assuming that there still is an element from
right to copy. Regardless of whether we added a value from right, we push s onto
the output vector, and continue until we've processed both input vectors—
remembering to return to our caller the picture that we've created.
It is important to note that the correct behavior of our program depends on the fact that
s is local to the while loop. Because s is declared inside the while, it is created, with
a null value, and destroyed on each trip through the loop.
5.9 Details
Containers and iterators: The standard library is designed so that similar operations
on different containers have the same interface and the same semantics. The containers
we have used so far are all sequential containers. We'll see in Chapter 7 that the library
also provides associative containers. All the sequential containers and the string type
provide the following operations:
container::iterator
container::const_iterator
The name of the type of the iterator on this container.
container::size_type
The name of the appropriate type to hold the size of the largest possible instance of
this container.
c.begin()
c.end()
Iterators referring to the first and (one past) the last element in the container.
c.rbegin()
c.rend()
Iterators referring to the last and (one beyond) the first element in the container that
grant access to the container's elements in reverse order.
container c;
container c(c2);
Defines c as a container that is empty or a copy of c2 if given.
container c(n);
Defines c as a container with n elements that are value-initialized (§7.2/125)
according to the type of T. If T is a class type, that type will control how to initialize
the elements. If T is a built-in arithmetic type, then the elements will be initialized to
0.
container c(n, t);
Defines c as a container with n elements that are copies of t.
container c(b, e);
Creates a container that holds a copy of the elements denoted by iterators in the range
[b, e).
c = c2
Replaces the contents of container c with a copy of the container c2.
c.size()
Returns the number of elements in c as a size_type.
c.empty()
Predicate that indicates whether c has no elements.
c.insert(d, b, e)
Copies elements denoted by iterators in the range [b, e) and inserts them into c
immediately before d.
c.erase(it)
c.erase(b, e)
Removes the element denoted by it or the range of elements denoted by [b, e)
from the container c. This operation is fast for list but can be slow for vector and
string, because for these types it involves copying all the elements after the one that
is removed. For list, iterators to the element(s) that are erased are invalidated. For
vector and string, all iterators to elements after the one erased are invalidated.
c.push_back(t)
Adds an element to the end of c with the value t.
Containers that support random access, and the string type, also provide the
following:
c[n]
Fetches the character at position n from the container c.
Iterator operations:
*it
Dereferences the iterator it to obtain the value stored in the container at the position
that it denotes. This operation is often combined with . to obtain a member of a class
object, as in (*it).x, which yields the member x of the object denoted by the
iterator it. * has lower precedence than . and the same precedence as ++ and —.
it->x
Equivalent to (*it).x, which returns the member x denoted by the object obtained
by dereferencing the iterator it. Same precedence as the . operator.
++i
it++
Increments the iterator so that it denotes the next element in the container.
b == e
b != e
Compares two iterators for equality or inequality.
The string type offers iterators that support the same operations as do iterators on
vectors. In particular, string supports full random access, about which we'll learn
more in Chapter 8. In addition to the operations on containers, string also provides:
s.substr(i, j)
Creates a new string that holds a copy of the characters in s with indices in the
range [i, i + j).
getline(is, s)
Reads a line of input from is and stores it in s.
s += s2
Replaces the value of s by s + s2.
The vector type offers the most powerful iterators, called random-access iterators, of
any of the library containers. We'll learn more about these in Chapter 8.
Although all the functions we've written have relied on dynamically allocating our
vector elements, there are also mechanisms for preallocating elements, and an
operation to direct the vector to allocate, but not to use, additional memory in order to
avoid the overhead of repeated memory allocations.
v.reserve(n)
Reserves space to hold n elements, but does not initialize them. This operation does
not change the size of the container. It affects only the frequency with which vector
may have to allocate memory in response to repeated calls to insert or push_back.
v.resize(n)
Gives v a new size equal to n. If n is smaller than the current size of v, elements
beyond n are removed from the vector. If n is greater than the current size, then
new elements are added to v and initialized as appropriate to the type in v.
The list type is optimized for efficiently inserting and deleting elements at any point
in the container. The operations on lists and list iterators include those described in
§5.9/96. In addition,
l.sort()
l.sort(cmp)
Sorts the elements in l using the < operator for the type in the list, or the predicate
cmp.
The header provides useful functions for manipulating character data:
isspace(c) true if c is a whitespace character.
isalpha(c) true if c is an alphabetic character.
isdigit(c) true if c is a digit character.
isalnum(c) true if c is a letter or a digit.
ispunct(c) true if c is a punctuation character.
isupper(c) true if c is an uppercase letter.
islower(c) true if c is a lowercase letter.
toupper(c) Yields the uppercase equivalent to c
tolower(c) Yields the lowercase equivalent to c
Exercises
5-0. Compile, execute, and test the programs in this chapter.
5-1. Design and implement a program to produce a permuted index. A permuted index
is one in which each phrase is indexed by every word in the phrase. So, given the
following input,
The quick brown fox
jumped over the fence
the output would be
The quick brown fox
jumped over the fence
The quick brown fox
jumped over the fence
jumped over the fence
The quick brown fox
jumped over the fence
The quick brown fox
A good algorithm is suggested in The AWK Programming Language by Aho, Kernighan,
and Weinberger (Addison-Wesley, 1988). That solution divides the problem into three
steps:
Read each line of the input and generate a set of rotations of that line. Each
rotation puts the next word of the input in the first position and rotates the
previous first word to the end of the phrase. So the output of this phase for the first
line of our input would be
The quick brown fox
quick brown fox The
brown fox The quick
fox The quick brown
Of course, it will be important to know where the original phrase ends and where
the rotated beginning begins.
1.
2. Sort the rotations.
Unrotate and write the permuted index, which involves finding the separator,
putting the phrase back together, and writing it properly formatted.
3.
5-2. Write the complete new version of the student-grading program, which extracts
records for failing students, using vectors. Write another that uses lists. Measure
the performance difference on input files of ten lines, 1,000 lines, and 10,000 lines.
5-3. By using a typedef, we can write one version of the program that implements
either a vector-based solution or a list-based one. Write and test this version of the
program.
5-4. Look again at the driver functions you wrote in the previous exercise. Note that it is
possible to write a driver that differs only in the declaration of the type for the data
structure that holds the input file. If your vector and list test drivers differ in any
other way, rewrite them so that they differ only in this declaration.
5-5. Write a function named center(const vector&) that returns a
picture in which all the lines of the original picture are padded out to their full width, and
the padding is as evenly divided as possible between the left and right sides of the
picture. What are the properties of pictures for which such a function is useful? How can
you tell whether a given picture has those properties?
5-6. Rewrite the extract_fails function from §5.1.1/77 so that instead of erasing
each failing student from the input vector v, it copies the records for the passing
students to the beginning of v, and then uses the resize function to remove the extra
elements from the end of v. How does the performance of this version compare with the
one in §5.1.1/77?
5-7. Given the implementation of frame in §5.8.1/93, and the following code fragment
vector v;
frame(v);
describe what happens in this call. In particular, trace through how both the width
function and the frame function operate. Now, run this code. If the results differ from
your expectations, first understand why your expectations and the program differ, and
then change one to match the other.
5-8. In the hcat function from §5.8.3/95, what would happen if we defined s outside
the scope of the while? Rewrite and execute the program to confirm your hypothesis.
5-9. Write a program to write the lowercase words in the input followed by the
uppercase words.
5-10. Palindromes are words that are spelled the same right to left as left to right. Write
a program to find all the palindromes in a dictionary. Next, find the longest palindrome.
5-11. In text processing it is sometimes useful to know whether a word has any
ascenders or descenders. Ascenders are the parts of lowercase letters that extend above
the text line; in the English alphabet, the letters b, d, f, h, k, l, and t have ascenders.
Similarly, the descenders are the parts of lowercase letters that descend below the line;
In English, the letters g, j, p, q, and y have descenders. Write a program to determine
whether a word has any ascenders or descenders. Extend that program to find the
longest word in the dictionary that has neither ascenders nor descenders.
6
Using library algorithms
As we saw in Chapter 5, many container operations apply to more than one type of
container. For example, vector, string , and list allow us to insert elements by
calling insert and remove elements by calling erase . These operations have the
same interface for each type that supports them. For that matter, many container
operations also apply to the string class.
Every container—as well as the string class—provides companion iterator types, which
let us navigate through a container and examine its elements. Again, the library ensures
that every iterator that supplies an operation does so through the same interface. For
example, we can use the ++ operator to advance any type of iterator from one element
to the next; we can use the * operator to access the element associated with any type
of iterator; and so on.
In this chapter, we'll see how the library exploits these common interfaces to provide a
collection of standard algorithms. By using these algorithms, we can avoid writing (and
rewriting) the same code over and over again. More important, we can write programs
that are smaller and simpler than we would write otherwise—sometimes astonishingly
so.
Like containers and iterators, algorithms also use consistent interface conventions. This
consistency lets us learn a few of the algorithms and then apply that knowledge to
others as the need arises. In this chapter, we'll use several of the library algorithms to
solve problems related to processing string s and student grades. Along the way, we'll
cover most of the core concepts in the algorithm library.
Unless we say otherwise, the header defines all the algorithms that we
introduce in this chapter.
6.1 Analyzing strings
In §5.8.2/94, we used a loop to concatenate two character pictures:
for (vector::const_iterator it = bottom.begin();
it != bottom.end(); ++it)
ret.push_back(*it);
We noted that this loop was equivalent to inserting a copy of the elements of bottom at
the end of ret , an operation that vectors provided directly:
ret.insert(ret.end(), bottom.begin(), bottom.end());
This problem has an even more general solution: We can separate the notion of copying
elements from that of inserting elements at the end of a container, as follows:
copy(bottom.begin(), bottom.end(), back_inserter(ret));
Here, copy is an example of a generic algorithm, and back_inserter is an example of
an iterator adaptor.
A generic algorithm is an algorithm that is not part of any particular kind of container,
but instead takes a cue from its arguments' types about how to access the data it uses.
The standard library's generic algorithms usually take iterators among their arguments,
which they use to manipulate the elements of the underlying containers. So, for
example, the copy algorithm takes three iterators, which we'll call begin, end , and
out , and copies all the elements in the range [begin, end) to a sequence of
elements starting at out and extending as far as necessary. In other words,
copy(begin, end, out);
has the same effect as
while (begin != end)
*out++ = *begin++;
except that the while body changes the values of the iterators, and copy doesn't.
Before we describe iterator adaptors, we should note that this loop depends on the use
of the postfix version of the increment operators. These operators differ from the prefix
versions, which we have used up to now, in that begin++ returns a copy of the original
value of begin , incrementing the stored value of begin as a side effect. In other
words,
it = begin++;
is equivalent to
it = begin;
++begin;
The increment operators have the same precedence as * , and they are both rightassociative,
which means that *out++ has the same meaning as *(out++) . Thus,
*out++ = *begin++;
is equivalent to the more verbose
{ *out = *begin; ++out; ++begin; }
Let's return to iterator adaptors , which are functions that yield iterators with
properties that are related to their arguments in useful ways. The iterator adaptors are
defined in . The most common iterator adaptor is back_inserter , which
takes a container as its argument and yields an iterator that, when used as a
destination, appends values to the container. For example, back_inserter(ret) is an
iterator that, when used as a destination, appends elements to ret . Therefore,
copy(bottom.begin(), bottom.end(), back_inserter(ret));
copies all of the elements of bottom and appends them to the end of ret . After this
function completes, the size of ret will have increased by bottom.size() . Notice
that we could not call
// error—ret is not an iterator
copy(bottom.begin(), bottom.end(), ret);
because copy's third argument is an iterator, not a container. Nor could we call
// error - no element at ret.end()
copy(bottom.begin(), bottom.end(), ret.end());
This latter mistake is particularly insidious, because the program will compile. What it
does when you try to run it is another story entirely. The first thing copy will try to do is
assign a value to the element at ret.end() . There's no element there, so what the
implementation will do is anybody's guess.
Why is copy designed this way? Because separating the notions of copying elements
and expanding a container allows programmers to choose which operations to use. For
example, we might want to copy elements on top of elements that already exist in a
container, without changing the container's size. As another example, which we shall
see in §6.2.2/112, we might want to use back_inserter to append elements to a
container that are not merely copies of another container's elements.
6.1.1 Another way to split
Another function that we can write more directly using the standard algorithms is split
, which we saw in §5.6/88. The hard part of writing that function was dealing with the
indices that delimited each word in the input line. We can replace the indices by
iterators, and use standard-library algorithms to do much of the work for us:
// true if the argument is whitespace, false otherwise
bool space(char c)
{
return isspace(c);
}
// false if the argument is whitespace, true otherwise
bool not_space(char c)
{
return !isspace(c);
}
vector split(const string& str)
{
typedef string::const_iterator iter;
vector ret;
iter i = str.begin();
while (i != str.end()) {
// ignore leading blanks
i = find_if(i, str.end(), not_space);
// find end of next word
iter j = find_if(i, str.end(), space);
// copy the characters in [i, j)
if (i != str.end())
ret.push_back(string(i, j));
i = j;
}
return ret;
}
This code uses a lot of new functions, so it will take a bit of explanation. The key idea to
keep in mind is that it implements the same algorithm as the original, using i and j to
delimit each word in str . Once we've found a word, we copy it from str , and push the
copy onto the back of ret .
This time, i and j are iterators, not indices. We use typedef to abbreviate the iterator
type, so that we can use iter instead of the longer string::const_iterator .
Although the string type does not support all of the container operations, it does
support iterators. Therefore, we can use the standard-library algorithms on the
characters of a string , just as we can use them on the elements of a vector .
The algorithm that we use in this example is find_if . Its first two arguments are
iterators that denote a sequence; the third is a predicate, which tests its argument and
returns true or false . The find_if function calls the predicate on each element in
the sequence, stopping when it finds an element for which the predicate yields true .
The standard library provides an isspace function to test whether a character is a
space. However, that function is overloaded, so that it will work with languages, such as
Japanese, that use other character types, such as wchar_t (§1.3/14). It's not easy to
pass an overloaded function directly as an argument to a template function. The trouble
is that the compiler doesn't know which version of the overloaded function we mean,
because we haven't supplied any arguments that the compiler might use to select a
version. Accordingly, we'll write our own predicates, called space and not_space , that
make clear which version of isspace we intend.
The first call to find_if seeks the first nonspace character, which begins a word.
Remember that one or more spaces might begin a line or might separate adjacent words
in the input. We don't want to include these spaces in the output.
After the first call to find_if , i will denote the first nonspace, if any, in str . We use
i in the next call to find_if , which looks for the first space in [i, str.end()) . If
find_if fails to find a value that satisfies the predicate, it returns its second argument,
which, in this case, is str.end() . Therefore, j will be initialized to denote the blank
that separates the next word in str from the rest of the line, or, if we are on the last
word in the line, j will be equal to str.end() .
At this point, i and j delimit a word in str . All that's left is to use these iterators to
copy the data from str into ret . In the earlier version of split , we used
string::substr to create the copy. However, that version of split operated on
indices, not iterators, and there isn't a version of substr that operates on iterators.
Instead, we construct a new string directly from the iterators that we have. We do so by
using an expression, string(i, j) , that is somewhat similar to the definition of
spaces that we explained in §1.2/13. Our present example constructs a string that is
a copy of the characters in the range [i, j) . We push this new string onto the back
of ret .
It is worth pointing out that this version of the program omits the tests of the index i
against str.size() . Nor are there the obvious equivalent tests of the iterator against
str.end() . The reason is that the library algorithms are written to handle gracefully
calls that pass an empty range. For example, at some point the first call to find_if will
set i to the value returned by str.end() , but there is no need to check i before
passing it to the second call to find_if . The reason is that find_if will look in the
empty range [i, str.end()) and will return str.end() to indicate that there is no
match.
6.1.2 Palindromes
Another character-manipulation problem that we can use the library to solve succinctly
is determining whether a word is a palindrome. Palindromes are words that are spelled
the same way front to back as back to front. For example, "civic," "eye," "level,"
"madam," and "rotor" are all palindromes.
Here is a compact solution that uses the library:
bool is_palindrome(const string& s)
{
return equal(s.begin(), s.end(), s.rbegin());
}
The return statement in this function's body calls the equal function and the rbegin
member function, both of which we have not yet seen.
Like begin , rbegin returns an iterator, but this time it is an iterator that starts with
the last element in the container and marches backward through the container.
The equal function compares two sequences to determine whether they contain equal
values. As usual, the first two iterators passed to equal specify the first sequence. The
third argument is the starting point for the second sequence. The equal function
assumes that the second sequence is the same size as the first, so it does not need an
ending iterator. Because we pass s.rbegin() as the starting point for the second
sequence, the effect of this call is to compare values from the back of s to values in the
front. The equal function will compare the first character in s with the last. Then it will
compare the second to the next to last, and so on. This behavior is precisely what we
want.
6.1.3 Finding URLs
As the last of our examples of character manipulation, let's write a function that finds
Web addresses, called uniform resource locators (URLs), that are embedded in a
string . We might use such a function by creating a single string that holds the
entire contents of a document. The function would then scan the document and find all
the URLs in it. A URL is a sequence of characters of the form
protocol-name: //resource-name
where protocol-name contains only letters, and resource-name may consist of letters,
digits, and certain punctuation characters. Our function will take a string argument
and will look for instances of :// in that string . Each time we find such an instance,
we'll look for the protocol-name that precedes it, and the resource-name that follows it.
Because we want our function to find all the URLs in its input, we'll want it to return a
vector , with one element for each URL. The function executes by moving
the iterator b through the string , looking for the characters :// that might be a part
of a URL. If we find these characters, it looks backward to find the protocol-name, and it
looks forward to find the resource-name:
vector find_urls(const string& s)
{
vector ret;
typedef string::const_iterator iter;
iter b = s.begin(), e = s.end();
// look through the entire input
while (b != e) {
// look for one or more letters followed by ://
b = url_beg(b, e);
// if we found it
if (b != e) {
// get the rest of the URL
iter after = url_end(b, e);
// remember the URL
ret.push_back(string(b, after));
// advance b and check for more URLs on this line
b = after;
}
}
return ret;
}
We start by declaring ret , which is the vector into which we will put the URLs as we
find them, and by obtaining iterators that delimit the string . We will have to write the
url_beg and url_end functions, which will find the beginning and end of any URL in
the input. The url_beg function will be responsible for identifying whether a valid URL
is present and, if so, for returning an iterator that refers to the first character of the
protocol-name. If it does not identify a URL in the input, then it will return its second
argument (e in this case) to indicate failure.
If url_beg finds a URL, the next task is to find the end of the URL by calling url_end .
That function will search from the given position until it reaches either the end of the
input or a character that cannot be part of a URL. It will return an iterator positioned
one past the last character in the URL.
Thus, after the calls to url_beg and url_end , the iterator b denotes the beginning of
a URL, and the iterator after denotes the position one past the last character in the URL:
We construct a new string from the characters in this range, and push that string
onto the back of ret .
All that remains is to increment the value of b and to look for the next URL. Because
URLs cannot overlap one another, we set b to (one past) the end of the URL that we just
found, and continue the while loop until we've looked at all the input. Once that loop
exits, we return the vector that contains the URLs to our caller.
Now we have to think about url_beg and url_end . The url_end function is simpler,
so we'll start there:
string::const_iterator
url_end(string::const_iterator b, string::const_iterator e)
{
return find_if(b, e, not_url_char);
}
This function just forwards its work to the library find_if function, which we used in
§6.1.1/103. The predicate that we pass to find_if is one that we will write, named
not_url_char . It will return true when passed a character that cannot be in a URL:
bool not_url_char(char c)
{
// characters, in addition to alphanumerics, that can appear in a URL
static const string url_ch = "~;/?:@=&$-_.+!*'(),";
// see whether c can appear in a URL and return the negative
return !(isalnum(c) ||
find(url_ch.begin(), url_ch.end(), c) != url_ch.end());
}
Despite being small, this function uses a fair bit of new material. First is the use of the
static storage class specifier . Local variables that are declared to be static are
preserved across invocations of the function. Thus, we will create and initialize the
string url_ch only on the first call to not_url_char . Subsequent calls will use the
object that the first call created. Because url_ch is a const string , its value will
not change once we have initialized it.
The not_url_char function also uses the isalnum function, which the
header defines. This function tests whether its argument is an alphanumeric character
(a letter or a digit).
Finally, find is another algorithm that we haven't used yet. It is similar to find_if ,
except that instead of calling a predicate, it looks for the specific value given as its third
argument. As with find_if , if the value that we want is present, the function returns
an iterator denoting the first occurrence of the value in the given sequence. If the value
is not found, then find returns its second argument.
With this information in hand, we can now understand the not_url_char function.
Because we negate the value of the entire expression before we return it,
not_url_char will yield false if c is a letter, a digit, or any of the characters in
url_ch . If c is any other value, the function returns true .
Now the hard part begins: implementing url_beg . This function is messy, because it
must deal with the possibility that the input might contain :// in a context that cannot
be a valid URL. In practice, we'd probably have a list of acceptable protocol-names and
look only for those. For simplicity, though, we'll limit ourselves to being sure that one or
more letters precede the :// separator, and at least one character follows it:
string::const_iterator
url_beg(string::const_iterator b, string::const_iterator e)
{
static const string sep = "://";
typedef string::const_iterator iter;
// i marks where the separator was found
iter i = b;
while ((i = search(i, e, sep.begin(), sep.end())) != e) {
// make sure the separator isn't at the beginning or end of the line
if (i != b && i + sep.size() != e) {
// beg marks the beginning of the protocol-name
iter beg = i;
while (beg != b && isalpha(beg[-1]))
--beg;
// is there at least one appropriate character before and after the separator?
if (beg != i && !not_url_char(i[sep.size()]))
return beg;
}
// the separator we found wasn't part of a URL advance i past this separator
i += sep.size();
}
return e;
}
The easy part is to write the function header. We know that we'll be passed two iterators
denoting the range in which to look, and that we'll return an iterator that denotes the
beginning of the first URL in that range, if one exists. We also declare and initialize a
local string , which will hold the characters that make up the separator that identifies
a potential URL. Like url_ch in the not_url_char function (§6.1.3/107), this string
is static and const . Thus, we will not be able to change the string , and its value
will be created only on the first invocation of url_beg .
The function executes by placing two iterators into the string delimited by b and e :
The iterator i will denote the beginning of the URL separator, if any, and beg will
indicate the beginning of the protocol-name , if any.
The function first looks for the separator, by calling search , a library function that we
haven't used before. This function takes two pairs of iterators: The first pair denotes the
sequence in which we are looking, and the second pair denotes the sequence that we
wish to locate. As with other library functions, if search fails, it returns the second
iterator. Therefore, after the call to search , either i denotes (one past) the end of the
input string , or it denotes a : that is followed by //.
If we found a separator, the next task is to get the letters (if any) that make up the
protocol-name. We first check whether the separator is at the beginning or end of the
input. If the separator is in either of those places, we know that we don't have a URL,
because a URL has at least one character on each side of its separator. Otherwise, we
need to try to position the iterator beg . The inner while loop moves beg backward
through the input until it hits either a nonalphabetic character or the beginning of the
string . It uses two new ideas: The first is the notion that if a container supports
indexing, so do its iterators. In other words, beg[-1] is the character at the position
immediately before the one that beg denotes. We can think of beg[-l] as an
abbreviation for *(beg - 1) . We'll learn more about such iterators in §8.2.6/148. The
second new idea is the isalpha function, defined in , which tests whether
its argument is a letter.
If we were able to advance the iterator over as much as a single character, we assume
that we've found a protocol-name. Before returning beg , we still have to check that
there's at least one valid character following the separator. This test is more
complicated. We know that there is at least one more character in the input, because
we're inside the body of an if that compares the value of i + sep.size() with e .
We can access the first such character as i[sep.size()] , which is an abbreviation
for *(i + sep.size()) . We test whether that character can appear in a URL by
passing the character to not_url_char . This function returns true if the character is
not valid, so we negate the return to check whether the character is valid.
If the separator is not part of a URL, then the function advances i past the separator
and keeps looking.
This code uses the decrement operator , which we mentioned in the operator table in
§2.7/32, but which we have not previously used. It works like the increment operator,
but it decrements its operand instead. As with the increment operator, it comes in prefix
and postfix versions. The prefix version, which we use here, decrements its operand and
returns the new value.
6.2 Comparing grading schemes
In §4.2/61, we presented a grading scheme that based students' final grades, in part,
on their median homework scores. Devious students can exploit this scheme by
deliberately not turning in all their homework assignments. After all, the bottom half of
their homework grades has no effect on their final grade. If they've done enough
homework to ensure a good grade, why not stop doing homework altogether?
In our experience, most students do not exploit this particular loophole. However, we did
have occasion to teach one class that gleefully and openly did so. We wondered whether
the students who skipped homework had, on average, different final grades than those
who did all the homework. While we were thinking about how to answer that question,
we decided that it might be interesting to see what the answer would be if we used one
of two alternative grading schemes:
Using the average instead of the median, and treating those assignments that the
student failed to turn in as zero
Using the median of only the assignments that the student actually submitted
For each of these grading schemes, we wanted to compare the median grade of the
students who turned in all their homework with the median grade of the students who
missed one or more assignments. We wound up with a program that had to solve two
distinct subproblems:
Read all the student records, separating the students who did all the homework
from the others.
1.
Apply each of the grading schemes to all the students in each group, and report the
median grade of each group.
2.
6.2.1 Working with student records
Our first subproblem is to read and classify the student records. Fortunately, we already
have some code we can use in solving this part of the problem: We can use the
Student_info type from §4.2.1/61 and the associated read function from §4.2.2/62
to read the student data records. What we don't have yet is a function that checks
whether a student has done all the homework. Writing such a function is easy:
bool did_all_hw(const Student_info& s)
{
return ((find(s.homework.begin(), s.homework.end(), 0))
== s.homework.end());
}
This function looks in s.homework to see whether any of the values stored there is 0.
Because we give at least partial credit for any assignment that is turned in, a 0 grade
means that the assignment was not submitted. We compare the return from find with
homework.end(). As usual, find returns its second argument if it fails to find the value
that it seeks.
With these two functions, writing code to read and separate the student records is
simplicity itself. We'll read each student record, check whether the student did all the
homework, and append the record to one of two vector s, which, for want of a better
idea, we'll name did and didnt . While we're at it, we'll check that neither vector is
empty, so that we'll know that our analysis will actually tell us something useful:
vector did, didnt;
Student_info student;
// read all the records, separating them based on whether all homework was done
while (read(cin, student)) {
if (did_all_hw(student))
did.push_back(student);
else
didnt.push_back(student);
}
// check that both groups contain data
if (did.empty()) {
cout << "No student did all the homework!" << endl;
return 1;
}
if (didnt.empty()) {
cout << "Every student did all the homework!" << endl;
return 1;
}
The only new idea here is the empty member function, which yields true if the
container is empty and false otherwise. It is a better idea to use this function to check
for an empty container than it is to compare the size with 0, because for some kinds of
containers, it might be more efficient to check whether the container has any elements
than to figure out exactly how many elements there are.
6.2.2 Analyzing the grades
We now know how to read and classify student records into the did and didnt vectors.
The next step is to analyze them, which means we need to think a little about how to
structure the analysis.
We know that we have three analyses to perform, and each analysis has two parts,
which analyze separately the students who did and who didn't do all the homework.
Because we will do each analysis on two sets of data, we certainly want to make each
analysis its own function. However, there are some operations, such as reporting in a
common format, that we are going to want to do on pairs of analyses, rather than on
individual analyses. Evidently, we'll want to make writing the results of each pair of
analyses into a function as well.
The tricky part is that we want to call the function that writes the analysis results three
times, once for each kind of analysis. We want that function to call the appropriate
analysis function twice, once each for the did and didnt objects. However, we want
the function that generates the reports to call a different analysis function each time we
call it! How do we arrange that?
The easiest solution is to define three analysis functions and pass each one as an
argument to the reporting function. Remember that we've used such arguments already,
such as when we passed the compare function to the library sort routine in §4.2.2/64.
In this case, we want our output routine to take five arguments:
The stream on which to write the output
A string that represents the name of the analysis
The function to use for the analysis
Two arguments, each of which is one of the vector s that we want to analyze
For example, let's assume that the first analysis, which looks at the medians, is done by
a function called median_analysis . Then, we'd like to report the results for each
group of students by executing
write_analysis(cout, "median", median_analysis, did, didnt);
Before we define write_analysis , let's define median_analysis . We would like to
give that function a vector of student records, and we would like it to compute the
students' grades according to the normal grading scheme and to return the median of
those grades. We can define that function as follows:
// this function doesn't quite work
double median_analysis(const vector& students)
{
vector grades;
transform(students.begin(), students.end(),
back_inserter(grades), grade);
return median(grades);
}
Although this function might appear difficult at first glance, it introduces only one new
idea, namely the transform function. This function takes three iterators and a
function. The first two iterators specify a range of elements to transform; the third
iterator is the destination into which to put the result of running the function.
When we call transform , we are responsible for ensuring that the destination has
room for the values from the input sequence. In this case, there is no problem, because
we obtain the destination by calling back_inserter (§6.1/102), thereby arranging
that transform 's results will be appended to grades , which will automatically grow
as necessary to accommodate the results.
The fourth argument to transform is a function that transform applies to each
element of the input sequence to obtain the corresponding element of the output
sequence. Therefore, when we call transform in this example, the effect is to apply the
grade function to each element of students , and to append each grade to the
vector named grades . When we have all these students' grades, we call median ,
which we defined in §4.1.1/53, to compute their median.
There's only one problem: As the comment notes, this function doesn't quite work.
One reason that it doesn't work is that there are several overloaded versions of the
grade function. The compiler doesn't know which version to call, because we haven't
given grade any arguments. We know that we want to call the version from §4.2.2/63,
but we need a way to tell the compiler to do so.
The other reason is that the grade function will throw an exception if any student did
no homework at all, and the transform function does nothing about exceptions. If an
exception occurs, the transform function will be stopped at the point of the exception,
and control will return to median_analysis . Because median_analysis doesn't
handle the exception either, the exception will continue to propagate outward. The effect
will be that this function will also exit prematurely, passing control to its caller, and so
on, until control reaches an appropriate catch . If there is no such catch , as would be
likely in this case, the program itself is terminated, and the message that was thrown is
printed (or not, depending on the implementation).
We can solve both problems by writing an auxiliary function that will try the grade
function and handle the exception. Because we are calling the grade function explicitly,
rather than passing it as an argument, the compiler will be able to figure out which
version we mean:
double grade_aux(const Student_info& s)
{
try {
return grade(s);
} catch (domain_error) {
return grade(s.midterm, s.final, 0) ;
}
}
This function will call the version of grade from §4.2.2/63. If an exception occurs, we
will catch it and call the version of grade , from §4.1/52, that takes three double s
that represent the exam scores and overall homework grade. Thus, we'll assume that
students who did no homework at all got a 0 grade on their homework, but their exams
still count. Now, we can rewrite the analysis function to use grade_aux :
// this version works fine
double median_analysis(const vector& students)
{
vector grades;
transform(students.begin(), students.end(),
back_inserter(grades), grade_aux);
return median(grades);
}
Having seen what an analysis routine looks like, we are now in a position to define
write_analysis , which uses an analysis routine to compare two sets of students:
void write_analysis(ostream& out, const string& name,
double analysis(const vector&),
const vector& did,
const vector& didnt)
{
out << name << ": median(did) = " << analysis(did) <<
", median(didnt) = " << analysis(didnt) << endl;
}
Again, this function is surprisingly small, although it does introduce two new ideas. The
first is how to define a parameter that represents a function. The parameter definition
for analysis looks just like the function declaration that we wrote in §4.3/67.
(Actually, as we shall learn in §10.1.2/172, there is slightly more going on here than
meets the eye. The additional detail doesn't affect the current discussion directly.)
The other new idea is the return type, void. The built-in type void can be used only in
a few restricted ways, one of which is to name a return type. When we say a function
"returns" a void , we're really saying that it has no return value. We can exit from such
a function by executing a return statement with no value, such as
return;
or, as we do here, by falling off the end of the function. Ordinarily, we cannot just fall off
the end of a function, but the language allows functions that return void to do so. At
this point, we can write the rest of our program:
int main()
{
// students who did and didn't do all their homework
vector did, didnt;
// read the student records and partition them
Student_info student;
while (read(cin, student)) {
if (did_all_hw(student))
did.push_back(student);
else
didnt.push_back(student);
}
// verify that the analyses will show us something
if (did.empty()) {
cout << "No student did all the homework!" << endl;
return 1;
}
if (didnt.empty()) {
cout << "Every student did all the homework!" << endl;
return 1;
}
// do the analyses
write_analysis(cout, "median", median_analysis, did, didnt);
write_analysis(cout, "average", average_analysis, did, didnt);
write_analysis(cout, "median of homework turned in",
optimistic_median_analysis, did, didnt);
return 0;
}
All that remains is to write average_analysis and optimistic_median_analysis
.
6.2.3 Grading based on average homework grade
We would like the average_analysis function to compute the students' grades by
using the average homework grade, rather than the median. Therefore, the logical first
step is to write a function to compute the average of a vector , with the aim of using it
instead of median for grade computation:
double average(const vector& v)
{
return accumulate(v.begin(), v.end(), 0.0) / v.size();
}
This function uses accumulate , which, unlike the other library algorithms we've used,
is declared in . As this header's name implies, it offers tools for numeric
computation. The accumulate function adds the values in the range denoted by its first
two arguments, starting the summation with the value given by its third argument.
The type of the sum is the type of the third argument, so it is crucially important for us
to use 0.0 , as we did here, instead of 0. Otherwise, the result would be an int , and
any fractional part would be lost.
Having used accumulate to generate the sum of all the elements in the range, we
divide that sum by v.size() , which is the number of elements in the range. The
result of that division, of course, is the average, which we return to our caller.
Once we have the average function, we can use it to implement the average_grade
function to reflect this alternative grading policy:
double average_grade(const Student_info& s)
{
return grade(s.midterm, s.final, average(s.homework));
}
This function uses the average function to compute an overall homework grade, which
it then gives to the grade function from §4.1/52 to use in computing the final grade.
With this infrastructure in place, the average_analysis function is simplicity itself:
double average_analysis(const vector& students)
{
vector grades;
transform(students.begin(), students.end(),
back_inserter(grades), average_grade);
return median(grades);
}
The only difference between this function and median_analysis (§6.2.2/113) is its
name and its use of average_grade instead of grade_aux .
6.2.4 Median of the completed homework
The last analysis scheme, optimistic_median_analysis , gets its name from the
optimistic assumption that the students' grades on the homework that they didn't turn
in would have been the same as the homework that they did turn in. With that
assumption, we would like to compute the median of just the homework that each
student submitted. We'll call this computation an optimistic median, and we'll begin by
writing a function to compute it. Of course, we have to contend with the possibility that
a student did no homework at all, in which case we'll use 0 as the overall homework
grade:
// median of the nonzero elements of s.homework, or 0 if no such elements exist
double optimistic_median(const Student_info& s)
{
vector nonzero;
remove_copy(s.homework.begin(), s.homework.end(),
back_inserter(nonzero), 0);
if (nonzero.empty())
return grade(s.midterm, s.final, 0);
else
return grade(s.midterm, s.final, median(nonzero));
}
This function works by extracting the nonzero elements from the homework vector and
putting them into a new vector, called nonzero . Once we have the nonzero homework
grades, we call the version of grade defined in §4.1/52 to compute the final score
based on the median of the homework assignments that were actually submitted.
The only new idea in this function is how we get values into nonzero , which we do by
calling the remove_copy algorithm. To understand the call to remove_copy , you may
find it useful to know that the library provides "copying" versions of many of the
algorithms. So, for example, remove_copy does what remove does, but copies its
results to an indicated destination.
The remove function finds all values that match a given value and "removes" those
values from the container. All the values in the input sequence that are not "removed"
will be copied into the destination. We'll have more to say shortly about what "remove"
means in this context.
The remove_copy function takes three iterators and a value. As with most algorithms,
the first two iterators denote the input sequence. The third denotes the beginning of the
destination for the copy. As with copy , the remove_copy algorithm assumes that
there is enough space in the destination to hold all the elements that are copied. We call
back_inserter to grow nonzero as needed.
We should now be able to see that the effect of the remove_copy call is to copy into
nonzero all the nonzero elements in s.homework . We then check whether v is empty,
and if not, we do the normal grade calculation based on the median of the nonzero
grades. If v is empty, then we use 0 as the homework grade.
Of course, to complete our analysis, we need to write an analysis function to call our
optimistic_median function. We leave doing so as an exercise.
6.3 Classifying students, revisited
In Chapter 5, we looked at the problem of copying records with failing grades into a
separate vector and then removing those records from the existing vector. The
obvious, easy approach to this problem proved to have abysmal performance as the
input size grew. We went on to show how to solve the performance problem by using a
list instead of a vector, but we also promised to revisit the problem and show an
algorithmic solution that would perform similarly to the revised data structure.
We can use the algorithm library to demonstrate two other solutions. The first is slightly
slower because it uses a pair of library algorithms and visits every element twice. We
can do better by using a more specialized library algorithm that will let us solve the
problem in a single pass.
6.3.1 A two-pass solution
Our first approach will use a strategy similar to the one that we used in §6.2.4/115,
when we wanted only the nonzero homework grades. In that case, we didn't want to
change homework itself, so we used remove_copy to put copies of the nonzero
homework grades into a separate vector. In our current problem, we need both to
copy and remove the nonzero elements:
vector
extract_fails(vector& students) {
vector fail;
remove_copy_if(students.begin(), students.end(),
back_inserter(fail), pgrade);
students.erase(remove_if(students.begin()), students.end(),
fgrade), students.end());
return fail;
}
The interface to the program is identical to that from §5.3/82, which presented the
obvious vector-based solution that used iterators instead of indices. As in that
solution, we'll use the vector that we were passed to hold grades for students who
passed, and define fail to hold the failing grades. There the similarities end.
In the original program, we used an iterator named iter to march through the
container, copying the records with failing grades into fail, and using the erase
member to erase them from students. This time, we use the remove_copy_if
function to copy the failing grades into fail. That function operates as did the
remove_copy function that we used in §6.2.4/116, except that it uses a predicate as
its test, rather than a value. We give it a predicate that inverts the result of calling
fgrade (§5.1/75):
bool pgrade(const Student_info& s)
{
return !fgrade(s);
}
When we pass a predicate to remove_copy_if, we are asking it "remove" each
element that satisfies the predicate. In this context, "removing" an element means not
copying it, so we copy only those elements that do not satisfy the predicate. Therefore,
passing pgrade to remove_copy_if copies only the student records with failing
grades.
The next statement is somewhat complicated. First, we call remove_if to "remove" the
elements that correspond to failing grades. Again, the quotes around "remove" are
because nothing is actually removed. Instead, remove_if copies all the elements that
do not satisfy the predicate—in this case, all the student records with passing grades.
This call is tricky to understand because remove_if uses the same sequence as its
source and destination. What it really does is copy to the beginning of the sequence the
elements that don't meet the predicate. For example, suppose we started with seven
students with grades as follows:
Then the call to remove_if would leave the first two records untouched, because
they're already in the right places. It would "remove" the next two records by treating
them as free space to be overwritten by the next records that should be kept. So, when
it sees the fifth record, which represents a student who passed, it would copy that
record into the now free position that used to hold the first of the "removed" failing
records, and so on:
The result in this case would be to copy the four passing records to the beginning of the
sequence, leaving the remaining three records untouched. So that we can know how
much of the sequence is still relevant, remove_if returns an iterator that refers to one
past the last element that it did not "remove":
Next, we need to erase these unneeded records from students. We have not used
this version of erase before. It takes two iterators, and erases all the elements in the
range delimited by those iterators. If we erase the elements between the iterator
returned from the call to remove_if and students.end(), we are left with just the
passing records:
6.3.2 A single-pass solution
Our first algorithmic solution performs pretty well, but we should be able to do slightly
better. The reason is that the solution in §6.3.1/117 calculates the grade for every
element in students twice: once from remove_copy_if and a second time from
remove_if.
Although there is no library algorithm that does exactly what we want, there is one that
approaches our problem from a different angle: It takes a sequence and rearranges its
elements so that the ones that satisfy a predicate precede the ones that do not satisfy
it.
There are really two versions of this algorithm, which are named partition and
stable_partition. The difference is that partition might rearrange the elements
within each category, and stable_partition keeps them in the same order aside
from the partitioning. So, for example, if the student names were already in alphabetical
order, and we wanted to keep them that way within each category, we would need to
use stable_partition rather than partition.
Each of these algorithms returns an iterator that represents the first element of the
second section. Therefore, we can extract the failing grades this way:
vector
extract_fails(vector& students)
{
vector::iterator iter =
stable_partition(students.begin(), students.end(), pgrade);
vector fail(iter, students.end());
students.erase(iter, students.end());
return fail;
}
To understand what is going on here, let's start with our hypothetical input data again:
After calling stable_partition, we would have
We construct fail from a copy of the failing records, which are the ones in the range
[iter, students.end())., and then erase those elements from students.
When we ran our algorithm-based solutions, they had roughly the same overall
performance as the list-based solution. As expected, once the input was large enough,
the algorithm and list-based solutions were substantially better than the vector
solution that used erase. The two algorithmic solutions are good enough that the time
consumed by the input library dominated the timings for input files up to about 75,000
records. To compare the effects of the two strategies in extract_fails, we separately
analyzed the performance of just this portion of the program. Our timings confirmed
that the one-pass algorithm ran about twice as fast as the two-pass solution.
6.4 Algorithms, containers, and iterators
There is a fact that is crucial to understand in using algorithms, iterators, and
containers:
Algorithms act on container elements—they do not act on containers.
The sort, remove_if, and partition functions all move elements to new positions
in the underlying container, but they do not change the properties of the container itself.
For example, remove_if does not change the size of the container on which it
operates; it merely copies elements around within the container.
This distinction is especially important in understanding how algorithms interact with the
containers that they use for output. Let's look in more detail at our use of remove_if in
§6.3.1/117. As we've seen, the call
remove_if(students.begin(), students.end(), fgrade)
did not change the size of students. Rather, it copied each element for which the
predicate was false to the beginning of students, and left the rest of the elements
alone. When we need to shorten the vector to discard those elements, we must do so
ourselves. In our example, we said
students.erase(remove_if(students.begin(), students.end(), fgrade),
students.end());
Here, erase changes the vector by removing the sequence indicated by its
arguments. This call to erase shortens students so that it contains only the elements
we want. Note that erase must be a member of vector, because it acts directly on the
container, not just on its elements.
Similarly, it is important to be aware of the interaction between iterators and algorithms,
and between iterators and container operations. We've already seen, in §5.3/83 and
§5.5.1/86, that container operations such as erase and insert invalidate the iterator
for the element erased. More important, in the case of vectors and strings,
operations such as erase or insert also invalidate any iterator denoting elements
after the one erased or inserted. Because these operations can invalidate iterators,
we must be careful about saving iterator values if we are using these operations.
Similarly, functions such as partition or remove_if, which can move elements
around within the container, will change which element is denoted by particular
iterators. After running one of these functions, we cannot rely on an iterator continuing
to denote a specific element.

6.5 Details
Type modifiers:
static type variable;
For local declarations, declares variable with static storage class. The value of
variable persists across executions of this scope and is guaranteed to be initialized
before the variable is used for the first time. When the program exits from the scope,
the variable keeps its value until the next time the program enters that scope. We'll
see in §13.4/244 that the meaning of static varies with context.
Types: The built-in type void can be used in a restricted number of ways, one of which
is to indicate that a function yields no return value. Such functions can be exited
through a return; that has no value or by falling off the end of the function.
Iterator adaptors are functions that yield iterators. The most common are the adaptors
that generate insert_iterators, which are iterators that grow the associated
container dynamically. Such iterators can be used safely as the destination of a copying
algorithm. They are defined in header :
back_inserter(c)
Yields an iterator on the container c that appends elements to c. The container must
support push_back, which the list, vector, and the string types all do.
front_inserter(c)
Like back_inserter, but inserts at the front of the container. The container must
support push_front, which list does, but string and vector do not.
inserter(c, it)
Like back_inserter, but inserts elements before the iterator it.
Algorithms: Unless otherwise indicated, defines these algorithms:
accumulate(b, e, t)
Creates a local variable and initializes it to a copy of t (with the same type as t, which
means that the type of t is crucially important to the behavior of accumulate), adds
each element in the range [b, e) to the variable, and returns a copy of the variable
as its result. Defined in .
find(b, e, t)
find_if(b, e, p)
search(b, e, b2, e2)
Algorithms to look for a given value in the sequence [b, e). The find algorithm
looks for the value t; the find_if algorithm tests each element against the predicate
p; the search algorithm looks for the sequence denoted by [b2, e2).
copy(b, e, d)
remove_copy(b, e, d, t)
remove_copy_if(b, e, d, p)
Algorithms to copy the sequence from [b, e) to the destination denoted by d. The
copy algorithm copies the entire sequence; remove_copy copies all elements not
equal to t; and remove_copy_if copies all elements for which the predicate p fails.
remove_if(b, e, p)
Arranges the container so that the elements in the range [b, e) for which the
predicate p is false are at the front of the range. Returns an iterator denoting one past
the range of these "unremoved" elements.
remove(b, e, t)
Like remove_if, but tests which elements to keep against the value t.
transform(b, e, d, f)
Runs the function f on the elements in the range [b, e), storing the result of f in d.
partition(b, e, p) stable_partition(b, e, p)
Partitions the elements in the range [b, e), based on the predicate p, so that
elements for which the predicate is true are at the front of the container. Returns an
iterator to the first element for which the predicate is false, or e if the predicate is
true for all elements. The stable_partition function maintains the input order
among the elements in each partition.
Exercises
6-0. Compile, execute, and test the programs in this chapter.
6-1. Reimplement the frame and hcat operations from §5.8.1/93 and §5.8.3/94 to
use iterators.
6-2. Write a program to test the find_urls function.
6-3. What does this program fragment do?
vector u(10, 100);
vector v;
copy(u.begin(), u.end(), v.begin());
Write a program that contains this fragment, and compile and execute it.
6-4. Correct the program you wrote in the previous exercise to copy from u into v.
There are at least two possible ways to correct the program. Implement both, and
describe the relative advantages and disadvantages of each approach.
6-5. Write an analysis function to call optimistic_median.
6-6. Note that the function from the previous exercise and the functions from
§6.2.2/113 and §6.2.3/115 do the same task. Merge these three analysis functions into
a single function.
6-7. The portion of the grading analysis program from §6.2.1/110 that read and
classified student records depending on whether they did (or did not) do all the
homework is similar to the problem we solved in extract_fails. Write a function to
handle this subproblem.
6-8. Write a single function that can be used to classify students based on criteria of
your choice. Test this function by using it in place of the extract_fails program, and
use it in the program to analyze student grades.
6-9. Use a library algorithm to concatenate all the elements of a vector.
7
Using associative containers
All the containers that we have used until now have been sequential containers, whose
elements remain in the sequence that we choose for them. When we use push_back or
insert to add elements to a sequential container, each element will stay where we put
it until we do something to the container that reorders the elements.
Some kinds of programs are hard to write efficiently if we restrict ourselves to
sequential containers. For example, if we have a container of integers, and we wish to
write a program that determines whether any element of the container has the value 42,
we have two plausible strategies—neither of which is ideal. One alternative is to inspect
every element of the container until we find 42 or run out of elements. This approach is
straightforward, but potentially slow—especially if the container has many elements. The
other alternative is for us to keep the container in an appropriate order and devise an
efficient algorithm to find the element we seek. This approach can yield fast searches,
but such algorithms are not easy to devise. In other words, we must live with a slow
program, or come up with our own sophisticated algorithm. Fortunately, as we'll see in
this chapter, the library offers a third alternative.
7.1 Containers that support efficient look-up
Instead of storing data in a sequential container, we can use an associative container.
Such containers automatically arrange their elements into a sequence that depends on
the values of the elements themselves, rather than the sequence in which we inserted
them. Moreover, associative containers exploit this ordering to let us locate particular
elements much more quickly than do the sequential containers, without our having to
keep the container ordered by ourselves.
Associative containers offer efficient ways to find an element that contains a particular
value, and might contain additional information as well. The part of each container
element that we can use for these efficient searches is called a key. For example, if we
were keeping track of information about students, we might use the student's name as
the key, so that we could find students efficiently by name.
In the sequential containers, the closest that we have seen to a key is the integer index
that accompanies every element of a vector. However, even these indices are not
really keys, because every time we insert or delete a vector element, we implicitly
change the index of every element after the one that we touched.
The most common kind of associative data structure is one that stores key-value pairs,
associating a value with each key, and that lets us insert and retrieve elements quickly
based on their keys. When we put a particular key-value pair into the data structure,
that key will continue to be associated with the same value until we delete the pair.
Such a data structure is called an associative array. Many languages, such as AWK,
Perl, and Sno-bol, have associative arrays built in. In C++, associative arrays are part of
the library. The most common kind of associative array in C++ is called a map, and,
analogous with other containers, it is defined in the

9.3 Protection
By defining the grade and read functions as members, we have fixed half of our
problem: Users of type Student_info no longer have to manipulate the internal state
of the object directly. However, they still can do so. We would like to hide the data, and
allow users to access the data only through our member functions.
C++ supports data hiding by allowing authors of types to say which members of those
types are public, and hence accessible to all users of the type, and which parts are
private, and inaccessible to users of the type:
class Student_info {
public:
// interface goes here
double grade() const;
std::istream& read(std::istream&);
private:
// implementation goes here
std::string name;
double midterm, final;
std::vector homework;
};
We've made a couple of changes to our definition of Student_info: We've said class
instead of struct, and we've added two protection labels. Each protection label
defines the accessibility of all the members that follow the label. Labels can occur in any
order within the class, and can occur multiple times.
By putting name, homework, midterm, and final after a private label, we have
made these data elements inaccessible to users of the Student_info type. References
to these members from nonmember functions are now illegal, and the compiler will
generate a diagnostic message to the effect that the member is private or inaccessible.
The members in the public section are fully available; any user may call read or grade.
What about the use of class instead of struct? We can use either keyword to define a
new type. The only difference between saying struct and class is the default
protection, which applies to every member defined before the first protection label. If we
say class Student_info, then every member between the first { and the first
protection label is private. If, instead, we write struct Student_info, then every
member declared between the { and the first protection label is public. For example,
class Student_info {
public:
double grade() const;
// etc.
};
is equivalent to
struct Student_info {
double grade() const; // public by default
// etc.
};
and
class Student_info {
std::string name; // private by default
// other private members
public:
double grade() const;
// other public members
};
is equivalent to
struct Student_info {
private:
std::string name;
// other private members
public:
double grade() const;
// other public members
};
In each of these definitions, we're saying that we'll allow users to get at the member
functions of Student_info objects, but we will not allow them to access the data
members.
There is no difference between what we can do with a struct or a class. In fact,
there is no way, short of reading the code, for our users to distinguish whether we used
struct or class to define our class type. Our choice of struct or class can have a
useful documentation role. In general, our programming style is to reserve struct to
denote simple types whose data structure we intend to expose. For that reason, we used
struct to define our original Student_info data type in Chapter 4. Now that we
intend to build a type that will control access to its members, we use class to define our
Student_info type.
9.3.1 Accessor functions
At this point, we've hidden our data members, so that users can no longer modify the
data in a Student_info object. Instead, they must use the read operation to set the
data members, and the grade function to find out the final grade for a given
Student_info. There's one more operation that we must still provide: We must give
users a way to get at the student's name. For example, think a bit about the program in
§4.5/70, in which we wrote a formatted report of student grades. That program needs
to access the student's name in order to generate the report. Although we want to allow
read access, we do not want to allow write access. Doing so is straightforward:
class Student_info {
public:
double grade() const;
std::istream& read(std::istream&) // must change definition
std::string name() const { return n; } // added
private:
std::string n; // changed
double midterm, final;
std::vector homework;
};
Instead of giving our users access to the name data member, we've added a member
function, also named name, that will give (read-only) access to the corresponding data
value. Of course, we have to change the data member's name to avoid confusing it with
the name of the function.
The name function is a const member function, which takes no arguments and which
returns a string that is a copy of n. By copying n, rather than returning a reference to
it, we ensure that users can read but not change the value of n. Because we need only
read access to the data, we declare the member function as const.
When we defined grade and read, we did so outside the class definition. When we
define a member function as part of the class definition, as we have done here with the
name function, we are hinting to the compiler that it should avoid function-call overhead
and expand calls to the function inline (§4.6/72) if possible.
Functions such as name are often called accessor functions. This nomenclature is
potentially misleading, because it implies that we are granting access to a part of our
data structure. Indeed, historically, such functions were often introduced to allow easy
access to hidden data, thus breaking the encapsulation that we were trying to achieve.
Accessors should be provided only when they are part of the abstract interface of the
class. In the case of Student_info, our abstraction is that of a student and a
corresponding final grade. Therefore, the notion of a student name is part of our
abstraction, and it is fitting to provide a name function. On the other hand, we do not
provide accessors to the other grades—midterm, final, or homework. These grades
are an essential part of our implementation, but they are not part of our interface.
Having added the name member function, we can now write the compare function:
bool compare(const Student_info& x, const Student_info& y)
{
return x.name() < y.name();
}
This function looks a lot like the version from §4.2.2/64. The only difference is how we
get at the student's name. In the original version, we could access the data member
directly; here, we must invoke the name function, which returns the student's name.
Because compare is part of our interface, we should include a declaration for this
function in the same header that defines Student_info and we should include this
definition in the associated source file that contains the definitions of the member
functions.
9.3.2 Testing for empty
Having hidden our members and provided the appropriate accessor function, we have
one remaining problem: There is still a reason that a user might want to look directly at
an object's data. For example, consider what happens if we call grade on an object for
which read has not been called:
Student_info s;
cout << s.grade() << endl; // exception: s has no data
Because we haven't called read to give s any values, the homework member of s will
be empty, and the call to grade will throw an exception. Although our users can catch
the exception, they have no way to detect the problem in advance, so that they can
avoid making the call at all.
Using the original Student_info structure from Chapter 4, users could test the
homework member to determine whether a call to grade would succeed. If homework
turned out to be empty, then they knew not to call grade. This approach worked, but at
the cost of forcing users to know about the structure of the object in order to perform
the test. We can do better, by offering the same test in a more abstract form:
class Student_info {
public:
bool valid() const { return !homework.empty(); }
// as before
};
The valid function will tell the user whether the object contains valid data, with a value
of true indicating that the student did at least one homework assignment, and
therefore that it is possible to compute the student's grade. Our users can call valid to
determine whether subsequent operations will succeed. For example, before calling
grade, a user can check whether the object is valid, thereby avoiding a potential
exception.
9.4 The Student_info class
At this point we have resolved most of our objections to the original Student_info
structure, so it is worth reviewing what we've done:
class Student_info {
public:
std::string name() const { return n; }
bool valid() const { return !homework.empty(); }
// as defined in §9.2.1/157, and changed to read into n instead of name
std::istream& read(std::istream&);
double grade() const; // as defined in §9.2.1/158
private:
std::string n;
double midterm, final;
std::vector homework;
};
bool compare(const Student_info&, const Student_info&);
Users can change the state of a Student_info object only by calling the read member
function. They cannot reach inside an object and directly change any of the data
members. Instead, we provide operations that hide our implementation. Finally, all the
operations on Student_info objects are logically gathered together.
9.5 Constructors
Although our class is reasonably complete and usable as it stands, there is one more
thing to think about: We have not said anything about what happens when objects are
created.
We know that the library guarantees that when we create an object of a library class,
the object starts with an appropriate value. For example, when we define a string or a
vector without an initial value, we get an empty string or vector. Both string and
vector also allow us to give a new object an initial value, such as specifying a size or a
count and a fill character.
Constructors are special member functions that define how objects are initialized.
There is no way to call a constructor explicitly. Instead, creating an object of class type
calls the appropriate constructor automatically as a side effect.
If we do not define any constructors, the compiler will synthesize one for us. We'll have
more to say about synthesized operations in §11.3.5/201. What we need to know now is
what happens if we do not define any constructors. In this case, our users will be able to
define Student_info objects, but will not be able to initialize them explicitly, except as
a copies of other Student_info objects.
The synthesized constructor will initialize the data members to a value that depends on
how the object is being created. If the object is a local variable, then the data members
will be be default-initialized (§3.1/38). If the object is used to initialize a container
element, either as a side effect of adding a new element to a map, or as the elements of
a container defined to have a given size, then the members will be value-initialized
(§7.2/125). These rules are slightly complicated, but their essentials are:
If an object is of a class type that defines one or more constructors, then the
appropriate constructor completely controls initialization of the objects of that class.
If an object is of built-in type, then value-initializing it sets it to zero, and defaultinitializing
it gives it an undefined value.
Otherwise, the object can be only of a class type that does not define any
constructors. In that case, value- or default-initializing the object value- or defaultinitializes
each of its data members. This initialization process will be recursive if
any of the data members is of a class type with its own constructor.
As it stands, our Student_info class is in this third category: It is a class type, but we
do not explicitly say how to construct Student_info objects. So, if we define a local
Student_info variable, then the n and homework members will be automatically
initialized to the empty string and vector respectively, because they are class
objects with constructors. In contrast, default-initializing midterm and final will give
them undefined values, meaning they will hold whatever garbage happens to be in
memory when the object is created.
Given our simple operations, this behavior may appear harmless: None of our operations
uses the value of midterm or final without first initializing the object by calling read,
which assigns values to these members. However, it is normally good practice to ensure
that every data member has a sensible value at all times. For example, it is possible
that later we (or a subsequent maintainer of our code) will add operations that examine
these data members. If we don't initialize them in the constructor, then these new
operations might cause future failures. Moreover, as we'll see in §11.3.5/201, even
though we do not explicitly use midterm or final, there are synthesized operations on
the class that could do so. Any use other than writing to an undefined value is illegal
(§3.1/38), and so, strictly speaking, we must initialize these values.
In practice, we'll want to define two constructors: The first constructor takes no
arguments and creates an empty Student_info object; the second takes a reference
to an input stream and initializes the object by reading a student record from that
stream. This strategy allows our users to write code such as
Student_info s; // an empty Student_info
Student_info s2(cin); // initialize s2 by reading from cin
Constructors are distinguished from other member functions in two ways: They have the
same name as the name of the class itself, and they have no return type. Constructors
are similar to other functions in that we can define multiple versions of constructors that
differ in terms of the number or type of their arguments. With this knowledge, we might
update our class to add our two constructors:
class Student_info {
public:
Student_info() // construct an empty Student_info object
Student_info(std::istream&); // construct one by reading a stream
// as before
};
9.5.1 The default constructor
The constructor that takes no arguments is known as the default constructor. Its job is
normally to ensure that its object's data members are properly initialized. In the case of
Student_info objects, we want to initialize the data to indicate that we haven't yet
read a record: We'll want the homework member to be an empty vector, the n
member to be an empty string, and the midterm and final members to be
initialized to zero:
Student_info::Student_info(): midterm(O), final(0) { }
The constructor definition uses some new syntax. Between the : and the { is a
sequence of constructor initializers, which tell the compiler to initialize the given
members with the values that appear between the corresponding parentheses.
Therefore, this particular default constructor works by explicitly setting the midterm
and final grades to 0. Other than that, the constructor does no overt work: The body
of the function is empty. As we shall see, n and homework are implicitly initialized.
Understanding constructor initializers is crucial to understanding how to create and
1.
initialize objects. When we create a new class object, several steps happen in sequence:
1. The implementation allocates memory to hold the object.
2. It initializes the object, as directed by the constructor's initializer list.
3. It executes the constructor body.
The implementation initializes every data member of every object, regardless of whether
the constructor initializer list mentions those members. The constructor body may
change these initial values subsequently, but the initialization happens before the
constructor body begins execution. It is usually better to give a member an initial value
explicitly, rather than assigning to it in the body of the constructor. By initializing rather
than assigning a value, we avoid doing the same work twice.
We said that constructors exist to ensure that objects are created with their data
members in a sensible state. In general, this design goal means that every constructor
should initialize every data member. The need to give members a value is especially
critical for members of built-in type. If the constructor fails to initialize such members,
objects declared at local scope will be initialized with garbage, which is almost never
correct.
We can now understand why we said that the Student_info default constructor did no
other "overt" work. Although we explicitly initialized only midterm and final, the other
data members are initialized implicitly. Specifically, n is initialized by the string
default constructor, and homework is initialized by the vector default constructor.
9.5.2 Constructors with arguments
Our second Student_info constructor is even easier:
Student_info::Student_info(istream& is) { read(is); }
This constructor delegates the real work to the read function. The constructor has no
explicit initializer, so the homework and n members will be initialized by the default
constructors for vector and string respectively. The midterm and final members
will have explicit initial values only if the object is being value-initialized. This lack of
initialization doesn't matter, because read immediately gives these variables new values.
9.6 Using the Student_info class
Our new Student_info class is now quite different from the original Student_info
structure from Chapter 4. Not surprisingly, using the class is different from using the
original structure. After all, our objective was to prevent users from being able to
change our data values, which we accomplished by making them private. Instead, we
intend for users to write their programs in terms of the interface provided by our class.
As an example, we can rewrite our original main program from §4.5/70, which wrote
the final grades for the students in a formatted report, to use this version of the class:
int main()
{
vector students;
Student_info record;
string::size_type maxlen = 0;
// read and store the data
while (record.read(cin)) { // changed
maxlen = max(maxlen, record.name().size()) // changed
students.push_back(record);
}
// alphabetize the student records
sort(students.begin(), students.end(), compare);
// write the names and grades
for (vector::size_type i = 0;
i != students.size(); ++i) {
cout << students[i].name() // changed
<< string(maxlen + 1 - students[i].name.size(), ' ');
try {
double final_grade = students[i].grade(); // changed
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
}
return 0;
}
The changes here are to the calls of name, read, and grade. So, for example, the first
while loop now says
while (record.read(cin)) {
instead of
while (read(cin, record)) {
Our revised version calls the member read of the object record. The earlier version
called the global read function, passing it record as an explicit parameter. Both calls
have the same effect: The object record will be assigned values read from cin.
9.7 Details
User-defined types can be defined as either structs or classes. The only difference
is in the default protection that applies to members defined before the first protection
label: Members defined after struct are public; those defined after class are private.
Protection labels control access to members of a class type: public members are
generally accessible; private members are accessible only to members of the class.
Protection labels can appear in any order and multiple times within a class.
Member functions: Types may define member functions as well as data. Member
functions are implicitly called on behalf of a specific object. References to member data
or functions from within a member function are implicitly bound to that object.
Member functions can be defined inside or outside the class definition. Defining a
member function inside the class asks the implementation to expand calls to it inline,
thus avoiding function call overhead. Outside the class, the name of the function must
be qualified to indicate that it is from the class scope: class-name::member-name refers
to the member member-name from the class class-name.
Member functions can be defined as const by inserting the const keyword after the
parameter list. Such members may not change the state of the object on which they are
invoked. Only const member functions may be called for const objects.
Constructors are special member functions that define how objects of the type are
initialized. Constructors have the same name as the class and have no return value. A
class can define multiple constructors as long as they differ in the number or types of
their arguments. It is good practice for every constructor to ensure that every data
member has a sensible value on exit from the constructor.
Constructor initializer list: A constructor initializer is a comma-separated list of
member- name (value) pairs. Each member-name is initialized from the associated
value. Data members that are not explicitly initialized are implicitly initialized.
The order in which members are initialized is determined by the order of declaration in
the class, so care must be taken when using one class member to initialize another. It is
safer practice to avoid such interdependence by assigning values to these members
inside the constructor body and not initializing them in the constructor initializer.
Exercises
9-0. Compile, execute, and test the programs in this chapter.
9-1. Reimplement the Student_info class so that it calculates the final grade when
reading the student's record, and stores that grade in the object. Reimplement the
grade function to use this precomputed value.
9-2. If we define the name function as a plain, nonconst member function, what other
functions in our system must change and why?
9-3. Our grade function was written to throw an exception if a user tried to calculate a
grade for a Student_info object whose values had not yet been read. Users who care
are expected to catch this exception. Write a program that triggers the exception but
does not catch it. Write a program that catches the exception.
9-4. Rewrite your program from the previous exercise to use the valid function,
thereby avoiding the exception altogether.
9-5. Write a class and associated functions to generate grades for students who take
the course for pass/fail credit. Assume that only the midterm and final grades matter,
and that a student passes with an average exam score greater than 60. The report
should list the students in alphabetical order, and indicate P or F as the grade.
9-6. Rewrite the grading program for the pass/fail students so that the report shows all
the students who passed, followed by all the students who failed.
9-7. The read_hw function §4.1.3/57 solves a general problem (reading a sequence of
values into a vector) even though its name suggests that it should be part of the
implementation of Student_info. Of course, we could change its name—but suppose,
instead, that you wanted to integrate it with the rest of the Student_info code, in
order to clarify that it was not intended for public access despite its apparent generality?
How would you do so?
10
Managing memory and low-level data structures
Until now, we have been storing data either in variables or in containers, such as
vector, that come from the standard library. The reason for this strategy is that the
standard- library facilities are usually more flexible and easier to use than the facilities
that are part of the core language.
Once you know how to use the library, the logical next step is to understand how it
works. The key to this understanding turns out to involve core-language programming
tools and techniques that come in handy in other contexts as well. We use the term
low- level to refer to these ideas because they underlie the standard library and
because they correspond closely to the way typical computer hardware works. For these
reasons, they tend to be harder to use, and are more dangerous, but they sometimes
can be more efficient—provided that you understand them thoroughly—than are the
related ideas in the standard library. Because no library can solve all problems, many
C++ programs wind up using low-level techniques from time to time.
This chapter departs from our usual style of presenting problems before their solutions,
because the tools that we are going to present work at a low enough level that it is hard
to use any one tool by itself to solve useful problems. Instead, we are going to begin by
presenting two related ideas: arrays and pointers. Once we have done so, we'll show
how those ideas combine with new-expressions and delete-expressions to allow a form
of dynamic memory allocation that programmers can control more directly than they can
control the automatic memory management offered by library classes such as vector
and list.
Once we understand how arrays and pointers work, we will explore, in Chapter 11, how
the library uses these facilities to implement its containers.
10.1 Pointers and arrays
An array is a kind of container, similar to a vector but less powerful. A pointer is a kind
of random-access iterator that is essential for accessing elements of arrays, and has
other uses as well. Pointers and arrays are among the most primitive data structures in
C and C++. They are virtually inseparable from one another, in the sense that it is
impossible to do anything useful with an array without using pointers, and pointers
become much more useful in the presence of arrays.
Because these two notions are so closely intertwined, we shall explain them both before
trying to solve significant problems with either. It is easier to explain pointers without
understanding arrays than to explain arrays without understanding pointers, so we shall
discuss pointers first.
10.1.1 Pointers
A pointer is a value that represents the address of an object. Every distinct object has
a unique address, which denotes the part of the computer's memory that contains the
object. If you can access an object, you can obtain its address, and vice versa. For
example, if x is an object, then &x is the address of that object, and if p is the address
of an object, then *p is the object itself. The & in &x is an address operator, and is
distinct from the use of & to define reference types (§4.1.2/54). The * is a dereference
operator, which works analogously to the way * works when applied to any other
iterator (§5.2.2/81). If p contains the address of x, we also say that p is a pointer that
points to x. It is common to represent such a state of affairs with a diagram such as
As with other built-in types, a local variable that is a pointer has no meaningful value
until we give it one. Programmers frequently use the value 0 to initialize pointers,
because converting 0 to a pointer yields a value that is guaranteed to be distinct from a
pointer to any object. Moreover, the constant 0 is the only integer value that can be
converted to a pointer type. The resulting value, often called a null pointer, is often
useful in comparisons.
As with all C++ values, pointers have types. The address of an object of type T has type
"pointer to T," written as T* in definitions and similar contexts.
Suppose that x is an object of type int, defined as
int x;
and we want to define p to have a type that will allow p to contain the address of x. We
do so by saying that the type of p is "pointer to int," which we say implicitly by
defining *p to have type int:
int *p; // *p has type int
Here *p is a declarator, which is the part of a definition that defines a single variable.
Even though the * and the p are part of a single declarator, most C++ programmers
write this definition as
int* p; // p has type int*
to emphasize the notion that p has a particular type (i.e., int*). These two usages are
equivalent because spaces around the * are neutral. However, the latter usage conceals
a pitfall that is so important that it deserves special attention:
int* p, q; // What does this definition mean?
defines p as an object of type "pointer to int" and q as an object of type int. This
example is easier to understand if we view it this way:
int *p, q; // *p and q have type int
or, for that matter, this way:
int (*p), q; // (*p) and q have type int
Still better, we can make our intentions crystal clear by writing
int* p; // *p has type int
int q; // q has type int
We now know enough to write a simple program that uses pointers:
int main()
{
int x = 5;
// p points to x
int* p = &x;
cout << "x = " << x << endl;
// change the value of x through p
*p = 6;
cout << "x = " << x << endl;
return 0;
}
The output of this program will be
x = 5
x = 6
Immediately after we have defined p, the state of our variables is
It should be no surprise that x is 5 when we execute the first output expression. The
next statement changes the value of x to 6 by executing *p = 6. Remember, once p
contains the address of x, *p and x are two different ways of referring to the same
object. Thus, the x is 6 when the second output expression is executed.
It may be useful to think of a pointer to an object as an iterator that refers to the only
element of a "container" that contains that object and nothing else.
10.1.2 Pointers to functions
In §6.2.2/113, we saw a program that passed a function as an argument to another
function, and noted that there was slightly more going on there than met the eye. The
truth is that functions are not objects, and there is no way to copy or assign them, or to
pass them as arguments directly. In particular, there is no way for a program to create
or modify a function—only the compiler can do that. All that a program can ever do with
a function is call it or take its address.
Nevertheless, we can call a function with another function as an argument, as we did
when we passed median_analysis as an argument to write_analysis in
§6.2.2/112. What happens is that the compiler quietly translates such calls so as to use
pointers to functions instead of using functions directly. Pointers to functions behave
similarly to any other pointers. Once you have dereferenced such a pointer, however, all
you can do with the resulting function is call it—or take the function's address yet again.
Declarators for pointers to functions resemble other declarators. For example, just as we
wrote
int *p;
to say that *p has type int, thereby implying that p is a pointer, we might write
int (*fp)(int);
to say that if we dereference fp, and call it with an int argument, the result has type
int. By implication, fp is a pointer to a function that takes an int argument and
returns an int result.
Because all that you can do with a function is to take its address or call it, any use of a
function that is not a call is assumed to be taking its address, even without an explicit &.
Similarly, you can call a pointer to a function without dereferencing the pointer
explicitly. So, for example, if we have a function whose type matches fp, such as
int next(int n)
{
return n + 1;
}
then we can make fp point to next by writing either of the following two statements:
// these two statements are equivalent
fp = &next;
fp = next;
Similarly, if we have an int variable named i, we can use fp to call next, and thereby
increment i, by writing either
// these two statements are equivalent
i = (*fp)(i);
i = fp(i);
Finally, if we write a function that looks like it takes another function as a parameter,
the compiler quietly translates the parameter to be a pointer to a function instead. So,
for example, in the write_analysis function in §6.2.2/113, the parameter that we
wrote as
double analysis(const vector&)
could equivalently have been written as
double (*analysis)(const vector&)
However, this automatic translation does not apply to return values from functions. If
we wanted to write a function that returned a function pointer, of the same type as the
parameter to write_analysis, then we would have to say explicitly that the function
returns a pointer. One way to do so is to begin by using a typedef to define, say,
analysis_fp as the name of the type of an appropriate pointer:
typedef double (*analysis_fp)(const vector&);
Then we can use that type to declare our function:
// get_analysis_ptr returns a pointer to an analysis function
analysis_fp get_analysis_ptr();
The alternative
double (*get_analysis_ptr())(const vector&);
is arcane. In effect, we are saying that if you call get_analysis_ptr(), and
dereference the result, what you get is a function that takes a const
vector& and returns a double. Fortunately, functions that return
pointers to functions are rare! We use this syntax nowhere else in this book, but we
explain it in more detail in §A.l/295. Pointers to functions are most commonly used as
arguments to other functions. As an example, here is a sample implementation of the
find_if library function:
template
In find_if(In begin, In end, Pred f)
{
while (begin != end && !f(*begin))
++begin;
return begin;
}
In this example, Pred can potentially be any type at all, as long as f(*begin) has a
meaningful value. Suppose we have a predicate function, such as
bool is_negative(int n)
{
return n < 0;
}
and we use find_if to locate the first negative element in a vector named v:
vector::iterator i = find_if(v.begin(), v.end(), is_negative);
We are able to write is_negative instead of &is_negative only because the name
of a function turns into a pointer to the function automatically. Similarly, the
implementation of find_if is permitted to call f(*beg) instead of (*f)(*beg) only
because calling a function pointer automatically calls the function to which it points.
10.1.3 Arrays
An array is a kind of container that is part of the core language rather than part of the
standard library. Every array contains a sequence of one or more objects of the same
type. The number of elements in the array must be known at compile time, which
requirement implies that arrays cannot grow or shrink dynamically the way library
containers do.
Because arrays are not class types, they have no members. In particular, they do not
have the size_type member to name an appropriate type to deal with the size of an
array. Instead, the header defines size_t, which is a more general type.
The implementation defines size_t as the appropriate unsigned type large enough to
hold the size of any object. Thus, we can (and should) use size_t to refer to the size of
an array, similarly to the way we use size_type to deal with the size of a container.
For example, a three-dimensional geometry program might represent a point this way:
double coords[3];
knowing that the number of dimensions in physical space is unlikely to change any time
soon. A more experienced programmer might represent the point this way:
const size_t NDim = 3;
double coords[NDim];
taking advantage, for documentation purposes, of the fact that the value of NDim is
known at compilation time (because it is a const size_t that is initialized from a
constant). Using NDim instead of 3 distinguishes the 3 that represents the number of
dimensions from a 3 that represents, say, the number of sides in a triangle.
No matter how we define an array, there is always a fundamental relationship between
arrays and pointers: Whenever we use the name of an array as a value, that name
represents a pointer to the initial element of the array. We have defined coords as an
array, so using coords as a value gives us the address of the array's initial element. As
with any other pointer, we can dereference it with the * operator to get the object to
which it points, so that executing
*coords = 1.5;
sets the initial element of coords to 1. 5.
10.1.4 Pointer arithmetic
We now know how to define arrays, and how to obtain the address of the initial element
of an array. What about the other elements? Recall that in §10.1/169 we said that a
pointer was a kind of iterator. More specifically, a pointer is a random-access iterator.
This fact gives us a second fundamental property of arrays: If p points to the mth
element of an array, then p + n points to the (m + n)th element of the array and (p
- n) points to the (m - n)th element—assuming, of course, that these elements exist.
Continuing our example, and noting that, as usual, the initial element of coords has
number 0, we see that coord + 1 is the address of element number 1 of the coords
array (i.e., the element after the initial element), and coords + 2 is the address of
element number 2, which is also the last element because we defined coords to have
three elements.
What about coords + 3? That value represents the address of where element number
3 would be in the coords array if the element existed—but the element doesn't exist.
Nevertheless, coords + 3 is a valid pointer, even though it doesn't point to an
element. Analogous with vector and string iterators, adding n to the address of the
initial element of an n-element array yields an address that is not guaranteed to be the
address of any object, but it is one that we can use for comparisons. Moreover, the rules
that relate p, p + n, and p - n are valid even if one or more of those expressions
yields a value that is one (but not more than one) past the end of an array.
So, for example, we can copy the contents of coords into a vector by writing
vector v;
copy(coords, coords + NDim, back_inserter(v));
where, as before, NDim is just a fancy way of spelling 3. In this example, coords +
NDim does not point to an element, but it is a valid off-the-end iterator, and passing it
as the second argument to copy presents no difficulty.
As another example, because we can construct a vector from two iterators, we could
have constructed v directly as a copy of the elements in coords by writing
vector v(coords, coords + NDim);
In other words, suppose that a is an n-element array, that v is a vector, and that we
want to apply standard-library algorithms to elements of a. Then, wherever we might
use v.begin() and v.end() to give standard-library algorithms access to elements of
v, we should use a and a + n as arguments when we wish to apply these algorithms to
the elements of a.
If p and q are pointers to elements of the same array, then p - q is an integer that
represents the distance in elements between p and q. More precisely, p - q is defined
so that (p - q) + q is equal to p. Because p - q might be negative, it has a signed
integer type. Whether that type is int or long depends on the implementation, so the
library provides a synonym, named ptrdiff_t, to represent the appropriate type. Like
size_t, the ptrdiff_t type is defined in the header.
We saw in §8.2.7/150 that there is no guarantee of being able to compute an iterator
that refers to a point before the beginning of a container. Analogously, it is never
legitimate to compute an address that falls before the beginning of an array. In other
words, if a is an n-element array, then a+i is valid if and only if 0 = i = n, and a+i
refers to an element of a if and only if 0 = i < n (but not if i and n are equal).
10.1.5 Indexing
We said in §10.1/169 that pointers are random-access iterators for arrays. Therefore,
like all random-access iterators, they support indexing. Specifically, if p points to the
mth element of an array, p[n] is the m+nth element of the array—not the address of the
element, but the element itself.
Recall from §10.1.3/174 that the name of an array is the address of the initial element
of the array. This fact, together with the definition of p[n], implies that if a is an array,
a[n] is the nth element of the array. More formally, if p is a pointer and n is an integer,
then p[n] is equivalent to *(p + n).
In most languages, the behavior of indexing is fundamental and obvious. In C++, this
behavior is not a direct property of arrays. Rather, it is a corollary to the properties of
array names and pointers, and the fact that pointers supply the operations defined for
random-access iterators.
10.1.6 Array initialization
Arrays have an important property that the standard-library containers do not share:
There is a convenient syntax for giving an initial value to each element of an array.
Moreover, using this syntax often lets us avoid having to state the size of the array
explicitly.
For example, if we were writing a program that deals with dates, we might like to know
how many days are in each month. One way to do so would be the following:
const int month_lengths[] = {
31, 28, 31, 30, 31, 30, // we will deal elsewhere with leap years
31, 31, 30, 31, 30, 31
};
Here, we have given an initial value to each element that corresponds to the length of a
month, with January being month 0 and December being month 11. Now, we can use
month_lengths[i] to refer to the length of month i.
Note that we did not say explicitly how many elements the month_lengths array has.
Because we initialized it explicitly, the compiler will count elements for us—a task to
which it is much better suited than we.
10.2 String literals revisited
We have finally learned enough to understand the true meaning of string literals: A
string literal is really an array of const char with one more element than the number
of characters in the literal. That extra character is a null character (i.e., '\0') that the
compiler automatically appends to the rest of the characters. In other words, if we
define
const char hello[] = { 'H', 'e', 'l', 'l', 'o', '\0' };
then the variable hello has exactly the same meaning as the string literal "Hello",
except, of course, that the variable and the literal are two distinct objects and,
therefore, have different addresses.
The reason that the compiler inserts the null character is to allow the programmer to
locate the end of the literal given only the address of its initial character. The null
character acts as an end marker, so that the programmer can know where the string
ends. There is a library function in called strlen, which tells us how many
characters are in a string literal or other null-terminated array of characters, not
counting the null at the end. The strlen function might be implemented as follows:
// Example implementation of standard-library function
size_t strlen(const char* p)
{
size_t size = 0;
while (*p++ != '\0')
++size;
return size;
}
Recall from §10.1.3/174 that size_t is an unsigned integral type that is appropriate to
contain the size of any array, which makes it the appropriate type for size. The
strlen function counts characters in the array denoted by p up to but not including the
null. Because the variable hello has the same meaning as the string literal "Hello",
string s(hello);
will define a string variable named s that contains a copy of the characters stored in
hello, just as
string s("Hello");
defines a string variable named s that contains a copy of the characters in "Hello".
Moreover, because we can construct a string from two iterators, we can also write
string s(hello, hello + strlen(hello));
Here, using the name of the array hello yields a pointer to the initial character of the
hello array, and hello + strlen(hello) is a pointer to the '\0' that is at the end
of the array which is also one character past the o of hello. Because pointers are
iterators, we can construct a string from two pointers, similar to what we did in
§6.1.1/103, where we created a new string from two iterators. In both cases, the first
iterator refers to the initial character of the sequence that we wish to use to initialize the
string that we are constructing, and the second iterator refers to one past the last
character.
10.3 Initializing arrays of character pointers
We said in §10.2/176 that a string literal is just a convenient way of writing the address
of the initial character of a null-terminated sequence of characters. We said in
§10.1.6/176 that we can initialize the elements of an array by giving a sequence of
appropriate values, enclosed in curly braces, as an initializer. By combining these two
facts, we learn that we can initialize an array of character pointers by giving a sequence
of string literals.
This claim is quite a mouthful. To make it concrete, suppose that we wish to convert
numeric grades to letter grades according to the following rule:
If the grade is at least 97 94 90 87 84 80 77 74 70 60 0
then the letter grade is A+ A A- B+ B B- C+ C C- D F
Here is a program that does the conversion:
string letter_grade(double grade)
{
// range posts for numeric grades
static const double numbers[] = {
97, 94, 90, 87, 84, 80, 77, 74, 70, 60, 0
};
// names for the letter grades
static const char* const letters[] = {
"A+", "A", "A-", "B+", "B", "B-", "C+", "C", "C-", "D", "F"
};
// compute the number of grades given the size of the array
// and the size of a single element
static const size_t ngrades = sizeof(numbers)/sizeof(*numbers);
// given a numeric grade, find and return the associated letter grade
for (size_t i = 0; i < ngrades; ++i) {
if (grade >= numbers[i])
return letters[i];
}
return "?\?\?";
}
The definition of numbers uses the keyword static , which we saw in §6.1.3/107. In
the present context, it tells the compiler to initialize the letters and numbers arrays
only once, no later than the first time each array is used. Without the static , the
compiler would have initialized the array for each call, which would have slowed the
program needlessly. We have said that the array elements are const because we do
not intend to change them—which is what allows us to get away with initializing the
array only once.
The letters array is an array of constant pointers to const char . In this case, each
element points to the initial element of its respective letter-grade string literal.
The definition of ngrades introduces a new keyword, sizeof , which we use to
determine how many elements the numbers array has without having to count the
elements ourselves. If e is an expression, then sizeof(e) returns a size_t value that
tells us how much memory an object of the type of e consumes. It does so without
actually evaluating the expression, which is possible because it does not need to
evaluate the expression in order to determine its type, and because all objects of a
given type occupy the same amount of storage.
The sizeof operator reports its result in bytes , which are storage units whose exact
nature varies from one implementation to another. The only guarantees about bytes are
that a byte contains at least eight bits, every object occupies at least one byte, and that
a char occupies exactly one byte.
Of course, we want to determine how many elements the numbers array has, not how
many bytes it occupies. To do so, we divide the size of the entire array by the size of a
single element. Recall from §10.1.3/174 that because numbers is an array, *numbers
is an element of the array. It happens to be the initial element, but the particular
element is irrelevant in this context because all elements are the same size. What is
relevant is that sizeof(*numbers) is the size of a single element of the numbers
array, so that sizeof(numbers)/sizeof(*numbers) is the number of elements in
the array.
Once we have established our tables, determining the letter grade is simplicity itself. We
look sequentially at the elements of numbers until we find that grade is greater than or
equal to one of them. When we find the relevant element of numbers , we return the
corresponding element of letters . This element is a pointer, but we have already
seen in §10.2/176 that we can convert a character pointer to a string .
If we cannot find an appropriate letter grade, it means that our user gave us a negative
numeric grade, in which case we return a nonsense letter grade. The \ characters are
there because, as we explain in more detail in §A.2.1.4/302, C++ programs should not
contain two or more consecutive question marks. We must, therefore, use "?\?\? " to
represent ??? in a program.
10.4 Arguments to main
Now that we understand pointers and character arrays, we can understand how to pass
arguments to the main function. Most operating systems provide a way to pass a
sequence of character strings to main as an argument, if the main function is willing to
accept them. The way the author of main signals such willingness is by giving main two
parameters: an int and a pointer to a pointer to char. Like any parameters, these can
have arbitrary names, but programmers often call them argc and argv . The value of
argv is a pointer to the initial element of an array of pointers, one for each argument.
The value of argc is the number of pointers in the array of which argv points to the
initial element. The initial element of that array always represents the name by which
the program is called, so argc is always at least 1. The arguments, if any, occupy
subsequent elements of the array.
As an example, this program writes its arguments, if any, with spaces between them:
int main(int argc, char** argv)
{
// if there are arguments, write them
if (argc > 1) {
int i; // declare i outside the for because we need it after the loop finishes
for (i = 1; i < argc-1; ++i) // write all but the last entry and a space
cout << argv[i] << " "; // argv[i] is a char*
cout << argv[i] << endl; // write the last entry but not a space
}
return 0;
}
If we compile this program and put the resulting executable in a file called say, then by
asking the system to execute
say Hello, world
we will cause our program to write
Hello, world
In this case, argc will be 3, and the three elements of argv will be pointers to the
initial characters of arrays initialized with say, Hello , and world respectively. We can
visualize the value of argv this way:

10.5 Reading and writing files
The programs in this book use only cin and cout for their input and output. Larger
applications, however, often need to work with multiple files, both for input and output.
C++ offers a wide variety of facilities for doing so, of which we will discuss only a few.
10.5.1 The standard error stream
It is often useful for a program to be able to comment about what it is doing in a way
that is not part of its regular output. Such comments might notify the user about error
conditions, or might constitute a log of events that the program considers significant.
To make such comments easy to distinguish from ordinary output, the C++ library
defines a standard error stream, in addition to the standard input and output streams.
This stream is often merged with the standard output, but most systems provide a way
to separate them.
To write to the standard error stream, C++ programs can use either cerr or clog .
These output streams are both attached to the same destination. The difference
between them is how they handle buffering (§1.1/11).
The clog stream, as its name suggests, is intended for logging purposes. Accordingly, it
has the same buffering properties as cout : It saves characters and writes them when
the system decides that it is appropriate to do so. The cerr stream, on the other hand,
always writes its output immediately. This strategy guarantees that the output will
become visible as soon as possible, but it imposes what might be substantial overhead.
Therefore, to write an urgent complaint, you should use cerr ; to produce a running
commentary about what the program is doing, you should use clog .
10.5.2 Dealing with multiple input and output files
The standard input, output, and error streams might or might not be associated with
files. For example, a window system might run C++ programs with these streams
connected to a window associated with the program, and might use completely different
facilities to do so than it would to access disk files.
For this reason, the objects that the C++ standard library uses for file input and output
have different types than the objects that it uses to denote the standard input and
output streams. If you wish to work with an input or output file, you must create an
object of type ifstream or ofstream respectively. This requirement may seem to cause
needless difficulty. After all, we have seen that the library's input and output facilities
are all defined in terms of types istream and ostream . Does the library have another
set of definitions for ifstream and ofstream ?
Fortunately, the answer is no. As we shall see in Chapter 13, it is possible to say that
one type is similar enough to another that one can stand in for the other. The standard
library says exactly that, by defining ifstream to be a kind of istream and ofstream
to be a kind of ostream . As a result, it is possible to use an ifstream wherever the
library expects an istream and an ofstream wherever the library expects an ostream
. The definitions of both of these classes appear in header .
When we define an ifstream or ofstream object, we might expect to have to supply,
in the form of a string , the name of the file that we wish to use. In fact, we are
required to supply, not a string , but rather a pointer to the initial element of a nullterminated
character array. One reason for this curious requirement is to give programs
the option of using the input-output library without using the string facilities. Another
reason is historical: The input-output library predates the string class by several
years. A third reason is that this requirement makes it easier to interface with
operating-system input-output facilities, which typically use such pointers to
communicate. Whatever the reasons, the fact is that programs that deal with files must
ultimately express the files' names as pointers to null-terminated character arrays.
As an example, here is a program that copies a file named in to a file named out :
int main()
{
ifstream infile("in");
ofstream outfile("out");
string s;
while (getline(infile, s))
outfile << s << endl;
return 0;
}
This program takes advantage of the fact that a string literal is effectively a pointer to
the initial character of a null-terminated array. If we don't want to have to give the
name of the file as a literal, the best alternative is to store the file name in a string
and then use the c_str member function that we will describe in §12.6/224. So, for
example, if file is a string variable that contains the name of a file that we want to
read, we can create an ifstream object that will read it by defining it as
ifstream infile(file.c_str());
As a final example, here is a program that produces, on its standard output, a copy of
the contents of all the files whose names are given as arguments to main:
int main(int argc, char **argv)
{
int fail_count = 0;
// for each file in the input list
for (int i = 1; i < argc; ++i) {
ifstream in(argv[i]);
// if it exists, write its contents, otherwise generate an error message
if (in) {
string s;
while (getline(in, s))
cout << s << endl;
} else {
cerr << "cannot open file " << argv[i] << endl;
++fail_count;
}
}
return fail_count;
}
For each argument given to main (§10.4/179), the program creates an ifstream
object to read the file by that name. If the object appears false when used as a
condition, that means that the file does not exist, or that it cannot be read for some
reason. Accordingly, the program complains on cerr , and keeps a count of how many
failures it had. If the program created the ifstream object successfully, it reads the
file, one line at a time, into s, and writes the contents of each line on the standard
output.
When the program returns control to the system, it passes back the number of files that
it was unable to read. As usual, a return value of zero indicates success, which in this
case will indicate that we were able to read all the files.
10.6 Three kinds of memory management
So far, we have seen two distinct kinds of memory management, although we have not
discussed them explicitly. The first kind is usually called automatic memory
management, and is associated with local variables: A local variable occupies memory
that the system allocates when it encounters the variable's definition during execution.
The system automatically deallocates that memory at the end of the block that contains
the definition.
Once a variable has been deallocated, any pointers to it become invalid. It is the
programmer's responsibility to avoid using such invalid pointers. For example,
// this function deliberately yields an invalid pointer.
// it is intended as a negative example—don't do this!
int* invalid_pointer()
{
int x;
return &x; // instant disaster!
}
This function returns the address of the local variable x . Unfortunately, when the
function returns, doing so ends execution of the block that contains the definition of x ,
which deallocates x . The pointer that &x created is now invalid, but the function tries to
return it anyway. What happens at this point is anybody's guess. In particular, C++
implementations are not required to diagnose the error—you get what you get.
If we want to return the address of a variable such as x , one way to do so is to use the
other kind of memory management, by asking for x to be statically allocated:
// This function is completely legitimate.
int* pointer_to_static()
{
static int x;
return &x;
}
By saying that x is static , we are saying that we want to allocate it once, and only
once, at some point before the first time that pointer_to_static is ever called, and
that we do not want to deallocate it as long as our program runs. There is nothing wrong
with returning the address of a static variable; the pointer will be valid as long as the
program runs, and it will be irrelevant afterward.
However, static allocation has the potential disadvantage that every call to
pointer_to_static will return a pointer to the same object! Suppose we want to
define a function such that each time we call it, we get a pointer to a brand new object,
which stays around until we decide that we no longer want it. To do so, we use dynamic
allocation, which we request by using the new and delete keywords.
10.6.1 Allocating and deallocating an object
If T is the type of an object, new T is an expression that allocates an object of type T ,
which is default-initialized, and yields a pointer to this (unnamed) newly allocated
object. It is possible to give a specific value to use when initializing the object by
executing new T (args). The object stays around until the program either ends or
executes delete p (whichever happens first), where p is (a copy of) the pointer
returned by new. In order to delete a pointer, the pointer must point to an object that
was allocated by new , or be equal to zero. Deleting a zero pointer has no effect. As an
example,
int* p = new int(42);
will allocate an unnamed new object of type int , initialize the object to 42, and cause
p to point to that object. We can affect the value of the object by executing statements
such as
++*p; // p is now 43
after which the object has the value 43. When we're done with the object, we can
execute
delete p;
after which the space occupied by *p is freed and p becomes an invalid pointer, with a
value that we can no longer use until we have assigned a new value to it.
As another example, we might write a function that allocates an int object, initializes it,
and returns a pointer to it:
int* pointer_to_dynamic()
{
return new int(0);
}
which imposes on its caller the responsibility of freeing the object at an appropriate
time.
10.6.2 Allocating and deallocating an array
If T is a type and n is a non-negative integral value, new T[n] allocates an array of n
objects of type T and returns a pointer (which has type T* ) to the initial element of the
array. Each object is default-initialized, meaning that if T is a built-in type and the array
is allocated at local scope, then the objects are uninitialized. If T is a class type, then
each element is initialized by running its default constructor.
When T is a class type, there are two important implications of this initialization
process: First, if the class doesn't allow default-initialization, then the compiler will
reject the program. Second, each of the n elements in the array is initialized, which can
be a substantial execution overhead. In Chapter 11, we'll see that the standard library
provides a more flexible mechanism for dynamically allocating arrays. It is often
preferable to use that mechanism, rather than new, when dynamically allocating an
array.
Although every ordinary array is required to have at least one element, it is possible to
allocate an "array" with no elements by executing new T[n] with n equal to zero. In
this case, new has a little trouble returning a pointer to the initial element—because
there isn't one. What it does instead is return a valid off-the-end pointer, which we can
later use as an argument to delete[] , and which we can think of as being a pointer to
where the initial element would be if it existed.
The point of this curious behavior is to permit programs such as
T* p = new T[n];
vector v(p, p + n);
delete[] p;
to work even if n is zero. The fact that p doesn't point to an element when n is zero is
irrelevant; all that matters is that p and p + n are pointers that can legitimately be
compared with each other and found to be equal. In all cases, the vector will have n
elements. It is a great convenience for such programs to work properly even when n is
zero.
Note the use of delete[] in this example. The brackets are necessary to tell the
system to deallocate an entire array, rather than a single element. An array allocated by
new[] stays around until the program ends or until the program executes delete[] p
, where p is (a copy of) the pointer that new[] yielded. Before deallocating the array,
the system destroys each element, in reverse order.
As an example, here is a function that takes a pointer to the initial character of a nullterminated
character array such as a string literal, copies all the characters in the array
(including the null character at the end) into a newly allocated array, and returns a
pointer to the initial element of the new array:
char* duplicate_chars(const char* p) {
// allocate enough space; remember to add one for the null
size_t length = strlen(p) + 1;
char* result = new char[length];
// copy into our newly allocated space and return pointer to first element
copy(p, p + length, result);
return result;
}
Recall from §10.2/176 that strlen returns the number of characters in a nullterminated
array, excluding the null character at the end. We therefore add 1 to the
result of strlen to account for the null, and allocate that many characters. Because
pointers are iterators, we can use the copy algorithm to copy characters from the array
denoted by p into the array denoted by result . Because length includes the null
character at the end of the array, the call to copy copies that character as well as the
ones before it.
As before, this function imposes on its caller the obligation to free the memory that it
allocated. In general, finding an opportune time to free dynamically allocated memory is
far from easy. We shall discuss techniques for automating this task in §11.3.4/200.
10.7 Details
Pointers are random-access iterators that hold the addresses of objects. For example:
p = &s Makes p point to s.
*p = s2 Dereferences p and assigns a new value to the object to which p points.
vector (*sp)(const string&) = split;
Defines sp as a function pointer that points to the split function.
int nums[100];
Defines nums as an array of 100 ints.
int* bn = nums;
Defines bn as a pointer to the first element of the array nums.
int* en = nums + 100;
Defines en as a pointer to (one past) the last element of the array nums.
Pointers can point at single objects, arrays of objects, or functions. When a pointer
refers to a function, its value may be used only to call the function.
Arrays are fixed-size, built-in containers whose iterators are pointers. Uses of the name
of an array are automatically converted to a pointer to the initial element of the array. A
string literal is a null-terminated array of characters. Indexing an array is defined in
terms of pointer operations: For every array a and an index n, a[n] is the same as
*(a + n). If a is an array with n elements, then the range [a, a + n) represents all
the elements of a. Arrays can be initialized when they are defined:
string days[] = { "Mon", "Tues", "Wed", "Thu", "Fri", "Sat", "Sun" };
The implementation infers the size of days from the number of initializers.
The main function may (optionally) take two arguments. The first argument, an int,
says how many character arrays are stored in the second argument, which is a char**.
The second argument to main is sometimes written as
char* argv[]
which is equivalent to char**. This syntax is legal only in parameter lists.
Input-Output:
cerr
Standard error stream. Output is not buffered.
clog
Standard error intended for logging. Output is buffered.
ifstream(cp)
Input stream bound to the file named by the char* cp. Supports the operations on
istreams.
ofstream (cp)
Output stream bound to the file named by the char* cp. Supports the operations on
ostreams.
Input and output file streams are defined in .
Memory management:
new T
Allocates and default-initializes a new object of type T and returns a pointer to the
object.
new T(args)
Allocates and initializes a new object of type T using args to initialize the object.
Returns a pointer to the object.
delete p
Destroys the object to which p points and frees the memory used to hold *p. The
pointer must point at an object that was dynamically allocated.
new T[n]
Allocates and default-initializes an array of n new objects of type T. Returns a pointer
to the initial element in the array.
delete[] p
Destroys the objects in the array to which p points and frees the memory used to hold
the array. The pointer must point to the initial element of an array that was
dynamically allocated.
Exercises
10-0. Compile, execute, and test the programs in this chapter.
10-1. Rewrite the student-grading program from §9.6/166 to generate letter grades.
10-2. Rewrite the median function from §8.1.1/140 so that we can call it with either a
vector or a built-in array. The function should allow containers of any arithmetic type.
10-3. Write a test program to verify that the median function operates correctly.
Ensure that calling median does not change the order of the elements in the container.
10-4. Write a class that implements a list that holds strings.
10-5. Write a bidirectional iterator for your String_list class.
10-6. Test the class by rewriting the split function to put its output into a
String_list.
11
Defining abstract data types
In Chapter 9, we learned something about the basic language features required to
define new types. However, the Student_info type that we defined in that chapter did
not specify what should happen when objects of type Student_info are copied,
assigned, or destroyed. As we'll see in this chapter, the class author also controls all
these aspects of an object's behavior. What may be surprising is how essential the
correct definition of these operations is for creating a type that is intuitive and easy to
use.
Because we've used vectors extensively, we'll build a similar class to further our
understanding of how to design and implement classes. The implementation that we
present will be greatly simplified in terms of which operations we provide and our
attention to efficiency. Because it is a stripped-down version of the standard-library
vector class, we'll call our class Vec to avoid confusing it with the library class.
Although we are largely concerned with how to copy, assign, and destroy objects of class
Vec, we will find it useful to start by implementing some simpler member functions.
Once we have completed these other functions, we will come back and look at how we
can control copying, assigning, and destroying class objects.
11.1 The Vec class
When we design a class, we normally start by specifying the interface that we want to
provide. One way to determine the right interface is to look at the kind of programs we
want our users to be able to write. Because we want to implement a useful subset of the
standard vector class, a good place to start is to look at how we've used vectors:
// construct a vector
vector vs; // empty vector
vector v(100); // vector with 100 elements
// obtain the names of the types used by the vector
vector::const_iterator b, e;
vector::size_type i = 0;
// use size and the index operator to look at each element in the vector
for (i = 0; i != vs.size(); ++i)
cout << vs[i].name();
// return iterators positioned on the first and one past the last element
b = vs.begin(); e = vs.end();
Of course, this list of operations is but a subset of what the standard vector class
provides—but by implementing this subset, we can understand the language facilities
necessary to support much of the vector interface.
11.2 Implementing the Vec class
Having determined our operations, we next need to think about how to represent a Vec
.
The easiest implementation decision is that we need to define a template class . We
want to allow users to use Vec s to hold a variety of types. The template facility that we
described in §8.1.1/140 for functions also applies to classes. That facility let us write
one definition of a template function, and use that template to create versions that could
be run on a variety of types. Similarly, we can define a template class, and then use
that class to create a family of types that differ only with respect to the types used in
the template parameter list. We've already used such types, including vector , list ,
and map .
As with template functions, when we define a template class, we have to signal that the
class is a template, and list the type parameters that will be used in the class definition:
template class Vec {
public:
// interface
private:
// implementation
};
This code says that Vec is a template class, with one type parameter named T . As with
other class types, we can assume that there will be public and private parts that
define the interface and implementation respectively.
The next question we must resolve is what data we will store. Presumably we'll need
some storage to hold the elements in the Vec , and we'll want to keep track of the
number of elements that the Vec contains. The most obvious choice is to hold the
elements in a dynamically allocated array.
What information about the array do we need to store? We intend to implement the
begin , end , and size functions. This intention suggests that we might store the
address of the initial element, one past the address of the last element, and the number
of elements. However, we do not need to store all three of these items, because we can
compute any of them from the other two. Therefore, we'll make the arbitrary decision to
store only pointers to the first and (one past) the last element of the array, and to
compute the size as needed. We envision a data structure that looks like this:
With these implementation decisions made, we can update our Vec class:
template class Vec {
public:
// interface
private:
T* data; // first element in the Vec
T* limit; // one past the last element in the Vec
};
This class definition says that Vec is a template type, and that it takes a single type
parameter. In the body of the class definition, we'll call that type T . Whenever we use T
, the compiler will replace it with whatever type the user names when creating a Vec .
So for example, if we write
Vec v;
this definition will cause the compiler to instantiate (§8.1.2/142) a version of the Vec
class in which it replaces each reference to T by int . The code that the compiler
generates for that class will resolve expressions that involve T as if they had been
written using int . Thus, because we used the type parameter T in the declaration of
data and limit , the type of these pointers depends on the type of the objects that the
Vec will hold.
This type is not known until the definition of a Vec is instantiated. Once we say that we
want a Vec , the types of data and limit are known: They will be int* for this
instance of Vec . Similarly, if we also used Vec , then the compiler would
generate a second, different instantiation of Vec that bound T to string , thereby
giving data and limit the type string* in that instantiation.
11.2.1 Memory allocation
Because our class will allocate its array dynamically (§10.6.2/184), we might expect that
we should allocate space for our Vec by using new T[n] , where n is the number of
elements we want to allocate. However, remember that not only does new T[n]
allocate space, but it also initializes the elements by running the default constructor for
T . If we were to use new T[n] , then we would be imposing a requirement on T :
Users could create a Vec only if T has a default constructor. The standard vector
class imposes no such restriction. Because we want to emulate standard vector s, we
don't want to impose this restriction either.
It turns out that the library provides a memory allocation class that offers more detailed
control over memory allocation. This class will suit our needs exactly, if we use it instead
of new and delete . The class lets us allocate raw memory, and then—in a separate
step—build objects in that memory. Rather than diving right into the details of that
class, we will assume that eventually we shall have to write some utility functions that
will manage the memory for us. For now, we'll assume that these functions exist, and
we'll use them in completing the Vec class. As we use them, we'll get a better picture of
just what we would like them to do, so that when it's time to implement them, We will
know just what it is that we need to implement.
These new utility members will be part of the private implementation of our class.
They will be responsible for allocating and deallocating the memory that we need, and
for initializing and destroying the elements stored in the Vec . Thus, these functions will
manage our pointers—data and limit . Only these memory management functions will
give new values to these data members. The public members of Vec will read data
and 1imit , but will not change them.
When the public members need to do something, such as constructing a new Vec that
needs to change the value of data or limit , they will call an appropriate memorymanagement
function to do so. This strategy will let us partition our work: One set of
members will provide the interface to our user, and another set will deal with the
implementation details.
We'll come back to fill in the details of these utility functions in §11.5/203.
11.2.2 Constructors
We know that we must define two constructors:
Vec vs; // the default constructor
Vec vs(100); // constructor taking a size
The standard vector also provides a closely related third constructor, which takes a
size and an initial value to use to initialize the elements of the vector , and initializes
all the elements to copies of that value. This constructor is similar to the one that takes
a size alone, so we may as well implement this third constructor too.
The role of any constructor is to ensure that the object is correctly initialized. For Vec
objects, we need to initialize data and limit . Doing so involves allocating space to
hold the elements of the Vec and initializing those elements to an appropriate value. In
the case of the default constructor, we want to create an empty Vec , so we need not
allocate any space. For the constructors that take a size, we will allocate the given
amount of storage. If the user gives us an initial value as well as a size, we'll use that
value to initialize all the elements that we allocated. If the user gives us just a size, then
we'll use the default constructor for T to obtain a value to use in initializing the
elements. For now, we'll forward to our as yet unwritten memory-management functions
the job of initializing data and limit , and the related work of allocating and initializing
the elements:
template class Vec {
public:
Vec() { create(); }
explicit Vec(size_type n, const T& val = T()) { create(n, val); }
// remaining interface
private:
T* data;
T* limit;
};
The default constructor, which is the one that takes no arguments, needs to indicate
that the Vec is empty (i.e., that it has no elements). It does so by calling a member
called
create , that we'll have to write. When we return from create , we intend for both
data and limit to be set to zero.
Our second constructor uses a keyword that we have not yet seen, explicit, which we
will explain shortly. First, let's understand what the constructor does. Note that it uses a
default argument (§7.3/127) for the second parameter. Thus, the constructor effectively
defines two constructors: One takes a single argument of type size_type ; the other
takes a size_type and a const T& . In both cases we call a version of create that
takes a size and a value. We'll assume that this function, which we'll write in §11.5/203,
will allocate enough memory to hold n objects of type T , and will give those elements
the initial value specified by val . Our users will have supplied that value explicitly, or
else the default constructor for T will have generated it using the rules outlined in
§9.5/164 for value-initialization.
Now let's understand a bit about the use of explicit . This keyword makes sense only
in the definition of a constructor that takes a single argument. When we say that a
constructor is explicit, we're saying that the compiler will use the constructor only in
contexts in which the user expressly invokes the constructor, and not otherwise:
Vec vi(100); // ok, explicitly construct the Vec from an int
Vec vi = 100; // error: implicitly construct the Vec (§11.3.3/199) and copy it to Use of explicit can be crucial in other contexts in which a constructor might be used,
so we will discuss it further in §12.2/213. For consistency with the standard vector
class, we have made this constructor explicit even though none of our subsequent
examples in this chapter relies on this fact.
11.2.3 Type definitions
Following the convention used by the standard template classes, we want to provide
type names that our users can use, and that will hide the implementation details of how
we implement our class. Specifically, we need to provide typedef s for the const and
nonconst iterator types, and for the type that we use to denote the size of a Vec .
It turns out that the library containers also define another type, named value_type ,
that is a synonym for type of the objects that the container stores. Looking ahead, we
will want to add push_back to class Vec , so that users can dynamically grow their Vec
objects. If we also define value_type , then users will be able to use back_inserter
(which depends on both push_back and value_type ) to generate an output iterator
that can grow the Vec .
The only hard part of defining these types is deciding what types to choose. As we've
seen, iterators are objects that navigate among the objects in a container and let us
examine their values. Often iterators are themselves of class type. For example,
consider a class that implements a linked list. A logical strategy for such a class would
be to model a list as a set of nodes, where each node contains a value and a pointer to
the next node in the list. The iterator for such a class would contain a pointer to one of
these nodes and would implement the ++ operator to follow the pointer to the next node
in the list. Such an iterator would have to be implemented as a class type.
Because we used an array to hold the elements of a Vec , we can use plain pointers as
our Vec iterator type. Each such pointer will point into the underlying data array. As we
learned in §10.1/169, pointers support all the random-access-iterator operations. By
using a pointer as our underlying iterator type, we will provide full random-access
properties, which is consistent with the standard vector class.
What about the other types? The type of value_type is obvious: It must be T . But,
what about the type that represents the size? We know that size_t is big enough to
hold the number of elements in any array. Because we are storing Vec s in an array, we
can use size_t as the underlying type for Vec::size_type . These decisions give us
a class that now looks like
template class Vec {
public:
typedef T* iterator; // added
typedef const T* const_iterator; // added
typedef size_t size_type; // added
typedef T value_type; // added
Vec() { create(); }
explicit Vec(size_type n, const T& val = T()) { create(n, val); }
// remaining interface
private:
iterator data; // changed
iterator limit; // changed
};
In addition to adding the appropriate typedef s, we've also updated the class to use
our new types. By using the same names inside the class that we defined with our
typedef declarations, we make our code more readable and ensure the code will not
get out of sync if we subsequently change one of these types.
11.2.4 Index and size
We said that our users should be able to call size to find out how many elements are in
the Vec and to use the index operator to access the elements in the Vec . For example,
for (i = 0; i != vs.size(); ++i)
cout << vs[i].name();
From this usage we can see that the size function should be a member of class Vec ,
and that we'll need to define what it means to use the subscript operator, [] , on a Vec
. The size function is easiest: It takes no argument and should return the number of
elements in the Vec as a Vec::size_type . Before we define the index operation, we
need to know a bit more about how overloaded operators work.
We define an overloaded operator much as we define any other function: It has a name,
takes arguments, and specifies a return type.
We form the name of an overloaded operator by appending the operator symbol to the
word operator. Thus, the function we need to define will be called operator[] .
The kind of operator—whether it is a unary or binary operator—is part of what
determines how many parameters the corresponding function has. It the operator is a
function that is not a member, then the function has as many arguments as the
operator has operands. The first argument is bound to the left operand; the second is
bound to the right operand. If the operator is defined as a member function, then its left
operand is implicitly bound to the object on which the operator is invoked. Member
operator functions, therefore, take one less argument than the operator indicates.
In general, operator functions may be member or nonmember functions. However, the
index operator is one of a handful of operations that must be member functions. When
our user writes an expression such as vs[i] , that expression will call the member
named operator[] of vs , passing i as its argument.
We know that the operand must be an integral type large enough to denote the last
element in the largest possible Vec , and that this type is Vec::size_type . What
remains is to decide what type the index operator should return.
If we give it a bit of thought, we'll conclude that we should return a reference to the
element stored in the Vec . Doing so will allow users to write through the index as well
as read from it. Although our sample program uses the index only to read a value from
vs , it is reasonable to expect that users will also want write access to the elements.
With this analysis complete, we can update our class appropriately:
template class Vec {
public:
typedef T* iterator;
typedef const T* const_iterator;
typedef size_t size_type;
typedef T value_type;
Vec() { create(); }
explicit Vec(size_type n, const T& val = T()) { create(n, val); }
// new operations: size and index
size_type size() const { return limit - data; }
T& operator[](size_type i) { return data[i]; }
const T& operator[](size_type i) const { return data[i]; }
private:
iterator data;
iterator limit;
};
The size function calculates the number of elements in the Vec by subtracting the
pointers that delimit the array that holds our values. Remember from §10.1.4/175 that
subtracting two pointers yields the number of elements apart the locations are to which
the pointers refer (a value of type ptrdiff_t ). Returning that value from the size
function converts it to size_type , the function's return type, which is a synonym for
size_t (§10.1.3/174). Taking the size of a Vec doesn't change the Vec , so we
declare size as a const member function. Doing so lets us take the size of a const
Vec .
The index operator finds the corresponding position in the underlying array and returns
a reference to the element. By returning a reference, we allow the user to use the index
operation to change the values that are stored in the Vec . This ability to write to the
element implies that we need two versions: one for const Vec objects, the other for
nonconst objects of type Vec . Note that the const version returns a reference to
const . Doing so ensures that users may use the index only to read the Vec , not to
write to it. It is worth noting that we still return a reference, rather than returning a
value, for consistency with the standard vector . The reason to return a reference is
that if the objects stored in the container are large, it is more efficient to avoid copying
them.
It may be surprising that we can overload the index operator, because it appears that
both argument lists are the same; each appears to take a single parameter of type
size_type . However, every member function, including each of these operators, takes
an implicit parameter, which is the object on which it operates. Because the operations
differ regarding whether that object is const , we can overload the operation.
11.2.5 Operations that return iterators
Next, we need to consider the functions that return iterators. Our specification called for
us to implement the begin and end operations, which return an iterator positioned at
the beginning and one past the end of the Vec respectively:
template class Vec {
public:
typedef T* iterator;
typedef const T* const_iterator;
typedef size_t size_type;
typedef T value_type;
Vec() { create(); }
explicit Vec(size_type n, const T& val = T()) { create(n, val); }
T& operator[](size_type i) { return data[i]; }
const T& operator[](size_type i) const { return data[i]; }
size_type size() const { return limit - data; }
// new functions to return iterators
iterator begin() { return data; } // added
const_iterator begin() const { return data; } // added
iterator end() { return limit; } // added
const_iterator end() const { return limit; } // added
private:
iterator data;
iterator limit;
};
We offer two versions of the begin and end operations, which are overloaded based on
whether the Vec is const . The const versions return const_iterator s, so our
users will be able to read but not modify the Vec elements through the iterator.
At this point, our Vec class is still pretty basic, but the essentials are in place. In fact, if
we add only a few more operations, such as push_back and clear , we could use this
class instead of the standard vector for all the examples in this book. Unfortunately,
though, our Vec class fails to meet the vector specification in some critically important
ways, which we must now address.
11.3 Copy control
In the introduction to this chapter, we said that the class author controls what happens
when objects are created, copied, assigned, and destroyed. We've explained how to
create objects, but not how to control what happens when they are copied, assigned, or
destroyed. As we'll see, if we fail to define these operations, the compiler will synthesize
definitions for us. These synthesized operations are sometimes exactly what we need.
The rest of the time, the synthesized operations can lead to counterintuitive behavior,
and even to run-time failures.
C++ is the only language in widespread use that gives the programmer this level of
control over an object's behavior. Not surprisingly, getting these operations correct is
essential in building useful data types.
11.3.1 Copy constructor
Passing an object by value to a function, or returning an object by value from a function,
implicitly copies the object. For example,
vector vi;
double d;
d = median(vi); // copy vi into the parameter in median
string line;
vector words = split(line); // copy the return from split into words
Similarly, we can explicitly copy an object by using it to initialize another object:
vector vs;
vector v2 = vs; // copy vs into v2
Both explicit and implicit copies are controlled by a special constructor called the copy
constructor . Like other constructors, the copy constructor is a member function with
the same name as the name of the class. Because it exists to initialize a new object as a
copy of an existing object of the same type, it follows that the copy constructor takes a
single argument that has the same type as the class itself. Because we are defining
what it means to make a copy, including making copies of function arguments, this is
one case when it is essential that the parameter be a reference type! Furthermore,
copying an object should not change the object being copied from, so the constructor
takes a const reference to the object from which to copy:
template class Vec {
public:
Vec (const Vec& v); // copy constructor
// as before
};
Having declared the copy constructor, we have to figure out what it should do. In
general, copy constructors "copy" each data element from an existing object into the
new object. We say "copy" because sometimes copying involves more than just copying
the contents of a data element. For example, in our Vec class, we have two data
elements, both of which are pointers. If we copy the values of the pointers, then the
original and the copy will both point to the same underlying data. For example, assume
that v is a Vec , and that we want to copy v into v2 . If we copied the pointers, then
what we'd have is
Clearly, any change made to an element of one "copy" would result in changing the
value of the element of the other "copy" as well. That is, if we assigned a value to v[0]
, doing so would also change v2[0] . Is this behavior what we want?
As with other operations, we can answer this question by seeing what the standard
vector does. Recall the discussion in §4.1.1/53, in which we noted that we needed to
pass the vector to the median function by value so that the vector would be copied.
Making a copy ensured that changes made inside median would not propagate out of
the function. This analysis, and the behavior that we observe when we run the median
function, indicates that the standard vector class does not share the same underlying
storage once a copy is made. Instead, it arranges that each copy of a vector is
independent, so that changes to one are not reflected in the other:
Evidently, when we copy a Vec , we'll need to allocate new space and copy the contents
from the source into the newly allocated destination. As before, we'll assume that one of
our utility functions will handle the allocation and copy so that the copy constructor can
forward its work to that function:
template class Vec {
public:
Vec(const Vec& v) { create(v.begin(), v.end()); }
// as before
};
When we get around to writing it, the function will be yet another version of create —
this one taking a pair of iterators (i.e., pointers) and initializing the elements being
created from the elements in the range bounded by those pointers.
11.3.2 Assignment
Just as a class definition specifies what happens when objects of that class are copied,
the class definition also controls the behavior of the assignment operator . Although a
class may define several instances of the assignment operator—overloaded, as usual, by
differing types for its argument—the version that takes a const reference to the class
itself is special: It defines what it means to assign one value of the class type to
another. This version is typically called "the assignment operator," even if the class
defines several other versions of the operator= function. The assignment operator, like
the index operator, must be a member of the class. As with any other operator, the
assignment operator has a return value, and so it must define a return type. For
consistency with the built-in assignment operators, we return a reference to the lefthand
side:
template class Vec {
public:
Vec& operator=(const Vec&);
// as before
};
Assignment differs from the copy constructor in that assignment always involves
obliterating an existing value (the left-hand side) and replacing it with a new value (the
right- hand side). When we make a copy, we are creating a new object for the first time,
so there is no preexisting object to deallocate. Like the copy constructor, assignment
usually involves assigning each of the data values. Data members that are pointers
present the same issues for assignment as they did for copying. We'll want assignment
to ensure that each object has its own copy of the data from the right-hand side.
There's one last detail to consider before writing the code, and that is self-assignment.
It is possible that a user might wind up assigning an object to itself. As we shall see, it is
crucial that assignment operators deal correctly with self-assignment:
template
Vec& Vec::operator=(const Vec& rhs)
{
// check for self-assignment
if (&rhs != this) {
// free the array in the left-hand side
uncreate();
// copy elements from the right-hand to the left-hand side
create(rhs.begin(), rhs.end());
}
return *this;
}
This function uses a couple of new concepts, which we'll need to explain.
First is the syntax that we use to define a template member function outside the class
header. As with any template, we begin by signaling to the compiler that we are defining
a template, and naming the template parameters. Next comes the return type, which in
this case is Vec& . If we compare this definition with the corresponding declaration
in the header file, we'll see that we said the function returned a Vec& . We did not
explicitly name the type parameter in the return type. As a bit of syntactic sugar, the
language allows us to omit the type parameters when we are within the scope of the
template. Thus, inside the header file, we need not repeat because the template
parameter is implicit. When we name the return type, we are outside the scope of the
class, so we must explicitly state the template parameters, if any. Similarly, the name of
the function is Vec::operator= , not simply Vec::operator= . However, once
we have specified that it is a member of Vec that we are defining, we need no
longer repeat the template qualifiers. Hence, the argument is simply const Vec& ,
although we could have written the redundant const Vec& .
The other new aspect of this function is the use of a new keyword, this. The this
keyword is valid only inside a member function, where it denotes a pointer to the object
on which the member function is operating. For example, inside Vec::operator= , the
type of this is Vec* , because this is a pointer to the Vec object of which
operator= is a member. For a binary operator, such as assignment, this is bound to
the left-hand operand. Ordinarily, this is used when we need to refer to the object
itself, as we do here both in the initial if test and in the return.
We use this to determine whether the right- and left-hand sides of the assignment
refer to the same object. If they do, then they will have the same address. As we saw in
§10.1.1/170, &rhs yields a pointer that is the address of rhs . We explicitly test for
self-assignment by comparing that pointer and this , which points to the left-hand side.
If the objects are the same, then there's nothing to do in the assignment operator, and
we immediately fall through to the return statement. If the objects are different, we
need to free the old space and assign new values to each data element, copying the
contents from the right-hand side to the newly allocated array. Evidently, we will need
to write another of our utility functions, uncreate , which will destroy the elements
that had been in this Vec , and will free the storage that it had consumed. Once we call
uncreate to obliterate the old values, we can use the version of create that copies
from an existing Vec to allocate new space and copy the values from the right-hand to
the left-hand side.
It is crucially important that assignment operators correctly handle self-assignment,
which we did here by explicitly checking whether the left- and right-hand operands are
the same object. To see the importance, consider what would happen if we were to
remove this test from our assignment operator. In that case, we would always
uncreate the existing array from the left-hand operand, destroying the elements and
returning the space that had been used. However, if the two operands were the same
object, then the right operand would refer to this same space. Had we used the
elements from the right operand to create a new array for the left operand, the result
would have been a disaster: When we freed the space held by the left operand, we
would also have freed the space for the right-hand object. When create attempted to
copy the elements from rhs , those elements would have been destroyed and the
memory returned to the system.
Although it is most common to handle self-assignment through a direct check, such as
we did here, it is not universal, nor is it always the best approach. The important point is
to handle self-assignment correctly. How to do so is a matter of tactics.
The last interesting bit is the return statement, which dereferences this to obtain the
object to which it points. We then return a reference to that object. As usual in returning
a reference, it is crucial that the object to which the reference refers persist after the
function has returned. Returning a reference to a local object ensures disaster: The
referenced object will have gone away when the function returns, resulting in a
reference to garbage.
In the case of the assignment operator, we are returning a reference to the object that
is the left-hand side of the expression. That object exists outside the scope of the
assignment operator and hence is guaranteed to be around even after the function
returns.
11.3.3 Assignment is not initialization
Experience leads us to believe that the difference between assignment and initialization
is one of the trickier aspects of learning C++ well. Many programming languages, C
notably among them, do not expose the distinction, so programmers often are unaware
of the difference. The fact that the = symbol can be involved in both initialization and
assignment can make the distinction harder to grasp. When we use = to give an initial
value to a variable, we are invoking the copy constructor. When we use it in an
assignment expression, we're calling operator= . Class authors must be attuned to the
difference in order to implement the right semantics.
The key difference stems from two observations: Assignment (operator= ) always
obliterates a previous value; initialization never does so. Rather, initialization involves
creating a new object and giving it a value at the same time. Initialization happens
In variable declarations
For function parameters on entry to a function
For the return value of a function on return from the function
In constructor initializers
Assignment happens only when using the = operator in an expression. For example:
string url_ch = "~;/?:@=&$-_.+!*'()," // initialization
string spaces(url_ch.size(), ' ') ; // initialization
string y; // initialization
y = url_ch; // assignment
The first declaration creates a new object. Therefore, we know that we are initializing
that object, and hence that we will be invoking a constructor. The syntax
string url_ch = "~;/?:@=&$-_.+!*'(),"
says to create a string from the const char* that represents the string literal
"~;/?:@=&$-_.+!*'()," . To do so, the compiler will call the string constructor
that takes a const char* . That constructor can construct url_ch directly from the
string literal, or it can construct an unnamed temporary variable from the string literal,
and then call the copy constructor to construct url_ch as a copy of that temporary
variable.
The second declaration shows another form of initialization: giving one or more
constructor arguments directly. The compiler will call whichever constructor is most
appropriate for the number of arguments and their types. In this example, it will use the
string constructor that takes two arguments. The first argument says how many
characters the variable spaces is to have; the second tells what value to give each of
those characters. The effect will be to define spaces as having the same number of
characters as url_ch , with all of the characters being blank.
The third declaration is easier: We're invoking the default constructor to create an
empty string . The last statement is not a declaration at all. Instead, it is using the =
operator as part of an expression; hence, it is an assignment. That assignment will be
accomplished by running the string assignment operator.
A slightly more complicated example involves function arguments and return values. For
example, assume that line holds a line of input, and we call split from §6.1.1/103:
vector split(const string&); // function declaration
vector v; // initialization
v = split(line); // on entry, initialization of split's parameter from line;
// on exit, both initialization of the return value
// and assignment to v
The declaration of split is interesting because it defines a return type that is a class
type. Assigning a class type return value from a function is a two-step process: First, the
copy constructor is run to copy the return value into a temporary at the call site. Then
the assignment operator is run to assign the value of that temporary to the left-hand
operand. The distinction between initialization and assignment is important because
each one causes different operations to run:
Constructors always control initialization.
The operator= member function always controls assignment.
11.3.4 Destructor
We must still provide one more operation, which defines what happens when a Vec
object is destroyed. An object that is created in a local scope is destroyed as soon as it
goes out of scope; a dynamically allocated object is destroyed when we delete a
pointer to the object. For example, consider the split function from §6.1.1/103:
vector split(const string& str) {
vector ret;
// split str into words and store in ret
return ret;
}
When we return from split , the local variable ret goes out of scope and is destroyed.
Just as with copy and assignment, it is up to the class to say what happens when
objects are destroyed. Like constructors, which say how to create objects, there is a
special member function, called a destructor , that controls what happens when objects
of the type are destroyed. Destructors have the same name as the name of the class
prefixed by a tilde(~ ). Destructors take no arguments and have no return value.
The work of the destructor is to do any cleanup that should be done whenever an object
goes away. Typically, this cleanup involves releasing resources, such as memory, that
the constructor has allocated:
template class Vec {
public:
~Vec() { uncreate(); }
// as before
};
For Vec s, we allocate memory in the constructors, and so we must free it in the
destructor. This job is similar to what the assignment operator does to obliterate the old
left-hand side. Not surprisingly, we can call the same utility function from the
destructor, with the aim of destroying the elements and freeing the space that they
occupied.
11.3.5 Default operations
Some classes, such as the Student_info types that we defined in Chapters 4 and 9,
do not explicitly define a copy constructor, assignment operator, or destructor. A logical
question is: What happens when objects of such types are created, copied, assigned,
and destroyed? The answer is that if the class author does not specify these operations,
the compiler synthesizes default versions of the unspecified operations.
The default versions are defined to operate recursively—copying, assigning, or
destroying each data element according to the appropriate rules for the type of that
element. Members that are of class type are copied, assigned, or destroyed by calling
the copy constructor, assignment operator, or destructor for the data element. Members
that are of built-in type are copied or assigned by copying or assigning their value. The
destructor for built-in types has no work to do—even if the type is a pointer. In
particular, destroying a pointer through the default destructor does not free the space at
which the pointer points.
Now we can understand how the default Student_info operations execute. For
example, the copy constructor copies four data elements. To do so, it invokes the
string and vector copy constructors to copy the name and homework members
respectively. It copies the two double values, midterm and final , directly.
Finally, as we saw in §9.5/164, there is a default for the default constructor. If the class
defines no constructors at all, then the compiler will synthesize the default constructor,
which is the constructor that has no parameters. The synthesized default constructor
recursively initializes each data member in the same way as the object itself is
initialized: If the context requires default-initialization, it will default-initialize the data
members; if the context requires value-initialization, it will value-initialize the data
members.
It is important to note that if a class defines any constructor explicitly, even a copy
constructor, then the compiler will not synthesize a default constructor for that class.
Default constructors are essential in several contexts: One such context is in the
synthesized default constructor itself. In order to be used as a data member of a class
that relies on the synthesized default constructor, the data type must itself provide a
default constructor. Therefore, it is usually a good idea to give a class a default
constructor, either explicitly, as we did in Chapter 9, or implicitly, as we did in Chapter
4.
11.3.6 The rule of three
Classes that manage resources such as memory require close attention to copy control.
In general, the default operations will not suffice for such classes. Failure to control
every copy can confuse users of the class and often will lead to run-time errors.
Consider our Vec class, but pretend that we did not define the copy constructor,
assignment operator, or destructor. As we saw in §11.3.1/195, at best we will surprise
our users. Users of Vec will almost surely expect that once they've copied one Vec into
another, the two objects will be distinct. They will expect that operations on one Vec will
not have any effect on the data held by the other.
Even worse, though, is that if we do not define a destructor, then the default destructor
will be used. That destructor will destroy the pointer, but destroying a pointer does not
free the space to which it points. The result will be a memory leak: The space consumed
by Vec s will never be reclaimed.
If we fix the leak by providing a destructor, but we do not also add the copy constructor
and assignment operator, then we set things up so that a crash is likely. In such a
flawed implementation, it would be possible for two Vec s to share the same underlying
storage, as we illustrated in the first diagram in §11.3.1/196. When one of those objects
is destroyed, the destructor will destroy that shared storage. Any subsequent reference
through the undestroyed copy will lead to disaster.
Classes that allocate resources in their constructors require that every copy deal
correctly with those resources. Such classes almost surely need a destructor to free the
resources. If the class needs a destructor, it almost surely needs a copy constructor, as
well as an assignment operator. Copying or assigning objects of classes that allocate
resources usually allocates those resources in the same way that creating an object
from scratch does. To control every copy of objects of class T , you need
T::T(); one or more constructors, perhaps with arguments
T::~T() the destructor
T::T(const T&) the copy constructor
T::operator=(const T&) the assignment operator
Once we have defined these operations, the compiler will invoke them whenever an
object of our type is created, copied, assigned, or destroyed. Remember that objects
may be created, copied, or destroyed implicitly. Whether implicitly or explicitly, the
compiler will invoke the appropriate operation.
Because the copy constructor, destructor, and assignment operator are so tightly
coupled, the relationship among them has become known as the rule of three : If your
class needs a destructor, it probably needs a copy constructor and an assignment
operator too.
11.4 Dynamic Vecs
Before we implement our memory management functions, we should realize that our
Vec s are inferior to standard vector s in an important way: We do not provide a
push_back operation, and so our Vec s are of fixed size. Remember that push_back
pushes its argument onto the back of the vector and, in the process, increases the size
of the vector by one element.
We could add a push_back function that allocated new space to hold one more element
than the current Vec holds. We'd have to copy the existing elements into this new
space, constructing a new last element from the argument to push_back . We can see
that this strategy would be expensive if our users made many calls to push_back .
There is a classic approach to solving a problem such as this one: Allocate more storage
than we need. Only when we exhaust the preallocated storage will we go back for more.
For simplicity, whenever push_back needs to get more space, we'll allocate twice as
much storage as we currently use. So, if we create a Vec with 100 elements, and then
call push_back for the first time, it will allocate enough space to hold 200 elements. It
will copy the existing 100 elements into the first half of the newly allocated space and
construct a new, last element at the end of that sequence. The next 99 calls to
push_back will be satisfied without having to go back for more memory.
This strategy implies that we'll need to change how we keep track of the array that holds
our elements. We'll still need to keep track of the first element, but now we'll need two
"end" pointers. One will point (one past) the last constructed element, which,
equivalently, is a pointer to the first available element. The other pointer will point (one
past) the last allocated element. So, our Vec objects will now look like
Fortunately, only push_back and our memory management functions, which we have
not yet written, will need to know about this new member. Moreover, push_back itself
is pretty simple; it forwards the hard work to two of our memory-management
functions, named grow and unchecked_append , which we shall eventually have to
write.
template class Vec {
public:
void push_back(const T& val) {
if (avail == limit) // get space if needed
grow();
unchecked_append(val) ; // append the new element
}
private:
iterator data; // as before, pointer to the first element in the Vec
iterator avail; // pointer to (one past) the last constructed element
iterator limit; // now points to (one past) the last available element
// rest of the class interface and implementation as before
};
11.5 Flexible memory management
When we wrote our Vec class, we noted that we did not want to use the built-in new
and delete operations to manage our memory. The reason is that if we relied on these
operations, our Vec would be more restrictive than the standard vector. The new
operator does too much for our purposes: It both allocates and initializes memory. When
used to allocate an array of type T, it needs the default constructor for T. This approach
prevents us from offering our users as much flexibility as we would like to offer.
Using new would also be unduly expensive. If we use new, it always initializes every
element of a T array by using T::T(). If we wanted to initialize the Vec elements
ourselves, we would have to initialize each element twice—once by new, and again to
install the value that our user supplied. Even worse, consider the allocation strategy that
we propose to use for push_back. This strategy implies that we'll double the size of the
Vec each time we need to get more storage. We have no reason to want the extra
elements initialized. They'll be used only by push_back, which will use the space only
when we have a new element to construct in that space. If we used new to allocate the
underlying array, these elements would be initialized regardless of whether we ever use
them.
Instead of using the built-in new and delete operators, we can do better by using
standard-library facilities designed to support flexible memory management. The core
language itself does not have any notion of memory allocation, because the properties of
memory are too variable to wire into the language itself.
For example, modern computers have many kinds of memory. There may be many
different speeds of memory on the machine. There may be memory with special
properties, such as graphical buffers or shared memory. There may be memory that is
persistent across power failures. Because users might want to allocate any of these (or
other) kinds of memory, it is best left to the library to specify how we allocate and
manage memory. The standard library isn't required to support all these kinds of
memory either. However, by offering a library facility to manage memory, the standard
also offers a uniform interface to memory managers, even if those managers
themselves are tied to specific implementations. As with the decision to make inputoutput
a library rather than a language facility, the decision to make memory
management part of the library gives us greater flexibility in using these different kinds
of memory.
The header provides a class, called allocator, that allocates a block of
uninitialized memory that is intended to contain objects of type T, and returns a pointer
to the initial element of that memory. Such pointers are dangerous, because their type
suggests that they point to objects, but the memory doesn't really contain those objects
yet. The library also provides a way to construct objects in that memory, and to destroy
the objects again—all without deallocating the memory itself. It is up to the programmer
using the allocator class to keep track of which space holds constructed objects and
which space is still uninitialized.
The interesting part of the allocator class, for our purposes, comprises four member
functions and two associated nonmember functions:
template class allocator {
public:
T* allocate(size_t);
void deallocate(T*, size_t);
void construct(T*, const T&) ;
void destroy(T*);
// ...
};
template void uninitialized_fill(Out, Out, const T&);
template Out uninitialized_copy(In, In, Out);
The allocate member allocates typed but uninitialized storage sufficient to hold the
requested number of elements. By typed storage, we mean that it will be used to hold
values of type T, and that we will use pointers of type T* to address it. By uninitialized
storage, we mean that the memory is raw storage, and no objects have yet been
constructed in it. The deallocate member frees this uninitialized storage. It takes a
pointer to storage that was allocated by allocate, and a size that indicates how many
elements were allocated. The construct and destroy members construct or destroy a
single object in this uninitialized space. We call construct, passing it a pointer into
space that was allocated by allocate, and a value to copy into that space. The
destroy function runs the destructor for T on the element indicated by its argument.
The two companion functions that we need are named uninitialized_copy and
uninitialized_fill. These functions initialize elements in space that is allocated by
allocate. The uninitialized_fill function fills this uninitialized space with the
specified value. When the function completes, each of the elements in the range
specified by the first two arguments will be a newly constructed copy of the value
specified in the third argument. The uninitialized_copy function operates like the
library copy function, in that it copies values from a sequence specified by its first two
arguments into a target sequence denoted by its third argument. Like
uninitialized_fill, it assumes that the target range contains raw storage, rather
than elements that already hold values, and it constructs new objects in that memory.
As with any template, the actual type bound to T will be instantiated at compile time.
This instantiation will generate an appropriate allocator class for each type that uses
the class. In order to obtain an allocator of the right type, we'll add to our Vec
class an allocator member that will know how to allocate objects of type T. By
adding this member, and using its associated library functions, we can provide the same
kind of efficient, flexible memory management as the standard vector class provides.
11.5.1 The final Vec class
Our complete Vec class, including declarations but not definitions for the memory
management functions, now looks like
template class Vec {
public:
typedef T* iterator;
typedef const T* const_iterator;
typedef size_t size_type;
typedef T value_type;
Vec() { create(); }
explicit Vec(size_type n, const T& t = T()) { create(n, t); }
Vec(const Vec& v) { create(v.begin(), v.end()); }
Vec& operator=(const Vec&); // as defined in §11.3.2/196
~Vec() { uncreate(); }
T& operator[](size_type i) { return data[i]; }
const T& operator[](size_type i) const { return data[i]; }
void push_back(const T& t) {
if (avail == limit)
grow();
unchecked_append(t);
}
size_type size() const { return avail - data; }
iterator begin() { return data; }
const_iterator begin() const { return data; }
iterator end() { return avail; }
const_iterator end() const { return avail; }
private:
iterator data; // first element in the Vec
iterator avail; // (one past) the last element in the Vec
iterator limit; // (one past) the allocated memory
// facilities for memory allocation
allocator alloc; // object to handle memory allocation
// allocate and initialize the underlying array
void create() ;
void create(size_type, const T&);
void create(const_iterator, const_iterator);
// destroy the elements in the array and free the memory
void uncreate();
// support functions for push_back
void grow();
void unchecked_append(const T&);
};
All that remains is to implement the private members that handle memory allocation.
As we write these members, our program will be easier to understand if we remember
that whenever we have a valid Vec object, four things are always true:
1. data points at our initial data element, if we have any, and is zero otherwise.
2.
3.
4.
1.
2. data <= avail <= limit.
3. Elements have been constructed in the range [data, avail).
4. Elements have not been constructed in the range [avail, limit).
We shall call these conditions the class invariant. Much as we did with loop invariants
in §2.3.2/20, we intend to establish the class invariant as soon as we construct an
object of that class. If we do so, and we ensure that none of our member functions
falsifies the class invariant, we can be assured that the invariant will always be true.
Note that none of the public members is capable of falsifying the invariant, because
the only way to do so would be to change the value of data, avail, or limit, and
none of those member functions does so.
We shall begin by looking at the various create functions, which are responsible for
allocating memory, initializing elements in that memory, and setting the pointers
appropriately. In each case, we initialize whatever memory is allocated and so, after
running create, the pointers limit and avail are always equal: The last constructed
element is the same as the last allocated element. You should verify for yourself that
the class invariant is true after we have executed any of the following functions:
template void Vec::create()
{
data = avail = limit = 0;
}
template void Vec::create(size_type n, const T& val)
{
data = alloc.allocate(n);
limit = avail = data + n;
uninitialized_fill(data, limit, val);
}
template
void Vec::create(const_iterator i, const_iterator j)
{
data = alloc.allocate(j - i);
limit = avail = uninitialized_copy(i, j, data);
}
The version of create that takes no arguments creates an empty Vec, so its job is to
ensure that the pointers start out with zero values.
The version that takes a size and a value uses the size to allocate the appropriate
amount of memory. The allocate member of class allocator allocates enough
memory to hold the specified number of objects of type T. Thus, alloc.allocate(n)
allocates enough space to hold n objects. The allocate function returns a pointer to
the initial element, which we store in data. The memory returned by allocate is
uninitialized, so we arrange to initialize it by calling uninitialized_fill, which
copies its third argument into the sequence of uninitialized elements specified by its first
two arguments. When the function completes, it will have constructed new elements in
the space obtained by allocate and will have initialized each of these elements to
val.
The final version of create operates similarly to the other two, except that it calls
uninitialized_copy to initialize the space obtained from allocate. That function
copies elements from the sequence denoted by its first two arguments into a target
sequence of uninitialized elements denoted by its third argument. It returns a pointer to
(one past) the last element that it initialized, which is exactly the value that we need for
limit and avail.
The uncreate member has to undo what the create members did: It must run the
destructors on the elements, and return the space that the Vec used:
template void Vec::uncreate()
{
if (data) {
// destroy (in reverse order) the elements that were constructed
iterator it = avail;
while (it != data)
alloc.destroy(--it);
// return all the space that was allocated
alloc.deallocate(data, limit - data);
}
// reset pointers to indicate that the Vec is empty again
data = limit = avail = 0;
}
If data is zero, there's no work to do. If we were using delete, we might not bother to
compare data to zero, knowing that executing delete on a zero pointer is harmless.
However, unlike delete, the alloc.deallocate function requires a nonzero pointer,
even if no memory is being freed. Therefore, we must check whether data is zero.
If we have work to do, we march the iterator it through the constructed elements of
the Vec, calling destroy to destroy each element. We go backward through the Vec to
match the behavior of delete[], which destroys elements in reverse order. Once we've
destroyed the elements, we free all the space in the call to deallocate. This function
takes a pointer to the first element of the memory to free, and an integral value that
indicates how many elements of type T are to be freed. Because we want to return all
the space that was allocated, we deallocate the space between data and limit.
What's left is to implement the members used by push_back:
template void Vec::grow()
{
// when growing, allocate twice as much space as currently in use
size_type new_size = max(2 * (limit - data), ptrdiff_t(1));
// allocate new space and copy existing elements to the new space
iterator new_data = alloc.allocate(new_size);
iterator new_avail = uninitialized_copy(data, avail, new_data);
// return the old space
uncreate();
// reset pointers to point to the newly allocated space
data = new_data;
avail = new_avail;
limit = data + new_size;
}
// assumes avail points at allocated, but uninitialized space
template void Vec::unchecked_append(const T& val)
{
alloc.construct(avail++, val);
}
The job of grow is to allocate enough space to hold at least another element. It
allocates more than it needs, so that subsequent calls to push_back can use the
excess, avoiding the overhead of frequent memory allocations. In §11.4/202, we said
that our strategy would be to double the amount of space for each new allocation. Of
course, the Vec might currently be empty, so we cater to this possibility by allocating
the max of one element and twice the existing space. Remembering from §8.1.3/142
that the two arguments to max must have exactly the same type, we explicitly construct
an object with value 1 of type ptrdiff_t, which we know from §10.1.4/175 is the type
of limit - data.
We start by remembering in new_size how many elements we will allocate. We
allocate the appropriate space, and then call uninitialized_copy to copy the
elements from the current space into the newly allocated space. We then return the old
memory, and destroy the elements there by calling uncreate. Finally, we reset the
pointers so that data points to the first element in the newly allocated array, limit
points to (one past) the last constructed element in the Vec, and avail points to (one
past) the last allocated but as yet uninitialized element.
Note that it is essential that we save the values returned by allocate and
uninitialized_copy. The reason is that if we used those values immediately to reset
data and limit, then the subsequent calls to uncreate would destroy and free the
memory that we just allocated, rather than getting rid of the old space!
The unchecked_append function builds an element in the first location after the
constructed elements. It assumes that avail points at space that was allocated, but
has not yet been used to hold a constructed element. Because we call
unchecked_append only immediately after a previous call to grow, we know that this
call is safe.
11.6 Details
Template classes can be formed using the template facility described in §8.1.1/140:
template
class class-name { ... } ;
creates a template class named class-name that depends on the given type
parameters. These type-parameter names may be used inside the template wherever a
type is required. In the scope of the class, the template class may be referred to without
qualification; outside the class scope, class-name must be qualified with the type
parameters:
template
Vec& Vec::operator=(const Vec&) { ... }
Users specify the actual types when creating objects of template types: Vec
causes the implementation to instantiate a version of Vec binding the type parameter to
int.
Copy control: In general, classes control what happens when objects are created,
copied, assigned, or destroyed. Constructors are invoked as a side effect of creating or
copying objects; the assignment operator is invoked in expressions involving
assignment; and the destructor is run automatically when objects are destroyed or go
out of scope.
Classes that allocate resources in a constructor almost invariably must define the copy
constructor, the assignment operator, and the destructor. When we write an assignment
operator, it is essential for us to check for self-assignment. For consistency with the
built-in assignment operators, it is good practice to return a reference to the left-hand
operand.
Synthesized operations: If a class defines no constructors, the compiler will synthesize
the default constructor. If the class does not explicitly define them, the compiler will
synthesize the copy constructor, assignment operator, and/or destructor. The
synthesized operations are defined recursively: Each synthesized operator recursively
applies the appropriate operation for the data members of the class.
Overloaded operators are defined by defining a function named operator op, where
op is the operator being defined. At least one parameter must be of class type. When an
operator function is a member of a class, its left-hand operand (if it is a binary operator)
or its only operand (if it is a unary operator) is bound to the object on which it is
invoked. The index operator and the assignment operator must be class members.
Exercises
11-0. Compile, execute, and test the programs in this chapter.
11-1. The Student_info structure that we defined in Chapter 9 did not define a copy
constructor, assignment operator, or destructor. Why not?
11-2. That structure did define a default constructor. Why?
11-3. What does the synthesized assignment operator for Student_info objects do?
11-4. How many members does the synthesized Student_info destructor destroy?
11-5. Instrument the Student_info class to count how often objects are created,
copied, assigned, and destroyed. Use this instrumented class to execute the student
record programs from Chapter 6. Using the instrumented Student_info class will let
you see how many copies the library algorithms are doing. Comparing the number of
copies will let you estimate what proportion of the cost differences we saw are
accounted for by the use of each library class. Do this instrumentation and analysis.
11-6. Add an operation to remove an element from a Vec and another to empty the
entire Vec. These should behave analogously to the erase and clear operations on
vectors.
11-7. Once you've added erase and clear to Vec, you can use that class instead of
vector in most of the earlier programs in this book. Rewrite the Student_info
programs from Chapter 9 and the programs that work with character pictures from
Chapter 5 to use Vecs instead of vectors.
11-8. Write a simplified version of the standard list class and its associated iterator.
11-9. The grow function in §11.5.1/208 doubles the amount of memory each time it
needs more. Estimate the efficiency gains of this strategy. Once you've predicted how
much of a difference it makes, change the grow function appropriately and measure the
difference.
12
Making class objects act like values
Objects of built-in types generally behave like values: Whenever we copy an object of
such a type, the original and the copy have the same value but are otherwise
independent. Subsequent changes to one object do not affect the other. We can create
objects of these types, pass them to and from functions, copy them, or assign them to
other objects.
For most of the built-in types, the language also defines a rich set of operators and
provides automatic conversions between logically similar types. For example, if we add
an int and a double, the compiler automatically converts the int into a double.
When we define our own classes, we control the extent to which the resulting objects
behave like values. We saw in Chapters 9 and 11 that the class author controls what
happens when objects are created, copied, assigned, and destroyed. By defining copying
and assignment appropriately, the class author can arrange for objects of that class to
act like values. That is, the class author can arrange for each object to have state that is
independent of any other object. Our Vec and Student_info classes are examples of
types that act like values.
In this chapter, we shall see that the class author can also control conversions and
related operations on class objects, thereby providing classes whose objects behave
even more similarly to objects of built-in types. The standard-library string class is a
good example of such a type because of its rich set of operators and support for
automatic conversions. Accordingly, in this chapter, we will define a simplified version of
string, called Str, much as we defined a simplified version of vector in Chapter 11.
We will focus on the operators and conversions that let us write expressions involving
strings. In this chapter, we will not concern ourselves with efficiency. Instead, in
Chapter 14, we will revisit Str to understand techniques for managing more efficiently
the storage associated with each Str object.
We do not need to worry much about the implementation details of our Str class,
because we did most of the work already when we implemented the Vec class.
Accordingly, most of the discussion in this chapter will revolve around how to design an
appropriate interface to our class.
12.1 A simple string class
Let's start by defining a Str class that lets us create objects that behave approximately
as we would like:
class Str {
public:
typedef Vec::size_type size_type;
// default constructor; create an empty Str
Str() { }
// create a Str containing n copies of c
Str(size_type n, char c): data(n, c) { }
// create a Str from a null-terminated array of char
Str(const char* cp) {
std::copy(cp, cp + std::strlen(cp), std::back_inserter(data));
}
// create a Str from the range denoted by iterators b and e
template Str(In b, In e) {
std::copy(b, e, std::back_inserter(data));
}
private:
Vec data;
};
Our class delegates the work of managing its data to the Vec class that we wrote in
Chapter 11. That class is almost good enough to support our Str; it lacks only the
clear function that Chapter 11 had left as an exercise.
The Str class has four constructors, each of which arranges to create data as an
appropriately initialized Vec object.
The default constructor for Str implicitly invokes the Vec default constructor to create
an empty Str. Note that because our class has other constructors, we must explicitly
define the default constructor, even though it does exactly what the synthesized default
constructor would have done. The other three constructors take values, which we use to
construct or initialize data.
The constructor that takes a size and a character uses the corresponding Vec
constructor to construct data. It has no further work to do, so the constructor body is
empty.
The last two constructors are similar to each other. Their constructor initializers are
empty, which means that data is implicitly initialized as an empty Vec. Each
constructor asks copy to append the supplied characters to the initially empty data. For
example, the constructor that takes a const char* uses strlen to determine the
length of the string. From this length, it computes two iterators that denote the input
characters, and asks copy and back_inserter to append those characters to data.
Thus, the constructor will cause data to contain copies of the characters in the array
denoted by cp.
The most interesting constructor is the final one, which takes two iterators and creates a
new Str that contains a copy of the characters in the given sequence. Like the previous
constructor, it relies on copy and back_inserter to append the values in the range of
[b, e) to data. What's interesting about this constructor is that it is itself a template
function. Because it is a template, it effectively defines a family of constructors that can
be instantiated for different types of iterators. For example, this constructor could be
used to create a Str from an array of characters, or from a Vec.
It is important to note that the class does not define a copy constructor, assignment
operator, or destructor. Why not?
The answer is that the defaults work. The Str class itself does no memory allocation. It
can leave the details of memory management to the synthesized operations, which call
the corresponding Vec operations. One way to see that the defaults work is to note that
the Str class does not need a destructor. Indeed, if it had one, there would be no work
for it to do. In general, a class that needs no destructor doesn't need an explicit copy
constructor or assignment operator either (§11.3.6/201).
12.2 Automatic conversions
So far, we have defined a set of constructors and implicitly defined copying, assignment,
and destruction. These operations give Str valuelike behavior: When we copy a Str
object the original and the copy will be independent of each other. Our next problem is
to think about conversions. Values of built-in type can often be converted automatically
from one type to another. For example, we can initialize a double from an int , and
we can also assign an int to a double :
double d = 10; // convert 10 to double and use the converted value to initialize d
double d2;
d2 = 10; // convert 10 to double and assign the converted value to d
In the case of our Str class, we have defined how to construct a Str from a const
char* , so we can write
Str s("hello"); // construct s
This definition constructs s by explicitly asking for the constructor that takes a const
char* argument. We would also like to be able to write
Str t = "hello"; // initialize t
s = "hello"; // assign a new value to s
Remember from §11.3.3/199 that the = symbol has two different meanings in this last
example. The first statement defines t , so the = indicates initialization. This form of
initialization always requires the copy constructor, which takes a const Str& as its
argument. The second statement is an expression statement, not a declaration, so the =
is an assignment operator. The only assignment operator that is relevant for Str objects
is the one that the compiler defined for us, which also expects a const Str& as its
argument. In other words, each statement in this second example uses a string literal,
which has type const char* , where a const Str& is expected.
We might think, therefore, that we need to give class Str an additional assignment
operator with a parameter of type const char* , and figure out how to overload the
copy constructor. Fortunately, it turns out that we do not need to do so, because there is
already a constructor that takes a const char* , and that constructor also acts as a
user-defined conversion . User-defined conversions say how to transform to and from
objects of class type. As with built-in conversions, the compiler will apply user-defined
conversions to convert a value into the type that is needed.
A class can define conversions in two ways: It can convert from other types to its type,
or from its type to other types. We'll discuss this second form of conversion in
§12.5/222. The more common conversion defines how to convert other types to the type
that we are defining. We do so by defining a constructor with a single argument.
Our Str class already has such a constructor, namely the one that takes a const
char* . Therefore, the compiler will use this constructor when an object of type Str is
needed and an object of type const char* is available. The assignment of a const
char* to a Str is exactly such a situation. When we write s = "hello" ; what really
happens is that the compiler uses the Str(const char*) constructor to create an
unnamed local temporary of type Str from the string literal. It then calls the
(synthesized) assignment operator of class Str to assign this temporary to s.
12.3 Str operations
If we think about the kind of code we've written that used strings, we can see that we
used several operators:
cin >> s // use the input operator to read a string
cout << s // use the output operator to write a string
s[i] // use the index operator to access a character
s1 + s2 // use the addition operator to concatenate two strings
All these are binary operators, so that if we define them as functions, each function will
have two parameters, one of which may be implicit if the function is a member. As we
saw in §11.2.4/192, names for overloaded operators are formed by appending the
operator symbol to the word operator. Hence, operator>> is the name of the
function that overloads the input operator, operator[] names the index operation, and
so on.
We may as well start with the index operator, because in §11.2.4/192, we've already
seen how to implement this operation, and we know that it must be a member of the
class:
class Str {
public:
// constructors as before
char& operator[](size_type i) { return data[i]; }
const char& operator[](size_type i) const { return data[i]; }
private:
Vec data;
};
The index operators just forward their work to the corresponding Vec operations. It is
worth noting that, as we did for class Vec, we define two versions of the index
operator—one that can operate on const objects and the other that cannot. By
returning a reference to the character, the nonconst version gives write access to the
character that it returns. The const version returns a reference to a const char,
thereby preventing the user from writing to the underlying character. We return const
char& instead of a plain char for consistency with the standard string class.
What about the other operators? The most interesting problem in defining these
functions is deciding whether these operations should be members of the Str class. It
turns out that answering this question involves different issues for each of these kinds of
operators. We will address input-output operators first; then, in §12.3.3/218, we'll look
at the concatenation operator.
12.3.1 Input-output operators
In §9.2.2/159, we had to decide whether compare should be a member of
Student_info. We suggested that one way to decide was to ask whether the operation
affects the state of the object. The input operator certainly changes its object's state.
After all, we use the input operator to read a new value into a preexisting object.
Accordingly, we might think that we should make the input operator a member of the
Str class. However, doing so won't work as we might expect.
To see why, we have to remember (§11.2.4/192) how the operands of an expression are
bound to the parameters of the overloaded operator function. For a binary operation, the
left operand is always bound to the first parameter, and the right operand is bound to
the second. In the case of member operator functions, the first parameter (the left
operand) is always the one that is passed implicitly to the member function. Thus,
cin >> s;
is equivalent to
cin.operator>>(s);
which calls the overloaded >> operator defined for the object cin. This behavior implies
that the >> operator must be a member of class istream.
Of course, we do not own the definition of istream, so we cannot add this operation to
it. If instead we make operator>> a member of Str, then our users would have to
invoke the operation on behalf of a Str:
s.operator>>(cin);
or, equivalently,
s >> cin;
which would flout the conventions used throughout the library. Therefore, we can
conclude that the input—and, by analogy, the output—operator must be a nonmember.
We can now update our Str class appropriately, by adding declarations for the input
and output operators to Str.h:
std::istream& operator>>(std::istream&, Str&); // added
std::ostream& operator<<(std::ostream&, const Str&) ; // added
The output operator is easy to write: It will iterate through the Str, writing a single
character at a time:
ostream& operator<<(ostream& os, const Str& s)
{
for (Str::size_type i = 0; i != s.size(); ++i)
os << s[i];
return os;
}
The only catch is that this usage forces us to give Str a size function:
class Str {
public:
size_type size() const { return data.size(); }
// as before
};
Despite the simple form of the output operator, we should understand it thoroughly.
Each time through the loop, we invoke the Str::operator[] to fetch a character to
write. That operator, in turn, calls Vec::operator[] to obtain the actual value from
the underlying vector. Similarly, each time through the loop, we determine the size of
our Str object by calling s.size(), which calls the size member of the underlying Vec
object to determine that object's size.
12.3.2 Friends
The input operator isn't much harder to write than the output operator. It needs to read
and remember characters from the input stream. Each time we call the input operator, it
should read and discard any leading whitespace, and then read and remember
characters until it hits whitespace or end-of-file. Our input operator is a bit simplified—it
ignores some subtleties of the input-output library that are beyond the scope of this
book—but it does what we need done:
// this code won't compile quite yet
istream& operator>>(istream& is, Str& s)
{
// obliterate existing value(s)
s.data.clear();
// read and discard leading whitespace
char c;
while (is.get(c) && isspace(c))
; // nothing to do, except testing the condition
// if still something to read, do so until next whitespace character
if(is) {
do s.data.push_back(c); // compile error!, data is private
while (is.get(c) && !isspace(c));
// if we read whitespace, then put it back on the stream
if (is)
is.unget();
}
return is;
}
First we'll explain this function; then we'll explain why it doesn't compile.
We start by discarding any previous value that data might have, because reading into a
Str should obliterate whatever data were present. Next we need to read characters,
one at a time, from the given stream, until we encounter a character that is not whitespace.
Because we need to be able to detect whether we just read a whitespace
character, we use the get function on the input stream. Unlike the overloaded >>
operators, which ignore whitespace, the get function reads and returns whatever
character is next in the stream, including whitespace. Therefore, the while loop reads
characters until it finds one that is not whitespace, or it runs out of input. If the
character we read was white- space, then there is nothing to do but read again, so the
body of the while is empty.
The if test checks whether we exited the while because we read a nonwhitespace
character or because we ran out of input. If the former, we want to read characters until
we hit whitespace again, appending each character that we read to data. We do so in
the next statement, which is a do while loop (§7.4.4/136) that arranges to append to
data the character that we had already read in the previous while loop, and then
continues reading until we run out of input or hit a whitespace character. Each time it
reads a nonwhitespace character, it uses push_back to append that character to data.
We could fall out of the do while either because we can no longer read from is, or
because we encountered a whitespace character. If the latter, we have read one
character too many, which we put back onto the input stream by calling is.unget().
The unget function undoes the most recent get by backspacing the input stream by one
character. After the call to unget, the stream behaves as if the previous get had never
been done.
As the comments indicate, this code fails to compile. The problem is that operator>>
is not a member of class Str, so it cannot access the data member of s. We faced a
similar problem in §9.3.1/161, when the compare function needed access to the name
member from Student_info objects. We solved that problem by adding an accessor
function. In this case, giving read access to data isn't enough: The input operator
needs to be able to write data, not just read it. The input operator is a part of our
general Str abstraction, so giving it write access to data is fine. On the other hand, we
do not want all users to have write access to data, so we cannot solve our problem by
adding a public member that would let operator>> (and therefore any user) write to
data.
Rather than adding a (public) access function, we can say that the input operator is a
friend of class Str. A friend has the same access rights as a member. By making the
input operator a friend, we can allow it, along with our member functions, to access the
private members of class Str:
class Str {
friend std::istream& operator>>(std::istream&, Str&);
// as before
};
We've added a friend declaration to class Str. This declaration says that the version
of operator>> that takes an istream& and a Str& may access the private
members of Str. Once we have added this declaration to Str, our input operator will
compile.
A friend declaration may appear at any point in the class definition: It makes no
difference whether it follows a private or public label. Because a friend function has
special access privileges, it is part of the interface to the class. Therefore, it makes
sense to group friend declarations at the beginning of the class definition, near the
public interface for the class.
12.3.3 Other binary operators
What remains of our work on the Str class is to implement the + operator. Before we
can do so, we must make several decisions: Should the operator be a member? What
are its operands' types? What type should it return? As we shall see, these questions
turn out to have subtle implications.
For now, let's make some initial guesses about the answers. First, we know that we want
to be able to concatenate values that are of type Str. Second, we can observe that
concatenation does not change the value of either operand. These facts suggest that
there is no particular reason to decide to make the operator a member function. Finally,
we know that we want to be able to chain several concatenations into a single
expression in order to allow expressions such as
s1 + s2 + s3
where s1, s2, and s3 all have type Str. This usage suggests that the operator should
return a Str.
These decisions imply that we should implement concatenation as a nonmember:
Str operator+(const Str&, const Str&);
Before we launch into implementation, a bit of thought might suggest that if we offer
operator+, we might want to provide our users with operator+= as well. That is,
we'd like to let our users assign to s the value obtained by concatenating s and s1 in
either of these forms:
s = s + s1;
s += s1;
It turns out that the most convenient way to implement operator+ is to implement
operator+= first. Unlike the simple concatenation operator, the compound version
changes its left operand, so we make it a member of the Str class. After adding
definitions for the new concatenation operations, our final Str class looks like this:
class Str {
// input operator implemented in §12.3.2/216
friend std::istream& operator>>(std::istream&, Str&);
public:
Str& operator+=(const Str& s) {
std::copy(s.data.begin(), s.data.end(),
std::back_inserter(data));
return *this;
}
// as before
typedef Vec::size_type size_type;
Str() { }
Str(size_type n, char c): data(n, c) { }
Str(const char* cp) {
std::copy(cp, cp + std::strlen(cp), std::back_inserter(data));
}
template Str(In i, In j) {
std::copy(i, j, std::back_inserter(data));
}
char& operator[](size_type i) { return data[i]; }
const char& operator[](size_type i) const { return data[i]; }
size_type size() const { return data.size(); }
private:
Vec data;
};
// output operator implemented in §12.3:2/216
std::ostream& operator<<(std::ostream&, const Str&);
Str operator+(const Str&, const Str&);
Because we use a Vec for our underlying storage, implementing operator+= is trivial:
We call copy to append a copy of the right operand to the Vec that is the left operand.
As usual for assignment, we return a reference to the left object as our result. Now we
can implement operator+ in terms of operator+=:
Str operator+(const Str& s, const Str& t) {
Str r = s;
r += t;
return r;
}
Recall that concatenation is a nonmember function that will create a new Str. We
create this new Str by initializing a local variable named r to be a copy of s. That
initialization uses the Str copy constructor. Next, we invoke the += operator on r to
concatenate t, and then we return r (again through an implicit call to the copy
constructor) as the result.
12.3.4 Mixed-type expressions
We have defined the concatenation operator to take operands of type const Str&.
What about expressions that involve character pointers? For example, what if we wanted
to use our Str class to implement the program from §1.2/12? That program contained
code that looked like
const std::string greeting = "Hello, " + name + "!";
where name is a string. Analogously, we'd like to be able to write
const Str greeting = "Hello, " + name + "!";
where name is now a Str.
We know that the + operator is left-associative, which means that evaluating this
expression is equivalent to evaluating
"Hello, " + name
and applying the + operator to the result and "!". In other words, the expression is
equivalent to
("Hello, " + name) + "!"
By breaking down the expression into its components, we can see that we have two
different forms of +. In one case, we pass a string literal as the first operand and a Str
as the second. In the other, the left operand is a Str obtained as the result of a
concatenation, and the right operand is a string literal. Thus, in each case we are calling
+ on a const char* and a Str in some order.
In §12.3.3/218, we defined + with arguments of type Str, not const char*. However,
we know from §12.2/213 that by defining a constructor that takes a const char*, we
also defined a conversion operator from const char* to Str. Evidently, our Str class
handles these expressions already. In each case, the compiler will convert the const
char* argument to type Str, and then it will invoke operator+.
It is important to understand the implications of conversion operations. For example,
Str greeting = "Hello, " + name + "!";
gives greeting the same value as if we had written
Str temp1('Hello, "); // Str::Str(const char*)
Str temp2 = temp1 + name; // operator*(const Str&, const Str&)
Str temp3("!") // Str::Str(const char*)
Str greeting = temp2 + temp3; // operator*(const Str&, const Str&)
Seeing all these temporaries, we can imagine that this approach might be expensive. In
practice, because of the perceived cost of generating temporaries, commercial string
library implementations often take the more tedious route of defining specific versions of
the concatenation operator for every combination of operands, rather than relying on
automatic conversions.
12.3.5 Designing binary operators
It is important to appreciate the role of conversions in the design of binary operators. If
a class supports conversions, then it is usually good practice to define binary operators
as nonmember functions. By doing so, we preserve symmetry between the operands.
If an operator is a member of a class, then that operator's left operand cannot be the
result of an automatic conversion. The reason for this restriction is so that when a
programmer writes an expression such as x + y, the compiler does not have to
examine every type in the entire program to discover whether it is possible to convert x
to a type that has a member named operator+. Because of the restriction, the
compiler (and the programmer) has to look only at nonmember operator+ functions
and at operator+ functions that are members of the class of x.
The left operand of a nonmember operator, and the right operand of any operator,
follow the same rules as any ordinary function argument: The operand can be of any
type that can be converted to the parameter type. If we make the binary operator a
member function, we have introduced an asymmetry with respect to its operands: The
right operand can be the result of an automatic conversion, but the left operand cannot.
Such asymmetries are fine for intrinsically asymmetric operators such as +=, but in the
context of symmetric operands, they are confusing and error prone. It is almost always
desirable to treat both operands of such operators equivalently, which we can arrange
only by making the operator a nonmember function.
In the case of the assignment versions of binary operators, we want to constrain the left
operand to be of the class type. Otherwise, what would happen? If we allowed
conversions for the left operand, then we might convert that operand to the class type
and assign a new value to the resulting temporary. Because that value would be a
temporary object, once we completed the assignment we would have no way to access
the object to which we had just assigned! Therefore, like the assignment operator itself,
all of the compound- assignment operators should be members of the class.
12.4 Some conversions are hazardous
Recall that in §11.2.2/190, we defined as explicit the constructor that took a size.
Now t hat we know that constructors that take a single argument define conversions, we
can understand what happens when we make a constructor explicit: Doing so tells
the compiler to use that constructor only to construct objects explicitly. The compiler will
not use an explicit constructor to create objects implicitly by converting operands in
expressions or function calls.
To understand why explicit constructors might be useful, let's assume that we did
not declare the Vec constructor as explicit. Then we could implicitly build a Vec of a
given size. We could use this implicit conversion when calling a function, such as our
frame function from §5.8.1/93. Recall that that function takes a single parameter of
type const vector&, and produces a character picture that puts a frame
around the input vector. Suppose that frame used Vec instead of vector, that we
had not given Vec an explicit constructor, and that we were to execute
Vec p = frame(42);
What would happen? What should happen? More important, how could a user figure out
what will happen?
In this case, what would happen is that the user would get a framed picture with 42
empty rows. Was this behavior what the user intended? Isn't it more likely that the user
thought that the program would put a frame around the value 42? This kind of call is,
more likely than not, a mistake, and so our Vec class—and, for that matter, the
standard vector class—makes the constructor that takes an integer value explicit.
In general, it is useful to make explicit the constructors that define the structure of
the object being constructed, rather than its contents. Those constructors whose
arguments become part of the object usually should not be explicit.
As an example, the string and Str classes have constructors that take a single const
char* and are not explicit. Each constructor uses its const char* argument to
initialize the value of its object. Because the argument determines the value of the
resulting object, it is sensible to allow automatic conversions from a const char* in
expressions or function calls.
On the other hand, the vector and Vec constructors that take a single argument of
type Vec::size_type are explicit. These constructors use their argument value to
determine how many elements to allocate. The constructor argument determines the
structure of the object, but not its value.
12.5 Conversion operators
In §12.2/213, we saw that some constructors also define conversions. Class authors can
also define explicit conversion operators, which say how to convert an object from its
type to a target type. Conversion operators must be defined as members of a class. The
name of a conversion operator is operator followed by the target type name. Thus, if a
class has a member called operator double, that member says how to create a value
of type double from a value of the class type. For example,
class Student_info {
public:
operator double();
// ...
};
would say how to create a double from a Student_info object. The meaning of this
conversion would depend on the definition of the operator, which might be to convert
the object to its corresponding final grade. The compiler would use this conversion
operator any time we had an object of type Student_info but needed an object of
type double. So, for example, if vs were a vector, we could
calculate the average grade of all students as follows:
vector vs;
// fill up vs
double d = 0;
for (int i = 0; i != vs.size(); ++i)
d += vs[i]; // vs[i] is automatically converted to double
cout << "Average grade: " << d / vs.size() << endl;
Conversion operators are most often useful when converting from a class type to a builtin
type, but they can also be useful when converting to another class type for which we
do not own the code. In either case, we cannot add a constructor to the target type, so
we can define the conversion operator only as part of the class that we own.
In fact, we use this kind of conversion operator every time we write a loop that implicitly
tests the value of an istream. As we discussed in §3.1.1/39, we can use an istream
object where a condition is expected:
if (cin >> x) { /*...*/ }
which we saw was equivalent to
cin >> x;
if (cin) { /*...*/ }
We can now understand what happens in this expression.
As we know, the if tests a condition, which is an expression that yields a truth value.
The precise type of such a truth value is bool. Using a value of any arithmetic or
pointer type automatically converts the value to type bool, so we can use values of
these types as the expression in a condition. Of course, iostream is neither a pointer
nor an arithmetic type. However, the standard library defines a conversion from type
istream to void*, which is a pointer to void. It does so by defining
istream::operator void*, which tests various status flags to determine whether
the istream is valid, and returns either 0 or an implementation-defined nonzero void*
value to indicate the state of the stream.
We have not previously used the void* type. We said in §6.2.2/114 that the void type
could be used only in a few ways—the basis for a pointer being one of them. A pointer to
void is sometimes called a universal pointer, because it is a pointer that can point to
any type of object. Of course, you cannot dereference the pointer, because the type of
the object to yield isn't known. But one thing that can be done with a void* is to
convert it to bool, which is exactly how it is used in this context.
The reason that class istream defines operator void* rather than operator bool
is to allow the compiler to detect the following erroneous usage:
int x;
cin << x; // we should have written cin >> x;
If class istream were to define operator bool, this expression would use
istream::operator bool to convert cin to bool, and then convert the resulting
bool value to int, shift that value left by a number of bits equal to the value of x, and
throw the result away! By defining a conversion to void*, rather than to an arithmetic
type, the standard library still allows an istream to be used as a condition, but
prevents it from being used as an arithmetic value.
12.6 Conversions and memory management
Many C++ programs interface to systems written in C or assembly language that use
null-terminated arrays of characters to hold string data. As we saw in §10.5.2/180, the
C++ standard library itself uses this convention to obtain the names of input and output
files. Because of this convention, we might conclude that our Str class should provide a
conversion from Str to a null-terminated array of characters. If we did so, then our
users could (automatically) pass Str s to functions that operate on null-terminated
arrays. Unfortunately, as we shall see, doing so is fraught with memory-management
pitfalls.
Assuming that we wanted to provide a conversion from Str to char* , we'd probably
want to provide both const and nonconst versions:
class Str {
public:
// plausible, but problematic conversion operations
operator char*(); // added
operator const char*() const; // added
// as before
private:
Vec data;
};
With this rewrite, Str users could write code such as
Str S;
// ...
ifstream in(s); // wishful thinking: convert s and then open the stream named s
The only problem is that these conversions are almost impossible to implement well. We
can't just return data , most obviously because it's the wrong type: data is a
Vec , and we need an array of char . More subtly, even if the types matched,
returning data would violate class Str 's encapsulation: A user who obtained a pointer
to data could use that pointer to change the value of the string . Just as bad,
consider what happens when the Str is destroyed. If the user tries to use the pointer
after the Str object no longer exists, then the pointer will point to memory that has
been returned to the system and is no longer valid.
We can solve the encapsulation problem by providing only a conversion to const
char* , but doing so does not prevent a user from destroying the Str and then using
the pointer. We can solve this second problem by allocating new space for a copy of the
characters from data , and returning a pointer to this newly allocated space. The user
would then have to manage this space, freeing it when it is no longer needed.
As it turns out, this design won't work either. Conversions may happen implicitly, in
which case the user has no pointer to destroy! Look again at
Str s;
ifstream is(s); // implicit conversion—how can we free the array?
If the Str class had the proposed conversion, then when we passed s to the ifstream
constructor, we would implicitly convert the Str to the const char* that the
constructor expects. This conversion would allocate new space to hold a copy of the
value of s . However, there is no explicit pointer to this space, and so the user cannot
free it. Clearly, a design that mandates memory leaks cannot be right.
When we design a class, we want to avoid letting users trip up by writing innocuouslooking
code that gets them in trouble. Before the C++ standard was finished, many
library vendors offered various kinds of string s. Some surely provided implicit
conversions to character arrays, but did so in a way that allowed users to make one or
both of the potential errors outlined earlier.
The standard string library takes a different approach: It lets users get a copy of the
string in a character array, but makes them do so explicitly. The standard string
class provides three member functions for getting a char array from a string . The
first, c_str() , copies the contents of the string into a null-terminated char array.
The string class owns the array, and the user is expected not to delete the pointer.
Data in the array are ephemeral, and will be valid only until the next call of a member
function that might change the string . Users are expected either to use the pointer
immediately or to copy the data into storage that they will manage. The second
function, data() , is like c_str() , except that it returns an array that is not null
terminated. Finally, the copy function takes a char* and an integer as arguments, and
copies as many characters as indicated by the integer into space pointed to by the
char* , which space the user must allocate and free. We leave the implementation of
these functions as an exercise.
Note that both c_str and data share the pitfalls of the implicit conversion to const
char* . On the other hand, because users must request the conversion explicitly, they
are more likely to know about the functions that they call. This knowledge should include
the pitfalls inherent in retaining a copy of the pointer. If the library allowed implicit
conversions, it would be easier for users to stumble into these problems. They might not
even be aware that they had caused a conversion to take place, so they might be less
likely to understand why things failed when they did.
12.7 Details
Conversions are defined by nonexplicit constructors that take a single argument, or
by defining a conversion operator of the form operator type-name(), where typename
names the type to which the class type can be converted. Conversion operators
must be members. If two classes define conversions to each other's types, ambiguities
can result.
friend declarations can occur anywhere within the class definition, and allow the
friend to access private members of the class granting friendship.
template
class Thing {
friend std::istream& operator>>(std::istream&, Thing&);
// ...
};
As we will see in §13.4.2/246, classes can also be named as friends.
Template functions as members: A class may have template functions as members.
The class itself might or might not be a template. A class that has a template member
function effectively has an unbounded family of member functions with the same name.
Template member functions are declared and defined as are any other template
functions.
string operations
s.c_str()
Yields a const char* that points to a null-terminated array. The data in the array are
valid only until the next string operation that might modify s. The user may not
delete the pointer and should not hold a copy of it because the contents pointed to
have a limited lifetime.
s.data()
Similar to s.c_str(), but the array is not null terminated.
s.copy(p, n)
Copies up to an integral number n of characters from s into space pointed to by the
character pointer p. The user is responsible for ensuring that p points at space that is
sufficient to hold n characters.
Exercises
12-0. Compile, execute, and test the programs in this chapter.
12-1. Reimplement the Str class, but choose an implementation strategy that requires
that the class manage the storage itself. For example, you might store an array of char
and a length. Consider what implications this change in design has for copy control. Also
consider the cost of using Vec, (e.g., in storage overhead).
12-2. Implement the c_str, data, and copy functions.
12-3. Define the relational operators for Str. In doing so, you will want to know that
the header defines a function called strcmp, which compares two character
pointers. The function returns a negative integer if the null-terminated character array
denoted by the first pointer is less than the second, zero if the two strings are equal, or
a positive value if the first string is greater than the second.
12-4. Define the equality and inequality operators for Str.
12-5. Implement concatenation for Str so as not to rely on conversions from const
char*.
12-6. Give Str an operation that will let us implicitly use a Str object as a condition.
The test should fail if the Str is empty, and should succeed otherwise.
12-7. The standard string class provides random-access iterators to manipulate the
string's characters. Add iterators and the iterator operations begin and end to your
Str class.
12-8. Add the getline function to the Str class.
12-9. Use class ostream_iterator to reimplement the Str output operator. Why
didn't we ask you to reimplement the input operator using class istream_iterator?
12-10. Having seen in §12.1/212 how Str defined a constructor that takes a pair of
iterators, we can imagine that such a constructor would be useful in class Vec. Add this
constructor to Vec, and reimplement Str to use the Vec constructor instead of calling
copy.
12-11. If you add the operations listed in these exercises, then you can use this Str
class in all the examples in this book. Reimplement the operations on character pictures
from Chapter 5 and the split functions from §5.6/87 and §6.1.1/103.
12-12. Define the insert function that takes two iterators for the Vec and Str
classes.
12-13. Provide an assign function that could be used to assign the values in an array
to a Vec.
12-14. Write a program to initialize a Vec from a string.
12-15. The read_hw function from §4.1.3/57 checked the stream from which it read to
determine whether the function had hit end-of-file, or had encountered an invalid input.
Our Str input operator does no such check. Why? Will it leave the stream in an invalid
state?
13
Using inheritance and dynamic binding
The last several chapters have explored how we can build our own data types. This
ability is one of the foundations of object-oriented programming (OOP). This chapter will
begin a look at the the other key components of OOP—inheritance and dynamic binding.
In Chapter 9, we described a small class intended to encapsulate the operations used to
solve the grading problem from Chapter 4. This chapter will revisit that problem. This
time, we'll assume that there's a change in specification: Students can take the course
for undergraduate or graduate credit. Obtaining graduate credit requires that the
students do some extra work. We'll assume that in addition to the homework and exams
that all students must complete, graduate students also have to write a thesis. As we'll
see, this change in problem specification lends itself to an object-oriented solution,
which we'll use to explore the language features that C++ offers to support OOP.
Our objective is to write new classes that will mirror these new requirements. We'd also
like our previous solution to the grading problem, from §9.6/166, to continue to work.
That is, we'd like classes that allow us to generate the final grade report by reading a
file of grade records and writing a formatted report using the original code.
13.1 Inheritance
In our grading problem we know that a record for graduate credit is the same as for
undergraduate credit, except that it has additional properties related to the thesis. Such
contexts are natural places for inheritance . Inheritance is one of the cornerstones of
OOP. The basic idea is that we can often think of one class as being just like another,
except for some extensions. In this problem, all students must complete the homework
and exams, but some must also do a thesis . What we'd like is to define two classes:
one to represent the core requirements and the other to represent the requirements for
graduate credit.
We mostly know how to write the first of these classes: It is similar to our previous
Student_info class, which we'll rename Core for reasons that will become fully
apparent in §13.4/243. For now, what's useful to know is that Core no longer
represents all kinds of students, but only those students who meet the core
requirements for the course.
We'd like to reserve Student_info as a name that denotes any kind of student. In
addition to its name change, we'll add a private utility function to our Core class to read
the portion of a student's record that all students have in common:
class Core {
public:
Core();
Core(std::istream&);
std::string name() const;
std::istream& read(std::istream&);
double grade() const;
private:
std::istream& read_common(std::istream&);
std::string n;
double midterm, final;
std::vector homework;
};
Class Grad will capture the extra requirements for obtaining graduate credit:
class Grad: public Core {
public:
Grad();
Grad(std::istream&);
double grade() const;
std::istream& read(std::istream&);
private:
double thesis;
};
This definition says that we're defining a new type named Grad , which is derived from
or inherits from Core , or, equivalently, that Core is a base class of Grad . Because
Grad inherits from Core , every member of Core is also a member of Grad —except for
the constructors, the assignment operator, and destructor. The Grad class can add
members of its own, as we do here with the data member thesis and the constructors
for Grad . It can redefine members from the base class, as we do with the grade and
read functions. However, a derived class cannot delete any of the base class' members.
The use of public in public Core says that the fact that Grad inherits from Core is
part of its interface, rather than part of its implementation. That is, Grad inherits the
public interface to Core , which becomes part of the public interface to Grad . The
public members of Core are effectively public members of Grad . For example, if
we have a Grad object, we can call its name member function to obtain the student's
name, even though Grad does not define its own name function.
The Grad class differs from the Core class in that it keeps track of a grade for the
thesis, and uses a different algorithm for calculating the final grade. Thus, Grad objects
will have five data elements. Four of them are inherited from Core ; the fifth is a
double value named thesis . It will have two constructors and four member
functions, two of which redefine the corresponding members of Core , and the name
and read_common functions, which it inherits from Core .
13.1.1 Protection revisited
As it stands, all four data elements and the read_common function in Core are
inaccessible to member functions in Grad . We said that the members of Core were
private . Only the class and its friends may access private members. Unfortunately,
in order to write the Grad versions of the grade and read functions, we need access to
some of these private members. We can fix this problem by rewriting the Core class
using a protection label that we have not seen before:
class Core {
public:
Core() ;
Core(std::istream&);
std::string name() const;
double grade() const;
std::istream& read(std::istream&);
protected:
std::istream& read_common(std::istream&);
double midterm, final;
std::vector homework;
private:
std::string n;
};
We still say that n is private , but now the read_common function and the midterm,
final , and homework data members are protected. The protected label gives
derived classes, such as Grad , access to the protected members of their constituent
base-class objects, but keeps these elements inaccessible to users of the classes.
Because n is a private member of Core , only the members and friend s of class Core
may access n . Grad has no special access to n ; it can access n only through public
member functions of Core .
The read, name , and grade functions are public members of Core , and as such
they are available to all users of class Core —including classes derived from Core .
13.1.2 Operations
To complete our classes, we need to implement four constructors: the default
constructor and the constructor that takes an istream , once for each class. We must
also implement six operations: the name and read_common operations in class Core ,
and the read and grade functions for both classes. We'll see how to write the
constructors in §13.1.3/231.
Before writing our code, we need to think a bit about how student records will be
structured. As before, we'll want to accommodate a variable number of homework
assignments, so those grades must come at the end of each record. Therefore, we'll
assume that our records will consist of a student's name followed by the midterm and
final exam grades. If the record is for undergraduate credit, then the homework grades
will follow immediately. If the record is for graduate credit, then the thesis grade will
follow the final exam, but precede the homework grades.
With this information, we know how to write the operations on Core :
string Core::name() const { return n; }
double Core::grade() const
{
return ::grade(midterm, final, homework);
}
istream& Core::read_common(istream& in)
{
// read and store the student's name and exam grades
in >> n >> midterm >> final;
return in;
}
istream& Core::read(istream& in)
{
read_common(in);
read_hw(in, homework);
return in;
}
The Grad::read function is similar, but reads the thesis before calling read_hw :
istream& Grad::read(istream& in)
{
read_common(in);
in >> thesis;
read_hw(in, homework);
return in;
}
Note that in the definition of Grad::read , we can refer to elements from the base
class without any special notation, because these elements are also members of Grad .
If we wanted to be explicit about the fact that the members were inherited from Core ,
we could use the scope operator to do so:
istream& Grad::read(istream& in)
{
Core::read_common(in);
in >> thesis;
read_hw(in, Core::homework);
return in;
}
Of course, the thesis member is unqualified because that member is a part of Grad
and not a part of Core . We could have written Grad::thesis , but not
Core::thesis .
The grade function changes to account for the effect of thesis . Our policy is that the
student receives the lesser of the grade obtained on the thesis and the grade that
would have been obtained if we just counted the exams and homework scores:
double Grad::grade() const
{
return min(Core::grade(), thesis);
}
Here we want to call the grade function in the base class in order to calculate the score
independently from the thesis grade. In this case, the use of the scope operator is
essential. Had we written
return min(grade(), thesis);
we would have (recursively) called the Grad version of grade , leading to disaster.
We use the min function from to determine which grade to return. The
min function operates like max except that it returns the smaller of its two operands. As
with max , those operands must be of exactly the same type.
13.1.3 Inheritance and constructors
Before we write the constructors for Core and Grad , we need to understand how the
implementation creates objects of a derived type. As with any class type, the
implementation begins by allocating space for the object. Next, it runs the appropriate
constructor to initialize the object. The fact that the object is of a derived type adds an
extra step to the construction process in order to construct the base-class part of the
object. Derived objects are constructed by
Allocating space for the entire object (base-class members as well as derived
members)
Calling the base-class constructor to initialize the base-class part(s) of the object
Initializing the members of the derived class as directed by the constructor
initializer
Executing the body of the derived-class constructor, if any
The only new part is how we select which base-class constructor to run. Not surprisingly,
we use the constructor initializer to specify the base-class constructor that we want. The
derived-class constructor initializer names its base class followed by a (possibly empty)
list of arguments. These arguments are the initial values to use in constructing the
base- class part; they serve to select the base-class constructor to run in order to
initialize the base. If the initializer does not specify which base-class constructor to run,
then the base- class default constructor is used to build the base-part of the object.
class Core {
public:
// default constructor for Core
Core(): midterm(O), final(0) { }
// build a Core from an istream
Core(std::istream& is) { read(is); }
// ...
};
class Grad: public Core {
public:
// both constructors implicitly use Core::Core() to initialize the base part
Grad(): thesis(0) { }
Grad(std::istream& is) { read(is); }
// ...
};
The constructors for Core are identical to the ones in §9.5.1/165 and §9.5.2/166: They
specify how to make a Core from nothing or from an istream . The constructors for
Grad say how to create a Grad from these same values, that is, either from no
argument or from an istream& . It is worth noting that there is no requirement that
the derived- class constructors take the same argument(s) as the constructors for the
base class.
The default constructor for Grad says that to make a Grad from nothing, the
implementation should construct its Core part and set the thesis member to 0. As it
stands, most of this work is implicit: First, because the constructor initializer is empty,
we implicitly invoke the default constructor for Core to initialize the midterm, final,
homework , and name members. In the same fashion, the Core default constructor
implicitly initializes name and homework through their default constructors, and
explicitly initializes only midterm and final . The only explicit action that the default
Grad constructor takes is to initialize the thesis member. Once this is done, there is
no other work for the constructor to do, so the function body is empty.
We make a Grad from an istream in much the same way that we make a Core from
an istream —namely, by calling the read member. Before doing so, though, we first
(implicitly) invoke the base-class default constructor to initialize the base part of the
object. Then, because this constructor is a member of class Grad , the read that is
called is Grad::read . We don't bother to initialize thesis because the read function
reads a value into thesis from is.
It is important to understand how derived-class objects are constructed. Executing
Grad g;
causes the system to allocate enough space to hold Grad 's five data elements, run the
Core default constructor to initialize the data members in the Core part of g , and then
run the default constructor for Grad . Similarly, if we execute
Grad g(cin);
then after allocating an appropriate amount of space, the implementation will run the
Core default constructor, followed by the Grad::Grad(istream&) constructor to read
values into the name, midterm, final, thesis , and homework members.
13.2 Polymorphism and virtual functions
We have not yet completely reimplemented the original Student_info abstraction.
That abstraction relied on a nonmember function to support part of its interface: It used
the compare function to compare two student records. This function is used by sort to
arrange records in alphabetical order.
Our new comparison function is identical to the one we wrote in §9.3.1/162 except for
the change in type name:
bool compare(const Core& c1, const Core& c2)
{
return c1.name() < c2.name();
}
We compare two student records by comparing their names. We delegate the real work
to the string library < operator. What is interesting about this code is that we can use
it to compare both Core records and Grad records, or even to compare a Core record
with a Grad record:
Grad g(cin); // read a Grad record
Grad g2(cin); // read a Grad record
Core c(cin); // read a Core record
Core c2(cin); // read a Core record
compare(g, g2); // compare two Grad records
compare(c, c2); // compare two Core records
compare(g, c); // compare Grad record with a Core record
In each of these calls to compare , the name member of class Core will be run to
determine the value to return from compare . Obviously, this is the right member to
call for class Core , but what about for Grad ? When we defined class Grad , we said
that it is inherited from Core , and we did not redefine the name function. Thus, when
we invoke g.name() for a Grad object g , we are invoking the name member that it
inherited from Core . That function operates the same way on a Grad as it does on a
Core : It fetches the underlying n field from the Core part of the object.
The reason that we can pass a Grad object to a function expecting a Core& is that we
said that Grad is inherited from Core , so every Grad object has a Core part:
Because every Grad object has a Core part, we can bind compare 's reference
parameters to the Core portions of Grad objects, exactly as we can bind them to plain
Core objects. Similarly, we could have defined compare to operate on pointers to Core
or on objects of type Core (as opposed to a reference to Core ). In either case, we
could still call the function on behalf of a Grad object. If the function took pointers, we
could pass a pointer to Grad . The compiler would convert the Grad* to a Core* , and
would bind the pointer to the Core part of the Grad object. If the function took a Core
object, then what would be passed is just the Core portion of the object. There can be
striking differences in behavior, depending on whether we pass an object itself, or a
reference or pointer to the object—as we shall now see.
13.2.1 Obtaining a value without knowing the object's type
Our compare function does the right thing when we call it with a Grad object as an
argument because the name function is shared by both Grad and Core objects. What if
we wanted to compare students, not on the basis of their names, but on the basis of
their final grades? For example, instead of producing a listing of final grades sorted by
name, we might need to produce a listing sorted by final grade.
As a first cut at solving this problem, we'd write a function that is similar to compare :
bool compare_grades(const Core& c1, const Core& c2)
{
return c1.grade() < c2.grade();
}
The only difference is that here we're invoking the grade function rather than the name
function. This difference turns out to be significant!
The difference is that Grad redefines the meaning of the grade function, and we have
done nothing to distinguish between these two versions of grade . When we execute
the compare_grades function, it will execute the Core::grade member, just as
compare executes Core::name . In this case, if we are operating on a Grad object,
then the version from Core gives the wrong answer, because the grade functions in
Core and Grad behave differently from each other. For Grad objects, we must run
Grad::grade in order to account for the thesis .
What we need is a way for compare_grades to invoke the right grade function,
depending on the actual type of object that we pass: If c1 or c2 refers to a Grad object,
then we want the Grad version of grade ; if the object is of type Core , then we want
the one from Core . We want to make that decision at run time. That is, we want the
system to run the right function based on the actual type of the objects passed to the
function, which is known only at run time.
To support this kind of run-time selection, C++ provides virtual functions:
class Core {
public:
virtual double grade() const; // virtual added
// ...
};
We now say that grade is a virtual function. When we call compare_grades , the
implementation will determine the version of grade to execute by looking at the actual
types of the objects to which the references c1 and c2 are bound. That is, it will
determine which function to run by inspecting each object that we passed as an
argument to compare_grades . If the argument is a Grad object, it will run the
Grad::grade function; if the argument is a Core object, it will run the Core::grade
function.
The virtual keyword may be used only inside the class definition. If the functions are
defined separately from their declarations, we do not repeat virtual in the definitions.
Thus, the definition of Core::grade() need not change. Similarly, the fact that a
function is virtual is inherited, so we need not repeat the virtual designation on the
declaration of grade within the Grad class. We do have to recompile our code with the
new Core class definition. Once we have done so, then because the base-class version
is virtual , we get the behavior that we need.
13.2.2 Dynamic binding
This run-time selection of the virtual function to execute is relevant only when the
function is called through a reference or a pointer. If we call a virtual function on
behalf of an object (as opposed to through a reference or pointer), then we know the
exact type of the object at compile time. The type of an object is fixed: It is what it is,
and does not vary at run time. In contrast, a reference or pointer to a base-class object
may refer or point to a base-class object, or to an object of a type derived from the base
class, meaning that the type of the reference or pointer and the type of the object to
which a reference or pointer is bound may differ at run time. It is in this case that the
virtual mechanism makes a difference.
For example, assume we rewrote compare_grades as follows:
// incorrect implementation!
bool compare_grades(Core c1, Core c2)
{
return c1.grade() < c2.grade();
}
In this version, we say that our parameters are objects, not references to objects. In
this case, we always know the type of objects represented by c1 and c2 : They are
Core objects. We can still call this function on behalf of a Grad object, but the fact that
the argument had type Grad is immaterial. In this case, what happens is that what we
pass is the base part of the object. The Grad object will be cut down to its Core part,
and a copy of that portion of the Grad object will be passed to the compare_grades
function. Because we said that the parameters are Core objects, the calls to grade are
statically bound —they are bound at compile time—to Core::grade .
This distinction between dynamic binding and static binding is essential to
understanding how C++ supports OOP. The phrase dynamic binding captures the notion
that functions may be bound at run time, as opposed to static bindings that happen at
compile time. If we call a virtual function on behalf of an object, the call is statically
bound— that is, it is bound at compile time—because there is no possibility that the
object will have a different type during execution than it does during compilation. In
contrast, if we call a virtual function through a pointer or a reference, then the
function is dynamically bound—that is, bound at run time. At run time, the version of
the virtual function to use will depend on the type of the object to which the
reference or pointer is bound:
Core c;
Grad g;
Core* p;
Core& r = g;
c.grade(); // statically bound to Core::grade()
g.grade(); // statically bound to Grad::grade()
p->grade(); // dynamically bound, depending on the type of the object to which p points
r.grade(); // dynamically bound, depending on the type of the object to which r refers
The first two calls can be statically bound: We know that c is a Core object, and that at
run time, c will still be a Core object. Therefore, the compiler can statically resolve this
call, even though grade is a virtual function. In the third and fourth calls, however,
we can't know the type of the object to which p or r refers until run time: They might
be Core or Grad objects. Hence, the decision as to which function to run in these cases
must be delayed until run time. The implementation makes that decision based on the
type of the object to which p points or to which r refers.
The fact that we can use a derived type where a pointer or reference to the base is
expected is an example of a key concept in OOP called polymorphism . This word, from
the Greek polymorphos, meaning "of many forms," was already in use in English in the
mid-nineteenth century. In a programming context, it refers to the ability of one type to
stand in for many types. C++ supports polymorphism through the dynamic-binding
properties of virtual functions. When we call a virtual through a pointer or
reference, we make a polymorphic call. The type of the reference (or pointer) is fixed,
but the type of the object to which it refers (or points) can be the type of the reference
(or pointer) or any type derived from it. Thus, we can potentially call one of many
functions through a single type.
One final note about virtual functions: These functions must be defined, regardless of
whether the program calls them. Nonvirtual functions may be declared but not defined,
as long as the program does not call them. Many compilers generate mysterious error
messages for classes that fail to define one or more virtual functions. If your program
evokes a message from the compiler that you do not understand, and that message
says that something is undefined, you should verify that you have defined all of your
virtual functions. You are likely to find that the error goes away when you do so.
13.2.3 Recap
Before we continue, it is probably worth summarizing where we are, and making one
slight additional change: We'll make the read function virtual as well. We'd like to be
able to have the choice of which read function to run depend on the type of the object
on which it is invoked. With that final change, let's look at our classes:
class Core {
public:
Core(): midterm(O), final (0) { }
Core(std::istream& is) { read(is); }
std::string name() const;
// as defined in §13.1.2/230
virtual std::istream& read(std::istream&);
virtual double grade() const;
protected:
// accessible to derived classes
std::istream& read_common(std::istream&);
double midterm, final;
std::vector homework;
private:
// accessible only to Core
std::string n;
};
class Grad: public Core {
public:
Grad(): thesis(0) { }
Grad(std::istream& is) { read(is); }
// as defined in §13.1.2/230; Note: grade and read are virtual by inheritance
double grade() const;
std::istream& read(std::istream&);
private:
double thesis;
};
bool compare(const Core&, const Core&);
We have defined two classes to encapsulate our two kinds of students. The first class,
Core , represents students meeting the core requirements for the course. Our second
class inherits from Core , adding the requirements for completing a thesis. We can
create Core or Grad objects in two ways. The default constructor creates a properly
initialized, empty object; the other constructor takes an istream& and reads initial
values from the specified stream. The operations let us read into an object, resetting its
values, and let us fetch the student's name or final grade. Note that in this version, we
have made both the grade and read functions virtual . Finally, our interface includes
a global, nonmember compare function that compares two objects by comparing
students' names.
13.3 Using inheritance to solve our problem
Now that we have classes that model our different kinds of students, we would like to
use these classes to solve the grading problem from §9.6/166. That program read a file
that contained student grade records, computed the final grade for each student, and
wrote a formatted report in alphabetical order by student name. We'd like to solve the
same problem, but do so for a file that contains records for both kinds of students.
Before solving the whole problem, we'll solve two simpler problems: We will write
programs that can read files that consist entirely of one kind of record or the other. Both
of these programs will look just like our original except for the type declarations:
int main()
{
vector students; // read and process Core records
Core record;
string::size_type maxlen = 0;
// read and store the data
while (record.read(cin)) {
maxlen = max(maxlen, record.name().size());
students.push_back(record);
}
// alphabetize the student records
sort(students.begin(), students.end(), compare);
// write the names and grades
for (vector::size_type i = 0; i != students.size(); ++i) {
cout << students[i].name()
<< string(maxlen + 1 - students[i].name.size(), ' ');
try {
double final_grade = students[i].grade(); // Core::grade
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
}
return 0;
}
We can write the same program to handle Grad records by changing the type
definitions:
int main()
{
vector students; // different type in the vector
Grad record; // different type into which to read
string::size_type maxlen = 0;
// read and store the data
while (record.read(cin)) { // read from Grad, not Core
maxlen = max(maxlen, record.name().size());
students.push_back(record);
}
// alphabetize the student records
sort(students.begin(), students.end(), compare);
// write the names and grades
for (vector::size_type i = 0; i != students.size(); ++i) {
cout << students[i].name()
<< string(maxlen + 1 - students[i].name.size(), ' ');
try {
double final_grade = students[i].grade(); // Grad::grade
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
}
return 0;
}
Of course, the functions that are run in each instance depend on the type of record,
and on the type of the objects contained in the vector. For example, the expression
record.read(cin)
calls Core::read or Grad::read, depending on the type of record. It is worth noting
that this call is statically bound: The dependence on the type of record has nothing to
do with the virtual nature of the read function. We are invoking the function on
behalf of an object, not a pointer or reference to an object. Thus, record is either a
Core or a Grad, depending on which version of the program is being run. However,
once we define record, its type is fixed, and so the call to record.read(cin) is
likewise bound at compile time. Similarly, the grade function that we call when
generating output,
students[i].grade()
will be statically bound to the one from class Core when we run the first program, and
to the one from Grad when we run the second program.
In both versions of the program, the uses of name() refer to the (nonvirtual) version
defined in class Core. That function is inherited by Grad, so that when we run it for a
Grad object, we are still running the version that we defined in Core. The compare
function that we pass to sort operates on references to Core. When the version of the
program that operates on Grad records runs, it will compare the Core parts.
Obviously, it is tedious to write separate versions of our program. What we really want
to do is to write a single version that can handle either Core or Grad objects.
In order to write a single function that can read a file of either Core records or Grad
records, we need to look closely at the code, and identify those places where the type of
the record matters. In order to write a single version of the program, we will have to
eliminate these type dependencies:
The definition of the vector in which we store the elements as we read them
The definition of the local temporary into which we read the records
The read function
The grade function
The remaining code is either type independent (the code to sort the vector or to
iterate through it) or it is invariant between the Grad and Core versions (such as the
name and compare functions). Because we defined the grade and read functions as
virtuals, we have already solved the last two parts of our problem.
It turns out that the first two subproblems—what type to use for the local temporary and
what type to store in the container—can be handled by the same strategy. It also turns
out that there are two different approaches to solving these subproblems. The first
approach is straightforward, so we'll look at it in the next section. The other solution
represents a common, important C++ programming idiom, which we'll cover in
§13.4/243.
13.3.1 Containers of (virtually) unknown type
The problem that we need to solve is to relax the type dependencies in the following:
vector students; // must hold Core objects, not polymorphic types
Core record; // Core object, not a type derived from Core
The type dependencies in this code should be fairly clear. When we define record, we
say exactly what type of object record is: It is a Core, because that's what we said it
was. Similarly, when we define students, we say that it is a vector that holds objects
of type Core. We'll have more to say about such containers in §13.6.1/249, but for now
what's important is to realize that when we define a vector, we are saying that
each object in the vector will be a Core object—not an object of a type derived from
Core.
When we outlined the type dependencies in our two programs, we noted that we'd
solved half the type issues because read and grade were virtual functions. The
other half of the problem is that, as written, our programs make statically bound calls to
these virtuals. In order to invoke the dynamic behavior that we need, we need to call
read and grade through a pointer or reference to Core. That way, the type of the
object bound can differ from the type of the pointer or reference. This observation leads
us to a solution to all four subproblems: We can write the program to manage pointers
instead of objects. We can have a vector, and we can define record to be a
pointer as well. In this way, we can obtain the dynamic behavior we need, while
eliminating the type dependence involved in the definitions of the vector and our local
temporary. Unfortunately, as we shall see, this solution pushes a lot of complexity onto
our users. For example, the obvious try at a solution doesn't work at all:
int main()
{
vector students;
Core* record;
while (record->read(cin)) { // crash!
// ...
}
}
The trouble with this program is that it fails horribly, because we never caused record
to point to an object!
We can fix this problem, but only by requiring that our users actively manage the space
consumed by the objects that they read from the file. Our users will also have to be able
to detect what kind of records the program is reading. We'll assume that each record will
contain an indicator to distinguish the kind of record that it contains: Records for
graduate students will start with a G, and those for an undergraduate will start with a U.
Before we rewrite the program to use pointers, there is one more problem that we must
solve: How do we sort a vector of pointers? The easy answer is that we'll need a new
comparison function that takes two pointers to Core objects. The tricky part is that we
cannot name this function compare. Recall that in §8.1.3/142 we discussed various
subtleties in getting the right types for values that we pass as template arguments. For
similar reasons, we cannot pass an overloaded function name as a template argument.
If we did so, the compiler would have no way to determine which version of the function
we wanted. Evidently, we'll need to write a comparison function that will give us a nonoverloaded
name to pass to sort:
bool compare_Core_ptrs(const Core* cp1, const Core* cp2)
{
return compare(*cp1, *cp2);
}
Having written a specialized comparison function, we can now rewrite the program:
// this code almost works; see §13.3.2/242
int main()
{
vector students; // store pointers, not objects
Core* record; // temporary must be a pointer as well
char ch;
string::size_type maxlen = 0;
// read and store the data
while (cin >> ch) {
if (ch == 'U')
record = new Core; // allocate a Core object
else
record = new Grad; // allocate a Grad object
record->read(cin); // virtual call
maxlen = max (maxlen, record->name().size()); // dereference
students.push_back(record);
}
// pass the version of compare that works on pointers
sort(students.begin(), students.end(), compare_Core_ptrs);
// write the names and grades
for (vector::size_type i = 0;
i != students.size(); ++i) {
// students[i] is a pointer that we dereference to call the functions
cout << students[i]->name()
<< string(maxlen + 1 - students[i]->name.size(), ' ');
try {
double final_grade = students[i]->grade();
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
delete students[i]; // free the object allocated when reading
}
return 0;
}
We have noted in comments the many differences between this code and our original.
These changes all result from the fact that we must manipulate pointers and not
objects.
The while loop changes to read the first character from the input, which we
subsequently test to determine which kind of record we are about to read. Once we
know what kind of object we need, we allocate an object of the appropriate type, and
use that object to read from the standard input. The read function is virtual, so the
right version will be called, depending on whether record points to a Grad or a Core
object. In both cases, read will give the object the values from the next input record.
Note that we must remember to dereference record, which is a pointer, to access
read. The code to calculate the length of the longest name also changes to dereference
the pointer, but otherwise the next few lines of code are unchanged.
When we get to the loop that does output, we have to remember that students[i]
yields a pointer. Once we have fetched students[i], we have a pointer that must
itself be dereferenced to get at the underlying object. As with the call to read, the call
to grade is a virtual call, so the right version of grade is automatically invoked to
calculate the grade, including a thesis if the object is a Grad and not otherwise. The
final change is to remember to return to the implementation the space that the object
consumed, which we do by calling delete on the pointer that students[i] contains.
13.3.2 Virtual destructors
Our program almost works. The only problem occurs when we delete the objects, as
we do inside the output loop. When we allocated these objects, we allocated both Grad
and Core objects, but we stored pointers to these objects as Core*, and not as Grad*
pointers. Thus, when we delete them, we are deleting a pointer to Core, and never a
pointer to Grad, even if the pointer actually points to a Grad object. Fortunately, this
problem is easily fixed.
When we call delete on a pointer, two things happen: The destructor is run on the
object, and the space that held the object is freed. When the program deletes the
pointer in students[i], it could be pointing at either a Core object or a Grad object.
Neither Core nor Grad explicitly defined a destructor, which means that when the
delete runs, it will invoke the synthesized destructor and then return the space that
the object consumed. The synthesized destructor will run the destructor for each data
element in the class. But when the delete is executed, which destructor should the
system run? Should the destructor destroy the members of a Core or a Grad? And
when the space is freed, how much space should be returned—enough to hold a Core or
a Grad?
These questions sound like the kind that the virtual mechanism can resolve—and
indeed it can. In order to have a virtual destructor, the class must have a destructor,
which we can then make virtual:
class Core {
public:
virtual ~Core() { }
// as before
};
Now when we execute delete students[i], the destructor that will be run will
depend on the type of object to which students[i] actually points. Similarly, the type
of the memory that we return to the system will be determined by the type to which
students[i] actually points.
Note that the body of the destructor is empty. The only work needed to destroy a Core
is to destroy its members, and the system does this work automatically. Empty,
virtual destructors are not uncommon. A virtual destructor is needed any time it is
possible that an object of derived type is destroyed through a pointer to base. If there is
no other reason for the destructor to be defined, then that destructor has no work to do
and should be empty.
There is no need to update the Grad class to add a destructor. As with all virtual
functions, the fact that the destructor is virtual is inherited. Because neither class has
any explicit work to do in order to destroy objects, there is no need to redefine the
destructor in the derived class. Because the derived class inherits the virtual property
of its base-class destructor, all we have to do is recompile the program.

13.4 A simple handle class
Although the approach that we have just seen is straightforward, it has problems: The
program has acquired a lot of extra complexity related to managing the pointers, and
has added several pitfalls that could lead to bugs. Our users have to remember to
allocate space for the records as they read them, and to remember to free that space
when they no longer need the data. The code is constantly dereferencing the pointers to
get at the underlying objects. Nevertheless, we have solved the problem of writing a
program that can read a file that contains both kinds of records intermixed.
What we'd like to do is find a way to preserve the good properties of our simpler
programs that dealt with either Core objects or Grad objects, and eliminate the
problems inherent in our new solution, which can process both kinds of records. It turns
out that there is a common programming technique, known as a handle class, that will
let us do so.
Our code became cluttered when we realized that we needed to be able to deal with
objects whose type we could not know until run time. We knew that each object would
be either a Core, or something derived from Core. Our solution used pointers, because
we could allocate a pointer to Core and then make that pointer point to either a Core
or a Grad object. The trouble with our solution is that it imposed error-prone
bookkeeping on our users. We can't eliminate that bookkeeping, but we can hide it from
our users by writing a new class that will encapsulate the pointer to Core:
class Student_info {
public:
// constructors and copy control
Student_info(): cp(0) { }
Student_info(std::istream& is): cp(0) { read(is); }
Student_info(const Student_info&);
Student_info& operator=(const Student_info&);
~Student_info() { delete cp; }
// operations
std::istream& read(std::istream&);
std::string name() const {
if (cp) return cp->name();
else throw std::runtime_error("uninitialized Student");
}
double grade() const {
if (cp) return cp->grade();
else throw std::runtime_error("uninitialized Student");
}
static bool compare(const Student_info& s1,
const Student_info& s2) {
return s1.name() < s2.name(); }
private:
Core* cp;
};
The idea here is that a Student_info object can represent either a Core or a Grad. In
that sense, it will act like a pointer. However, users of Student_info do not have to
worry about allocating the underlying object to which the Student_info is bound. The
class will take care of these tedious and error-prone aspects of our programs.
Each Student_info object will hold a pointer, called cp, that points to an object that
has either type Core or a type derived from Core. As we'll see in §13.4.1/245, in the
read function we'll allocate the object to which cp points. Therefore, both constructors
initialize cp to 0, indicating that the Student_info object is as yet unbound. In the
constructor that takes an istream, we call the Student_info::read function. That
function will allocate a new object of the appropriate type, and will give that object the
value that it reads from the indicated istream.
We know from the rule of three (§11.3.6/201) that we will need a copy constructor,
assignment operator, and destructor to manage the pointer. The work that the
destructor must do is easy: It just has to destroy the object that the constructors
allocated. Because we gave Core a virtual destructor in §13.3.2/242, the destructor for
Student_info will operate correctly whether the object being destroyed is a Grad
object or a Core object. We'll define the copy constructor and assignment operator in
§13.4.2/246.
Because users will write programs in terms of Student_info objects rather than Core
or Grad objects, the Student_info class must provide the same interface as the Core
class. For both name and grade, there is nothing special for Student_info to do, and
so these functions forward their work to the underlying Core or Grad object to which cp
points.
However, cp could be 0. It will be 0 if a user creates a Student_info object using the
default constructor, and then does not read into it. If cp is 0, we can't just forward the
calls to the underlying object. Instead, we'll throw a runtime_error to indicate that a
problem has occurred.
It is important to remember that the Core::grade function is virtual, which means
that when we call it through cp, the version that is called at run time will depend on the
type of object to which cp points. For example, if cp points to a Grad object, then we'll
run the Grad::grade operation.
The only other function in the interface is the compare operation, which turns out to
have a couple of interesting properties. First, recall that for the Core classes, compare
was a global, nonmember function, whereas here, we implement it as a static member
function. Static member functions differ from ordinary member functions in that they
do not operate on an object of the class type. Unlike other member functions, they are
associated with the class, not with a particular object. As such, they cannot access the
nonstatic data members of objects of the class: There is no object associated with the
function, so there are no members to use.
For our purposes, static member functions have one significant advantage: Their
names are within the scope of their class. So, when we say that compare is a static
member we are defining a function named Student_info::compare. Because the
function has a qualified name, it does not overload the compare that we used to
compare Core objects. Thus, our users will be able to call sort, passing
Student_info::compare, and the compiler can know which function they want.
The other interesting thing about this function is its implementation. The function uses
the Student_info::name function to get at the names stored in the records. It is
worth thinking about what is happening here. The call to Student_info::name will
call Core::name if cp is set. If cp is 0, then name will throw an exception, which
propagates out to compare's caller. Because compare uses the public interface to
Student_info, that function doesn't need to check cp directly. As with other userlevel
code, it passes that problem along to the Student_info class.
13.4.1 Reading the handle
The read function has three responsibilities: It must free the object, if any, to which
this handle is bound. It must then decide what kind of object we are about to read, and
it must allocate the right kind of object, which it can initialize from the stream it was
given:
istream& Student_info::read(istream& is) {
delete cp; // delete previous object, if any
char ch;
is >> ch; // get record type
if (ch == 'U') {
cp = new Core(is);
} else {
cp = new Grad(is);
}
return is;
}
The read function starts by freeing the existing object (if any) to which the handle
object was previously bound. We do not need to check whether cp is 0 before calling
delete, because the language guarantees that it is harmless to delete a pointer with
value 0. Having freed the old value, we are ready to read the new one. We start by
reading and testing the first character on the line. Based on that character, we create an
object of the appropriate type, initializing that object by running the appropriate
constructor that takes an istream. These constructors call their own read functions to
read values from the input stream into the newly created object. After the object is
constructed, we store the pointer to it in cp. To finish up, we return the stream that we
were given.
13.4.2 Copying handle objects
The copy constructor and assignment operator are necessary to manage the Core
pointer. The constructor allocates this pointer as a side effect of calling read. When we
copy a Student_info, we will want to allocate a new object and initialize it with the
values from the object from which we are copying. However, there is a snag: What kind
of object are we copying? There is no obvious way to know whether the Student_info
object that we're copying points to a Core object or an object of a type derived from
Core.
The way we solve this problem is to give Core and its derived classes a new virtual
function. That function creates a new object that holds copies of the values in the
original:
class Core {
friend class Student_info;
protected:
virtual Core* clone() const { return new Core(*this); }
// as before
};
The clone function does exactly what we described, in a surprisingly succinct fashion.
We allocate a new Core object and use Core's copy constructor to give that new object
the appropriate values. Remember that the Core class did not explicitly define a copy
constructor. Nonetheless, we know from §11.3.5/201 that one exists: The compiler
synthesized a default copy constructor, which copies each member from the existing
Core object into the newly created one.
Because we created the clone function as an artifact of our implementation, we did not
add it to the public interface of Core. The fact that clone is protected means that we
must nominate Student_info as a friend of Core, so that Student_info objects
can access the clone function. Class friendship is similar to the friend functions that
we saw in §12.3.2/216. There, we learned that friend functions have access to the
private and protected members of the class. Naming a class as a friend has the
same effect as making all of the members of that class friends. That is, by adding
friend class Student_info;
to the definition of Core, we are saying that all the member functions in
Student_info may access all the private and protected members of class Core.
Having added the virtual function clone to the base class, we have to remember to
redefine the function in the derived class, so that when we clone a derived object, we
will allocate a new Grad object:
class Grad {
protected:
Grad* clone() const { return new Grad(*this); }
// as before
};
As with Core::clone, we allocate a new object as a copy of *this, but here we return
a Grad* rather than a Core*. Ordinarily, when a derived class redefines a function
from the base class, it does so exactly: the parameter list and the return type are
identical.
However, if the base-class function returns a pointer (or reference) to a base class, then
the derived-class function can return a pointer (or reference) to a corresponding derived
class.
We do not need to nominate Student_info as a friend of Grad, even though
friendship is not inherited, because our Student_info class never refers to
Grad::clone directly; it does so only through virtual calls to Core::clone, which
it can access by virtue of its friendship with Core.
With these changes in place, we can now implement copying and assignment:
Student_info::Student_info(const Student_info& s) : cp(0)
{
if (s.cp) cp = s.cp->clone();
}
Student_info& Student_info::operator=(const Student_info& s)
{
if (&s != this) {
delete cp;
if (s.cp)
cp = s.cp->clone();
else
cp = 0;
}
return *this;
}
In the copy constructor, we initialize the pointer cp to 0, and conditionally call clone if
there is something there to clone. If not, cp will remain equal to 0, indicating that the
handle is unbound. Similarly, the assignment operator calls clone conditionally. Of
course, we have other work to do in the assignment operator before calling clone. First
we must guard against self-assignment, by testing whether the addresses of our two
operands are the same. If we are assigning different objects, then we must free the
object to which cp currently points before making cp point to the newly created object.
Neither the copy constructor nor the assignment operator does anything special if cp is
0, because it is perfectly legitimate to copy or assign an unbound handle.
13.5 Using the handle class
Having finished the handle class, we can now use it to allow our initial program from
§9.6/166 to work with but one change:
int main()
{
vector students;
Student_info record;
string::size_type maxlen = 0;
// read and store the data
while (record.read(cin)) {
maxlen = max(maxlen, record.name().size());
students.push_back(record);
}
// alphabetize the student records
sort(students.begin(), students.end(), Student_info::compare);
// write the names and grades
for (vector::size_type i = 0;
i != students.size(); ++i) {
cout << students[i].name()
<< string(maxlen + 1 - students[i].name.size(), ' ');
try {
double final_grade = students[i].grade();
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
}
return 0;
}
The input loop now reads and processes two kinds of records. One kind of record
represents a student who is completing only the core requirements for the course; the
other kind represents a student who wants graduate credit. The loop works because the
read function for Student_info reads either kind of record. That function first reads
the character that says what kind of record we're about to read, and then allocates an
object of the appropriate type, initializing the object from the input stream. It constructs
the underlying Core or Grad object by reading the data, and stores a pointer to the
newly created object in record. We copy the Student_info object into the vector,
which copies the object as a side effect of running the Student_info copy constructor.
The next step is to sort the data, which we do by invoking sort, passing it the
Student_info::compare function. That function calls the base-class name function to
compare the names in the objects.
The output loop remains unchanged. On each trip through the loop, students[i]
denotes a Student_info object. That object contains a pointer to an object that is
either a Core or a Grad. When we call the grade function for Student_info, that
function will use the pointer to call the (virtual) grade function on the underlying
object. The type of object to which the handle points will determine which version to call
at runtime.
Finally, the objects that were allocated inside the read for the Student_info function
will be automatically freed when we exit main. On exiting main, the vector will be
destroyed. The destructor for vector will destroy each element in students, which
will cause the destructor for Student_info to be run. When that destructor runs, it will
delete each of the objects allocated in read.
13.6 Subtleties
Although the ideas behind inheritance and dynamic binding are powerful, they can
appear mysterious, at least at first. Now that we've seen examples that use these ideas,
let's look at some of the associated subtleties that often cause trouble.
13.6.1 Inheritance and containers
In section §13.3.1/239, we noted that when we say that we want to store Core objects
in a container, we're saying that the container will hold Core objects and nothing but
Core objects. This assertion might have been surprising: It might seem that we should
be able to store Core objects or objects derived from Core in the container. However,
if we think about our own implementation of Vec from Chapter 11, we know that at
some point, the Vec has to allocate storage for the objects that it contains. When we
allocate that storage, we say which exact type to allocate. There is no virtual -like
mechanism that determines what kind of object is needed and allocates enough space to
hold that object.
What may be more surprising is what would happen if we persisted in defining a
vector into which we intended to place either Core objects or Grad objects.
The answer is that we could do so, but the results would probably be a surprise; for
example
vector students;
Grad g(cin); // read a Grad
students.push_back(g); // store the Core part(!) of g in students
We are allowed to store the Grad object in students , because we can use a Grad
object wherever a reference to a Core object is required. The push_back function
takes a reference to the vector 's element type, so we can pass g to push_back .
However, when we put the object into students , only the Core portion of g is copied!
As in §13.2.2/235, this behavior is actually what we asked for, although it can be
surprising—especially when encountered for the first time. What will happen is that
push_back will expect that it was given a Core object, and will construct a Core
element, copying only the Core parts of the object, ignoring whatever is specific to the
Grad class .
13.6.2 Which function do you want?
It is important to realize that when a base- and derived -class function have the same
name, but they don't match exactly in number and types of parameters, they behave as
if they were completely unrelated functions. For example, we might add to our hierarchy
an accessor function that we could use to change a student's final exam-grade. For
Core students, this function should set only the final grade; for Grad students, the
function should take two parameters, the second one being used to set the thesis:
void Core::regrade(double d) { final = d; }
void Grad::regrade(double d1, double d2) { final = d1; thesis = d2; }
If r is a reference to a Core , then
r.regrade(100); // ok, call Core::regrade
r.regrade(100, 100); // compile error, Core::regrade takes a single argument
This second call is an error even if r refers to an object of type Grad . The type of r is a
reference to Core , and the version of regrade in Core takes one value of type
double . What may be more surprising is what happens if r is a reference to a Grad :
r.regrade(100); // compile error, Grad::regrade takes two arguments
r.regrade(100, 100); // ok, call Grad::regrade
Now when we look for a function to call, r is a Grad . The regrade function that
applies to Grad objects takes two arguments. Even though there is a base-class version
that takes a single argument, that version is effectively hidden by the existence of
regrade in the derived class. If we want to run the version from the base class, we
must call it explicitly:
r.Core::regrade(100); // ok, call Core::regrade
If we want to use regrade as a virtual function, we must give it the same interface
in the base and derived classes, which we can do by giving the Core version an extra,
unused parameter with a default argument:
virtual void Core::regrade(double d, double = 0) { final = d; }
13.7 Details
Inheritance allows us to model classes that are similar to one another with exceptions:
class base {
public:
// common interface
protected:
// implementation members accessible to derived classes
private:
// implementation accessible to only the base class
};
// public interface of base is part of the interface for derived
class derived: public base { ... };
Classes that derive from the base class may redefine operations from the base class and
may add members of their own. Classes can also inherit privately:
class priv_derived: private base { ... };
which is rare and normally is used for implementation convenience only.
An object of a (publicly) derived type can be used where an object, reference, or
pointer to a base class object is expected.
Derivation chains can be several layers deep:
class derived2: public derived { ... };
Objects of type derived2 have a derived part, which in turn has a base part. Thus,
derived2 objects have the properties of derived and base class objects.
Derived-class objects are constructed by allocating enough space for the entire object,
constructing the base-class part(s), and then constructing the derived class. Which
derived constructor is run depends, as usual, on the arguments used in creating the
derived-class object. That constructor, through its constructor-initializer list, can pass
arguments to be used when constructing its immediate base class. If the constructor
initializer does not explicitly initialize its base, then the base's default constructor is run.
Dynamic binding refers to the ability to select at run time which function to run based
on the actual type of the object on which the function is called. Dynamic binding is in
effect for calls to virtual functions made through a pointer or a reference. The fact
that a function is virtual is inherited, and need not be repeated in the derived classes.
Derived classes are not required to redefine their virtual functions. If a class does not
redefine a virtual, then it inherits the nearest definition for that function. However,
any virtual functions that the class does contain must be defined. It is often a source
of mysterious error messages from compilers to declare but not define a virtual
function.
Overriding: A derived-class member function overrides a function with the same name in
the base class if the two functions have the same number and types of parameters and
both (or neither) are const. In that case, the return types must also match, except that
as in §13.4.2/246, if the base-class function returns a pointer (or reference) to a class,
the derived-class function can return a pointer (or reference) to a derived class. If the
argument lists don't match, the base- and derived-class functions are effectively
unrelated.
virtual destructors: If a pointer to the base class is used to delete an object that
might actually be a derived-class object, then the base class needs a virtual
destructor. If the class has no other need for a destructor, then the virtual destructor
still must be defined and should be empty:
class base {
public:
virtual ~base(){ }
};
As with any other function, the virtual nature of the destructor is inherited by the
derived classes, and there is no need to redefine the destructor in the derived classes.
Constructors and virtual functions: While an object is under construction, its type
is the type of the class that is being constructed—even if the object is part of a derivedclass
object. Thus, calls to virtual functions from inside a constructor are statically
bound to the version for the type being constructed.
Class friendship: A class can designate another as its friend; doing so grants
friendship to all the member functions of the other class. Friendship is neither inherited
nor transitive; friends of friends and classes derived from friends have no special
privileges.
Static members exist as members of the class, rather than as an instance in each
object of the class. Therefore, the this keyword is not available in a static member
function. Such functions may access only static data members. There is a single
instance of each static data member for the entire class, which must be initialized,
usually in the source file that implements the class member functions. Because the
member is initialized outside the class definition, you must fully qualify the name when
you initialize it:
value-type class-name::static-member-name = value;
says that the static member named static-member-name from the class classname
has type value-type and is given the initial value value.
Exercises
13-0. Compile, execute, and test the programs in this chapter.
13-1. Annotate the Core and Grad constructors to write the constructor's name and
argument list when the constructor is executed. For example, you should add a
statement such as
cerr << "Grad::Grad(istream&)" << endl;
to the Grad constructor taking an istream& parameter. Then write a small program
that exercises each constructor. Predict beforehand what the output will be. Revise your
program and predictions until your predictions match what is actually written.
13-2. Given the Core and Grad classes defined in this chapter, indicate which function
is called for each of these invocations:
Core* p1 = new Core;
Core* p2 = new Grad;
Core s1;
Grad s2;
p1->grade();
p1->name();
p2->grade();
p2->name();
s1.grade();
s1.name();
s2.name();
s2.grade();
Check whether you are correct by adding output statements to the name and grade
functions that indicate which function is being executed.
13-3. The class that we built in Chapter 9 included a valid member that allowed users
to check whether the object held values for a student record or not. Add that
functionality to the inheritance-based system of classes.
13-4. Add to these classes a function that will map a numeric grade to a letter grade
according to the grading policy outlined in §10.3/177.
13-5. Write a predicate to check whether a particular student met all the relevant
requirements. That is, check whether a student did all the homework, and if a graduate
student, whether the student wrote a thesis.
13-6. Add a class to the system to represent students taking the course for pass/fail
credit. Assume that such students need not do the homework, but might do so. If they
do, the homework should participate in determining whether they passed or failed,
according to the normal formula. If they did no homework, then the grade is the
average of their midterm and final grades. A passing grade is 60 or higher.
13-7. Add a class to the system to represent students auditing the course.
13-8. Write a program to generate a grade report that can handle all four kinds of
students.
13-9. Describe what would happen if the assignment operator in §13.4.2/247 failed to
check for self-assignment.
14
Managing memory (almost) automatically
When we built our Student_info handle class in Chapter 13, we combined two
separable abstractions. Not only was that class an interface to the operations on student
records, but it also managed a pointer to an implementation object. Combining two
independent abstractions into a single class is often a sign of weak design.
What we'd like is to be able to define a class that is similar to Student_info, but that
is strictly an interface class. Such interface classes are common in C++, especially when
they interface to an inheritance hierarchy. We will arrange for our interface class to
delegate the implementation details to another class, which behaves like a pointer but
also manages the underlying memory. Once we have separated the interface class from
the pointerlike class, we should be able to use a single pointerlike class with multiple
interface classes.
As we'll see, we can also use classes such as these to improve the performance of
programs that manage memory often. By arranging for several pointerlike objects to
refer to a single underlying object where appropriate, we can avoid copying objects
unnecessarily.
Much of this chapter revolves around the answer to a single question: What does it
mean to copy an object? At first glance, this question seems to have an obvious answer:
A copy is a distinct object that has all the properties of the original object. However, the
moment it becomes possible for one object to refer to another, the question becomes
more complicated: If an object x refers to an object y, does copying x cause y to be
copied too?
Sometimes the answer to this latter question is obvious: If y is a member of x, the
answer must be yes, and if x is nothing more than a pointer that happens to point to y,
the answer is no. In this chapter we'll define three different versions of our pointerlike
class, each of which differs from the others in how it defines copying.
These questions about copying, and the very idea of a pointerlike class, are fairly
abstract notions. Because we will implement these abstractions, it is not surprising that
this chapter is by far the most abstract in the book. As a result, it is likely to
require—and repay—careful study.
14.1 Handles that copy their objects
Let's think again about the grading problem that we solved in Chapter 13. In solving
that problem, we needed to store and process a collection of objects representing
different types of students. These objects were of one of two types related by
inheritance, with the possibility of more types being added later. Our first solution, in
§13.3.1/239, used pointers to let us store a mixed collection of objects. Each of these
pointers might point to a Core object, or to an object of a type derived from Core .
User code was responsible for allocating the objects dynamically, and for remembering
to free them. The program was cluttered with details related to managing the pointers,
and was generally tricky.
The problem is that a pointer is a primitive, low-level data structure. Programming with
pointers is notoriously error prone. Many of the problems with pointers arise because
pointers are independent of the objects to which they point, leading to pitfalls:
Copying a pointer does not copy the corresponding object, leading to surprises if
two pointers inadvertently point to the same object.
Destroying a pointer does not destroy its object, leading to memory leaks.
Deleting an object without destroying a pointer to it leads to a dangling pointer,
which causes undefined behavior if the program uses the pointer.
Creating a pointer without initializing it leaves the pointer unbound, which also
causes undefined behavior if the program uses it.
In §13.5/247, we solved the grading problem again, this time using the Student_info
handle class. Because this class managed the pointers, our users' code dealt with
Student_info objects, rather than with pointers. However, the Student_info class
was intimately tied to the Core hierarchy: It contained operations that mirrored those in
the public interface of the Core classes.
What we want to do now is to separate these abstractions. We'll still use
Student_info to provide the interface, but it will rely on another class to manage the
"handle." That is, this other class will manage the pointers to the implementation
objects. The behavior of this new type can and will be independent of the type of the
objects to which the handle can be attached.
14.1.1 A generic handle class
Because we want our class to be independent of the type of object that it manages, we
already know that our class must be a template. Because we want it to encapsulate the
handle behavior, we will call it Handle . The properties that our class will provide are:
A Handle is a value that refers to an object.
We can copy a Handle object.
We can test a Handle object to determine whether it is bound to another object.
We can use a Handle to trigger polymorphic behavior when it points to an object of
a class that belongs to an inheritance hierarchy. That is, if we call a virtual
function through our class, we want the implementation to choose the function to
run dynamically, just as if we'd called the function through a real pointer.
Our Handle will have a restricted interface: Once you attach a Handle to an object, the
Handle class will take over memory management for that object. Users should attach
only one Handle to any object, after which they should not access the object directly
through a pointer; all access should be through the Handle . These restrictions will
allow Handle s to avoid the problems inherent in built-in pointers. When we copy a
Handle object, we'll make a new copy of the object, so that each Handle points to its
own copy. When we destroy a Handle , it will destroy the associated object, and doing
so will be the only straightforward way to free the object. We'll allow users to create
unbound Handles , but we will throw an exception if the user attempts to access the
object to which an unbound Handle refers (or, more accurately, doesn't refer). Users
who want to avoid the exception can test to see whether the Handle is bound.
These properties are like the ones that we implemented in the Student_info class.
The Student_info copy constructor and assignment operator called clone to copy the
associated Core object. The Student_info destructor destroyed the Core object as
well. The operations that used the underlying object checked before doing so to ensure
that the Student_info was bound to a real object. What we want is a class that
encapsulates this behavior, but that we can use to manage an object of any type:
template class Handle {
public:
Handle(): p(0) { }
Handle(const Handle& s): p(0) { if (s.p) p = s.p->clone(); }
Handle& operator=(const Handle&);
~Handle() { delete p; }
Handle(T* t): p(t) { }
operator bool() const { return p; }
T& operator*() const;
T* operator->() const;
private:
T* p;
};
The Handle class is a template class, so that we can create Handles that refer to any
type. Each Handle object holds a pointer to an object; the other operations manage
this pointer. Aside from variable-name changes, the first four functions are identical to
their versions in Student_info . The default constructor sets the pointer to zero to
indicate that the Handle is unbound. The copy constructor (conditionally) calls the
associated object's clone function to create a new copy of the object. The Handle
destructor frees the object. The assignment operator , like the copy constructor,
(conditionally) calls clone to create a new copy of the object:
template
Handle& Handle::operator=(const Handle& rhs)
{
if (&rhs != this) {
delete p;
p = rhs.p ? rhs.p->clone() : 0;
}
return *this;
}
The assignment operator , as usual, starts by checking for self-assignment, doing
nothing if the test succeeds. If the test fails, we continue by freeing the object that we
had been managing, and then making a copy of the right-hand object. The statement
that does the copy uses the conditional operator (§3.2.2/45) to decide whether it's safe
to call clone . If rhs.p is set, we call rhs.p->clone and assign the resulting pointer
to p . Otherwise, we set p to 0.
Because Handle models pointer behavior, we need a way to bind the pointer to an
actual object, which we do in the constructor that takes a T* . That constructor
remembers the pointer that it was given, thus binding the Handle to the object to
which t points. For example, if we define
Handle student(new Grad);
we construct a Handle object named student that contains a Core* pointer, which we
initialize to point to the object of type Grad that we just created:
Finally, we define three operator functions. The first of these, operator bool() lets
users test the value of a Handle in a condition. The operation returns true if the
Handle is bound to an object, and false otherwise. The other two define operator*
and operator-> , which give access to the object bound to the Handle :
template
T& Handle::operator*() const
{
if (p)
return *p;
throw runtime_error("unbound Handle");
}
template
T* Handle::operator->() const
{
if (p)
return p;
throw runtime_error("unbound Handle");
}
Applying the built-in unary * operator to a pointer yields the object to which the pointer
points. Here we define our own * , so that * of a Handle object yields the value that
results from applying the built-in * operator to the pointer member of that Handle
object. Given our student object, *student will yield the result of applying * to
student.p (assuming we could access the p member). In other words, the result of
*student will be a reference to the Grad object that we created when we initialized
student .
The -> operator is a bit more complicated. Superficially, -> looks like a binary operator,
but in fact it behaves differently from ordinary binary operators. Like the scope or dot
operators, the -> operator is used to access a member whose name appears in its right
operand from an object named by its left operand. Because names are not expressions,
we have no direct access to the name that our user requested. Instead, the language
requires that we define -> to return a value that can be treated as a pointer. When we
define operator-> , we are saying that if x is a value of type that defines operator-
> then
x->y
is equivalent to
(x.operator->())->y
In this case, operator-> returns the pointer that its object holds. So for student ,
student->y
is equivalent to
(student.operator->())->y
which, because of the way we defined operator-> , is equivalent to
student.p->y
(ignoring the fact that protection would not ordinarily allow us to access student.p
directly). Thus, the -> operator has the effect of forwarding calls made through a
Handle object to the underlying pointer that is a member of the Handle object.
One of our objectives was for Handle to preserve the polymorphic behavior associated
with built-in pointers. Having seen the definitions of operator* and operator-> , we
can see that we have reached our goal. These operations yield either a reference or a
pointer, through which we obtain dynamic binding. For example, if we execute
student->grade() , we're calling grade through the p pointer inside student . The
particular version of grade that is run depends on the type of the object to which p
points. Assuming that student still points to the Grad object with which it was
initialized, this call would be to Grad::grade . Similarly, because operator* yields a
reference, if we evaluate (*student).grade() , then we are calling grade through a
reference, and so the implementation will decide which particular function to call at run
time.
14.1.2 Using a generic handle
We could use Handle s to rewrite the pointer-based grading program from
§13.3.1/241:
int main()
{
vector< Handle > students; // changed type
Handle record; // changed type
char ch;
string::size_type maxlen = 0;
// read and store the data
while (cin >> ch) {
if (ch == 'U')
record = new Core; // allocate a Core object
else
record = new Grad; // allocate a Grad object
record->read(cin); // Handle::->, then virtual call to read
maxlen = max(maxlen, record->name().size()); // Handle::->
students.push_back(record);
}
// compare must be rewritten to work on const Handle&
sort(students.begin(), students.end(), compare_Core_handles);
// write the names and grades
for (vector< Handle >::size_type i = 0;
i != students.size(); ++i) {
// students[i] is a Handle, which we dereference to call the functions
cout << students[i]->name()
<< string(maxlen + 1 - students[i]->name.size(), ' ');
try {
double final_grade = students[i]->grade();
streamsize prec = cout.precision();
cout << setprecision(3) << final_grade
<< setprecision(prec) << endl;
} catch (domain_error e) {
cout << e.what() << endl;
}
// no delete statement
}
return 0;
}
This program stores Handle objects instead of Core* objects and so, as we did
in §13.3.1/240, we'll need to write a non-overloaded comparison operation that operates
on const Handle& that we can pass to sort . We leave the implementation as
an exercise, but assume that it is named compare_Core_handles .
The only other differences are in the output loop. Dereferencing students[i] yields a
Handle , which has an operator-> that we use to access the name and grade
functions through the underlying Core* . For example, students[i]->grade() uses
the overloaded -> , so it effectively calls students[i].p->grade() . Because grade
is virtual , we'll run the version that is appropriate to the type of the object to which
students[i].p points. Moreover, because Handle takes care of memory
management for us, we no longer need to delete the objects to which the elements of
students refer.
More important, we can also reimplement Student_info , which can now become a
pure interface class, delegating the work of managing pointers to Handle :
class Student_info {
public:
Student_info() { }
Student_info(std::istream& is) { read(is); }
// no copy, assign, or destructor: they're no longer needed
std::istream& read(std::istream&);
std::string name() const {
if (cp)
return cp->name();
else throw runtime_error("uninitialized Student");
}
double grade() const {
if (cp) return cp->grade();
else throw runtime_error("uninitialized Student");
}
static bool compare(const Student_info& s1,
const Student_info& s2) {
return s1.name() < s2.name();
}
private:
Handle cp;
};
In this version of Student_info , cp is a Handle rather than a Core* .
Therefore, we no longer need to implement the copy-control functions, because the
Handle manages the underlying object. The other constructors operate as before. The
name and grade functions look the same, but their execution relies on the conversion
to bool , which is invoked in the test on cp and in the overloaded operator-> from
the Handle class, which is used to get at the functions on the underlying objects.
To complete our reimplementation, we need to write the read function:
istream& Student_info::read(istream& is)
{
char ch;
is >> ch; // get record type
// allocate new object of the appropriate type
// use Handle(T*) to build a Handle from the pointer to that object
// call Handle::operator= to assign the Handle to the left-hand side
if (ch == U')
cp = new Core(is);
else
cp = new Grad(is);
return is;
}
This code looks like the earlier Student_info::read function, but its execution is
quite different. Most obviously, the delete statement is gone, because the assignment
to cp will free the object if appropriate. To understand this code, we need to trace
through it carefully. For example, when we execute new Core(is) , we get a Core*
object, which we implicitly convert to a Handle using the Handle(T*)
constructor. That Handle value is then assigned to cp using the Handle assignment
operator, which automatically delete s the object, if any, to which the Handle
previously referred. This assignment constructs and destroys an extra copy of the Core
object that we created, a copy that we will now see how to avoid.
14.2 Reference-counted handles
At this point, we have achieved our goal of separating the work of managing pointers
from our interface class. We can use Handles to implement a wide variety of interface
classes, none of which would have to worry about memory management. However, our
Handle class still has the problem that copying or assigning objects copies the
underlying data, even when it does not need to do so. The reason being that Handle
always copies the object to which the Handle is bound.
In general, we would like to be able to decide whether we want to make such copies. For
example, we might want objects that are copies of one another to share their underlying
information. Such classes do not need or want valuelike behavior. Other classes may not
have any way of changing state once an object is created. In such cases, there is no
reason to copy the underlying object. Copying such objects would waste time and space.
To support these kinds of classes, we'd like a kind of Handle that does not copy the
underlying object when the Handle itself is copied. Of course, if we allow multiple
Handles to be bound to the same underlying object, we'll still need to free that object
at some point. The obvious point at which to free the object is when the last Handle
that points to it goes away.
To this end, we will use a reference count, which is an object that keeps track of how
many objects refer to another object. Each target object will have a reference count
associated with it. We will arrange to increment the reference count each time we create
a new handle that refers to our target object, and to decrement the reference count
each time a referring object goes away. When the last referring object goes away, the
reference count will become zero. At that point we'll know that it is safe to destroy the
target object.
This technique can save a lot of unneeded memory management and copying of data.
We'll first build a new class, called Ref_handle, which will show how to add reference
counts to our Handle class. Then, in the next two sections, we'll explore how reference
counting can help us define classes that behave like values while sharing
representations.
To add reference counting to a class, we have to allocate a counter, and change the
operations that create, copy, and destroy the objects so that they update the counter
appropriately. Each object to which we have a Ref_handle will have a reference count
associated with it. The only question is where to store the counter. In general, we don't
own the source code for the types from which we want to make Ref_handles, so we
can't just add the counter to the object class type itself. Instead, we'll add another
pointer to our Ref_handle class to keep track of the count. Each object to which we
have attached a Ref_handle will also have an associated reference count that tracks
how many copies we have made of that object:
template class Ref_handle {
public:
// manage reference count as well as pointer
Ref_handle(): refptr(new size_t(1)), p(0) { }
Ref_handle(T* t): refptr(new size_t(1)), p(t) { }
Ref_handle(const Ref_handle& h): refptr(h.refptr), p(h.p)
{
++*refptr;
}
Ref_handle& operator=(const Ref_handle&);
~Ref_handle();
// as before
operator bool() const { return p; }
T& operator*() const
{
if (p)
return *p;
throw std::runtime_error("unbound Ref_handle");
}
T* operator->() const {
if (p)
return p;
throw std::runtime_error("unbound Ref_handle");
}
private:
T* p;
size_t* refptr; // added
};
We have added a second data member to our Ref_handle class, and updated the
constructors to initialize that new member. The default constructor and the constructor
that takes a T* create new Ref_handle objects, so they allocate a new reference count
(of type size_t) and set the value of that counter to 1. The copy constructor doesn't
create a new object. Instead, it copies the pointers from the Ref_handle object
that it was passed, and increments the reference count to indicate that there is one
more pointer to the T object than there was previously. Thus, the new Ref_handle
object points to the same T object, and to the same reference count, as the
Ref_handle object from which we copy. So, for example, if X is a Ref_handle
object, and we create Y as a copy of X, then the situation looks like this:
The assignment operator also manipulates the reference count instead of copying the
underlying object:
template
Ref_handle& Ref_handle::operator=(const Ref_handle& rhs)
{
++*rhs.refptr;
// free the left-hand side, destroying pointers if appropriate
if (--*refptr == 0) {
delete refptr;
delete p;
}
// copy in values from the right-hand side
refptr = rhs.refptr;
p = rhs.p;
return *this;
}
As always, it is important to protect against self-assignment, which we do by
incrementing the reference count of the right-hand side before decrementing the
reference count of the left-hand side. If both operands refer to the same object, the net
effect will be to leave the reference count unchanged, while ensuring that it will not
reach zero unintentionally.
If the reference count goes to zero when we decrement it, then the left operand is the
last Ref_handle bound to the underlying object. Because we are about to obliterate
the value of the left operand, we are about to delete the last reference to that object.
Therefore, we must delete the object, and its corresponding reference count before we
overwrite the values in refptr and p. We delete both p and refptr because both
were dynamically allocated objects; thus, we must free them to avoid a memory leak.
Having deleted the pointers, if needed, we then copy the values from the right-hand
side into the left-hand side and, as usual, return a reference to the left operand.
The destructor, like the assignment operator, checks whether the Ref_handle object
being destroyed is the last one bound to its T object. If so, the destructor deletes the
objects to which the pointers point:
template Ref_handle::~Ref_handle()
{
if (--*refptr == 0) {
delete refptr;
delete p;
}
}
This version of Ref_handle works well for classes that can share state between copies
of different objects, but what about classes, such as Student_info, that want to
provide valuelike behavior? For example, if we used the Ref_handle class to
implement Student_info, then after executing, say,
Student_info s1(cin); // initialize s1 from the standard input
Student_info s2 = s1; // "copy" that value into s2
the two objects s1 and s2 would refer to the same underlying object, even though it
might appear that s2 is a copy of s1. If we do anything to change the value of one of
these objects, we change the value of the other as well.
Our original Handle class defined in §14.1.1/255 provided valuelike behavior because it
always copied the associated object by calling clone. It should be easy to see that our
new Ref_handle class never calls clone at all. Because Ref_handle never calls
clone, handles of this type never copy the objects to which they are attached. On the
other hand, this version of Ref_handle certainly has the advantage of avoiding
needless copying of data. The trouble is that it does so by avoiding all copying, needless
or not. What can we do?
14.3 Handles that let you decide when to share data
So far, we have seen two possible definitions for our generic handle class. The first
version always copies the underlying object; the second never does so. A more useful
kind of handle is one that lets the program that uses it decide when it wants to copy the
target object and when it doesn't. Such a handle class preserves the performance of
Ref_handle, and allows the class author to provide the valuelike behavior of Handles.
Such a handle will preserve the useful properties of built-in pointers, but avoids many of
the pitfalls. Thus, we'll call this final handle class Ptr, to capture the notion that it is a
useful substitute for built-in pointers. In general, our Ptr class will copy the object if we
are about to change its contents, but only if there is another handle attached to the
same object. Fortunately, the reference count gives us a way to tell whether our handle
is the only one attached to its object.
The fundamentals of our Ptr class are the same as the Ref_handle class that we
developed in §14.2/260. All we need to do is to add one more member function to that
class to give control to the user:
template class Ptr {
public:
// new member to copy the object conditionally when needed
void make_unique() {
if (*refptr != 1) {
--*refptr;
refptr = new size_t(1);
p = p ? p->clone() : 0;
}
}
// the rest of the class looks like Ref_handle except for its name
Ptr(): refptr(new size_t(l)), p(0) { }
Ptr(T* t): refptr(new size_t(l)), p(t) { }
Ptr(const Ptr& h): refptr(h.refptr), p(h.p) { ++*refptr; }
Ptr& operator=(const Ptr&) ; // implemented analogously to §14.2/261
~Ptr(); // implemented analogously to §14.2/262
operator bool() const { return p; }
T& operator*() const;, // implemented analogously to §14.2/261
T* operator->() const; // implemented analogously to §14.2/261
private:
T* p;
size_t* refptr;
};
This new make_unique member does just what we want: If the reference count is 1, it
does nothing; otherwise, it uses the clone member of the object to which the handle is
bound to make a copy of that object, and sets p to point to the copy. If the reference
count is not 1, there must be at least one other Ptr that refers to the original object.
We therefore decrement the reference count associated with the original (which might
reduce it to 1 but not to 0). We then create a new reference count for our handle, and
for others that might be created in the future as copies of it. Because so far there is only
one Ptr attached to the copy that we're makings we initialize the counter to 1. Before
calling clone, we check whether the pointer to the object from which we're copying is
bound to an actual object. If so, we call the clone function to copy that object. When
we're done, we'll know that this Ptr is the only one that is attached to the object to
which p points. That object is either the same one as before (if the original reference
count was one) or a copy of it (if the reference count was greater than one).
We can use this latest revision of Ptr in the handle-based Student_info
implementation in §14.1.2/258. When we do, we'll discover that we don't need to
change this implementation of Student_info at all, because none of our operations
changes the value of the object without also replacing it. The only Student_info
operation that changes the value is the read function, but that function always assigns a
newly created value to its Ptr member. When it does so, the Ptr assignment operator
will either free the old value or keep it around, depending on whether there are other
objects that refer to the old value. In either case, the object into which we read will
have a new Ptr object and will, therefore, be the only user of that object. If our users
write code such as
Student_info s1;
read(cin, s1); // give s1 a value
Student_info s2 = s1; // copy that value into s2
read(cin, s2); // read into s2; changes only s2 and not s1
then the value of s2 is reset in the read call, but the value of s1 is unchanged.
On the other hand, had we added the virtual version of the regrade function
described in §13.6.2/249 to the Core hierarchy, and given Student_Info a
corresponding interface function, then that function would need to change to call
make_unique:
void Student_info::regrade(double final, double thesis)
{
// get our own copy before changing the object
cp.make_unique();
if (cp)
cp->regrade(final, thesis);
else throw run_time_error("regrade of unknown student");
}
14.4 An improvement on controllable handles
Useful as our controllable handle might be, it doesn't quite do all we want. For example,
suppose we want to use it to reimplement the Str class from Chapter 12. As we saw in
§12.3.4/219, we implicitly copy a lot of characters to form the new Str s that result
from concatenating two existing Str objects. By reference-counting the Str class, we
might think that we can avoid at least some of these copies:
// does this version work?
class Str {
friend std::istream& operator>>(std::istream&, Str&);
public:
Str& operator+=(const Str& s) {
data.make_unique();
std::copy(s.data->begin(), s.data->end(),
std::back_inserter(*data));
return *this;
}
// interface as before
typedef Vec::size_type size_type;
// reimplement constructors to create Ptrs
Str(): data(new Vec) { }
Str(const char* cp): data(new Vec) {
std::copy(cp, cp + std::strlen(cp),
std::back_inserter(*data));
}
Str(size_type n, char c): data(new Vec(n, c)) { }
template Str(In i, In j): data(new Vec) {
std::copy(i, j, std::back_inserter(*data));
}
// call make_unique as necessary
char& operator[](size_type i) {
data.make_unique();
return (*data)[i];
}
const char& operator[](size_type i) const { return (*data)[i]; }
size_type size() const { return data->size(); }
private:
// store a Ptr to a vector
Ptr< Vec > data;
};
// as implemented in §12.3.2/216 and §12.3.3/219
std::ostream& operator<<(std::ostream&, const Str&);
Str operator+(const Str&, const Str&);
We have preserved the interface to Str , but we have fundamentally changed the
implementation. Instead of holding a vector directly in each Str object, we store a
Ptr to the vector . This design allows multiple Str s to share the same underlying
character data. The constructors initialize this Ptr by allocating a new vector
initialized with the appropriate values. The code for the operations that read, but do not
change, data are unchanged from our previous version. Of course, these operations
now operate on a Ptr , so there is an indirection through the pointer stored in the Ptr
to get at the underlying characters that make up the Str . The interesting operations
are the ones that change the Str , such as the input operator, the compound
concatenation operator, and the nonconst version of the subscript operator.
For example, look at the implementation of Str::operator+= . It wants to append
data to the underlying vector , so it calls data.make_unique() . Once it has done
so, the Str object has its own copy of the underlying data, which it can modify freely.
14.4.1 Copying types that we can't control
Unfortunately, the definition of make_unique has a serious problem:
template
void Ptr::make_unique()
{
if (*refptr != 1) {
--*refptr;
refptr = new size_t(1);
p = p ? p->clone() : 0; // here is the problem
}
}
Look at the call to p->clone . Because we are using a Ptr< vector > , this
call will try to call the clone function that is a member of vector .
Unfortunately, no such function exists!
Yet the clone function has to be a member of the class to which we are attaching a
Ptr , because only in that way can it be a virtual function. In other words, making
clone a member is critically important to making it possible for Ptr to work across all
members of an inheritance hierarchy; yet doing so is impossible, because we can't
change the definition of the Vec class. That class is designed to implement a subset of
the interface to the standard vector class. If we add a clone member, we'll no longer
have a subset because we'll have added a member that vector does not have. What
can we do?
Solutions to tough problems such as this one often involve what we jokingly call the
fundamental theorem of software engineering: All problems can be solved by
introducing an extra level of indirection. The problem is that we are trying to call a
member function that does not exist, and we have no way to cause the member function
to exist. The solution, then, is not to call the member function directly but to define an
intermediary global function that we can both call and create. We will still call this new
function clone
template T* clone(const T* tp)
{
return tp->clone();
}
and change our make_unique member to call it
template
void Ptr::make_unique()
{
if (*refptr != 1) {
--*refptr;
refptr = new size_t(1);
p = p ? clone(p): 0; // call the global (not member) version of clone
}
}
It should be clear that introducing this intermediary function does not change the
behavior of make_unique . It still calls clone , which still calls the clone member of
the object that is being copied. However, make_unique now works through a level of
indirection:
It calls the nonmember clone function, which in turn calls the clone member for the
object to which p points. For classes such as Student_info that define clone , this
indirection buys us nothing. But for classes such as Str that hold Ptr s to types that do
not provide a clone function, the indirection is exactly what we need to make the whole
thing work. For these latter types, we can define yet another intermediary function:
// the key to making Ptr< Vec > work
template<>
Vec* clone(const Vec* vp)
{
return new Vec(*vp);
}
The use of template<> at the beginning of this function indicates that the function is a
template specialization . Such specializations define a particular version of a template
function for the specific argument type. By defining this specialization, we are saying
that clone behaves differently when we give it a pointer to a Vec than it
behaves when we give it any other pointer type. When we pass clone a Vec*
argument, the compiler will use this specialized version of clone . When we pass other
types of pointers, it will instantiate the general template form of clone , which calls the
member clone for the pointer that it was passed. Our specialized version uses the
Vec copy constructor to construct a new Vec from the one that we gave
it. It is true that this specialization of clone does not offer virtual behavior, but we do
not need it to do so because there are no classes derived from Vec .
What we have done, then, is to moderate our reliance on the clone member by
recognizing that the member might not exist. By introducing the extra indirection, we
have made it possible to specialize the clone template to do whatever is appropriate to
copy an object of a particular class, be it to use a clone member, to call a copy
constructor, or something else entirely. In the absence of a specialization, the Ptr class
will use the clone member, but it will do so only if there is a call to make_unique . In
other words
If you use Ptr but you don't use Ptr::make_unique , then it doesn't
matter whether T::clone is defined.
If you use Ptr::make_unique , and T::clone is defined, make_unique will
use T::clone .
If you use Ptr::make_unique , and you don't want to use T::clone
(perhaps because it doesn't exist), you can specialize clone to do whatever
you want.
The extra indirection has made it possible to control the behavior of Ptr in great detail.
All that remains is the hard part—deciding what you wanted to do in the first place.
14.4.2 When is a copy necessary?
One last part of this example is worth reviewing in detail. Look back at the definitions of
the two versions of operator[] . One of them calls data.make_unique ; the other
doesn't. Why the difference?
The difference relates to whether the function is a const member. The second version
of operator[] is a const member function, which means that it promises not to
change the contents of the object. It keeps this promise by returning a const char& to
its caller. Therefore, there is no harm in sharing its underlying Vec object with
other Str objects. After all, the user can't use the value obtained to change the value of
the Str .
In contrast, the first version of operator[] returns a char& , which means that a user
could use this return value to change the contents of the Str . If the user does so, we
want to limit the change to this Str and not propagate the change to any other Str s
that might happen to share the underlying Vec . We defend against the possibility of
changing the value of any other Str objects by calling make_unique on the Ptr before
returning a reference to a character of the Vec .
14.5 Details
Template specializations look like the template definitions that they are specializing,
but they omit one or more of the type parameters, replacing them with specific types
instead. The myriad uses of template specializations are beyond the scope of this book,
but you should know that they exist, and that they are useful for making decisions about
types during compilation.
Exercises
14-0. Compile, execute, and test the programs in this chapter.
14-1. Implement the comparison operation that operates on Ptr.
14-2. Implement and test the student grading program using Ptr objects.
14-3. Implement the Student_info class to use the final version of Ptr, and use that
version to implement the grading program from §13.5/247.
14-4. Reimplement the Str class to use the final version of Ptr.
14-5. Test the reimplemented Str class by recompiling and rerunning programs that
use Str, such as the version of split and the picture operations that use a Vec.
14-6. The Ptr class really solves two problems: maintaining reference counts, and
allocating and deallocating objects. Define a class that does reference counting and
nothing else; then use that class to reimplement the Ptr class.
15
Revisiting character pictures
Inheritance is most useful in modeling large, complex systems, which are well beyond
the scope of any introductory book. One of the reasons that we are so fond of the
character- picture example that we introduced in §5.8/91 is that such pictures lend
themselves to an object-oriented solution, yet we can implement them in only a few
hundred lines of code. We have used this example for many years, refining our code and
simplifying the presentation. In reviewing the example for this book, we were able to
remove nearly half the code by using the standard library and our generic handle class
from Chapter 14.
In §5.8/91, we wrote several functions that represented a character picture as a
vector, a strategy that entailed copying characters whenever we constructed
a new picture. Copying all those characters wastes time and space. For example, if we
were to concatenate two copies of a picture, we would then store three copies of each
character: one for the original, and one for each side of the newly concatenated picture.
Even more important, the solution in §5.8/91 discards all structural information about
the pictures. We have no idea how a given picture was formed. It might have been the
initial input from our user, or it might have been created by applying one or more
operations to simpler pictures. Some potentially useful operations require preserving a
picture's structure. For example, if we want to be able to change the frame characters in
a picture, we can do so only if we know which components of a picture were framed and
which ones were not. We cannot look only for instances of the frame characters,
because these characters might, coincidentally, have been part of an initial input
picture.
As we'll see in this chapter, by using inheritance and our generic handle class, we will be
able to preserve the structural information inherent in a picture, while at the same time
reducing the space consumption of our system dramatically.
15.1 Design
We are trying to solve two distinct problems. One is a design problem—we'd like to keep
structural information about how a picture was created. The other is an implementation
problem—we want to store fewer copies of the same data. Both problems result from
our decision to store a picture as a vector, so we must revisit that decision.
We can solve the implementation problem by managing our data with the Ptr class that
we developed in Chapter 14. That class will let us store the actual character data in a
single object, and then arrange for multiple pictures to share that same object. For
example, if we frame a given picture, we will no longer have to copy the characters of
the picture that we're framing. Instead, class Ptr will manage a reference count
associated with the data, which will indicate how many other pictures are using those
data.
The design problem is harder to solve. Each picture that we create has a structure,
which we want to retain. We form a picture either from an initial collection of characters,
or through one of three operations: frame, to produce a framed picture; and hcat or
vcat, to create pictures that are concatenated horizontally or vertically.
In other words, we have four similar kinds of pictures. Despite their similarity, we create
them differently, and we would like to keep track of the differences.
15.1.1 Using inheritance to model the structure
Our problem is a perfect match for inheritance: We have various kinds of data structures
that are similar to one another, but that differ in ways that we sometimes want to take
into account. Each of our data structures is a kind of picture, which implies that
inheritance is a sensible way to represent these data structures. We can define a
common base class that models the common properties of every kind of picture, and
then derive from that base class a separate class for each specific kind of picture that
we want to support.
We'll call the derived classes String_Pic, for pictures created from strings that our
user gives us; Frame_Pic, for a picture created by framing another picture; and
HCat_Pic and VCat_Pic, for pictures that are the result of concatenating two other
pictures horizontally or vertically respectively. By relating these classes through
inheritance, we can use virtual functions to write code that doesn't always need to
know the precise kind of picture on which it is operating. That way, our users can still
use any of our operations without knowing which kind of picture is being manipulated.
We will derive each of these classes from a common base class, which we shall call
Pic_base, resulting in the following inheritance hierarchy:
The next question to resolve is whether to make the inheritance hierarchy visible to our
users. There seems to be little reason to do so. None of our operations deals with
specific kinds of pictures; instead, they all deal with the abstract notion of a picture. So,
there is no need to expose the hierarchy. Moreover, because we intend to use a
reference- counting strategy, our users will find it more convenient if we hide the
inheritance and associated reference counting.
Instead of making our users deal directly with Pic_base and its associated derived
classes, we'll define a picture-specific interface class. Our users will access that class,
freeing them from having to be aware of any of our implementation's details. In
particular, using an interface class will hide the inheritance hierarchy, along with the fact
that our class relies on Ptr. Apparently, then we'll need to define six classes: the
interface class, the base class for our inheritance hierarchy, and the four derived
classes. We'll call the interface class Picture. Internally, Picture will use a Ptr to
manage its data.
What kind of Ptr? That is, what type of objects will the Ptr manage? It will manage our
implementation class, Pic_base. Thus, class Picture will have a single data member,
which will have type Ptr.
We said that we intend to conceal our use of Pic_base and its related hierarchy, so
that users will manipulate these objects only indirectly through class Picture, and will
not access any of these classes directly. It turns out that the most straightforward way
to hide these classes is to rely on the normal protection mechanisms. By giving these
classes an empty public interface, we can let the compiler enforce our decision that all
interactions with our pictures will be through class Picture.
To make these decisions concrete, let's write code that captures what we know so far:
// private classes for use in the implementation only
class Pic_base { };
class String_Pic: public Pic_base { };
class Frame_Pic: public Pic_base { };
class VCat_Pic: public Pic_base { };
class HCat_Pic: public Pic_base { };
// public interface class and operations
class Picture {
public:
Picture(const std::vector& =
std::vector());
private:
Ptr p;
};
Each Picture object will hold a (private) Ptr object. Class Pic_base
is the common base class for the four classes that will represent our four kinds of
pictures. The Ptr class will manage the reference counts to allow us to share the
underlying Pic_base objects. We will implement each operation on a Picture by
forwarding that operation through the Ptr to the underlying derived-class object. We
haven't thought yet about what these operations will be, so for now we've left the bodies
of Pic_base and its derived classes empty.
So far, the Picture class is pretty simple: The only operation is to create a Picture
from a vector of strings. We use a default argument (§7.3/127) to make that
vector optional. If a user constructs a Picture with no argument, then the compiler
will supply vector() as an argument automatically, which yields a
vector with no elements. Therefore, the effect of the default argument is to
allow us to use a definition such as
Picture p; // an empty Picture
to create a Picture with no rows.
Next, we need to think about how to represent our other Picture operations. We know
that we want to implement frame, hcat, and vcat. What we must decide is how to do
so, and whether these operations should be members of class Picture. The operations
do not change the state of the Picture on which they operate, so there is no strong
reason to make them members. Moreover, there is a strong reason not to do so: As we
saw in §12.3.5/220, by making them nonmembers we can allow conversions.
For example, because the Picture constructor that we have already written is not
explicit, users will be able to write
vector vs;
Picture p = vs;
and the implementation will convert vs into a Picture for us. If we wish to allow this
behavior—and we do—then we should also allow users to write expressions such as
frame(vs). If frame were a member, then users would not be able to write the
seemingly equivalent vs.frame(). Remember that conversions are not applied to the
left operand of the . operator, so this call would be interpreted as invoking the
(nonexistent) frame member of vs.
Moreover, we believe that our users will find it convenient to use an expression syntax
to build up complicated pictures. We consider it clearer to write,
hcat(frame(p), p)
than to write
p.frame().hcat(p)
because the first example reflects the symmetry of hcat's arguments and the second
example conceals it.
In addition to the functions that let us build Pictures, we will want to define an output
operator that can write the contents of a Picture. These decisions let us flesh out the
rest of our interface design:
Picture frame(const Picture&);
Picture hcat(const Picture&, const Picture&);
Picture vcat(const Picture&, const Picture&);
std::ostream& operator<<(std::ostream&, const Picture&);
15.1.2 The Pic_base class
The next step in our design is to fill in the details of the Pic_base hierarchy. If we look
back at our initial implementation, we'll see that we used the vector::size
function to determine how many strings were in a given picture, and we wrote a
separate width function (§5.8/91), which proved useful in padding the output. When
we think about how we will display a picture, we see that we are likely to need to be
able to perform these same operations on our classes that are derived from Pic_base.
These operations will need to be virtual, so that we can ask any kind of Pic_base
how many rows it has and how wide its widest row is. Furthermore, because our users
will use the output operator to write the contents of a particular Pic_base, we can infer
that we'll need another virtual function to display a given Pic_base on a given
ostream.
The only one of these operations that needs significant insight is display. It is easy to
decide that one of the parameters to display should be the stream on which to write
its output, but figuring out what other parameters display might take requires that we
think carefully about how it will operate. When we write a Picture, that Picture will
comprise one or more component parts, each of which is an object of a class derived
from Pic_base. If we think about writing a horizontally concatenated picture, it will be
apparent that each row of the output from a single Picture might involve writing the
corresponding row for more than one subpicture. In particular, we cannot write the
entire contents of one subpicture, and then the entire contents of the other. Instead, we
have to write the contents of each subpicture a row at a time, interleaved with the
corresponding rows of the other subpictures. We can conclude, therefore, that the
display function needs a parameter that says which row to write.
Similarly, when we display the left-hand part of a horizontally concatenated picture,
we'll need to tell the corresponding subpicture to pad each row to use the full width()
of itself on each line. We'll also need to tell a picture that is contained within a
Frame_Pic to pad to its widest extent. On the other hand, if we're displaying a
Picture that contains only a String_Pic, or a vertically concatenated Picture
composed only of String_Pics, then padding the output results only in writing a lot of
unneeded trailing blanks. So, as an optimization, we'll pass display a third argument
that indicates whether to pad the output.
These observations lead us to decide that the display function will take three
arguments: the stream on which to generate the output, the number of the row to write,
and a bool that will indicate whether to pad the picture to its full width. With these
decisions, we can fill in the details of the Pic_base family of classes:
class Pic_base {
// no public interface
typedef std::vector::size_type ht_sz;
typedef std::string::size_type wd_sz;
virtual wd_sz width() const = 0;
virtual ht_sz height() const = 0;
virtual void display(std::ostream&, ht_sz, bool) const = 0;
};
We start by defining shorthand names for the size_types that we'll need in our
implementation. Thinking ahead, we can see that the underlying data will still be a
vector, so the size_type member of vector will be the right
type to represent the height of a picture, and the one from string will be the one we
need for the width. We'll abbreviate these types as ht_sz and wd_sz respectively.
The other task is to define our virtual functions for the base class, which you'll notice
take a new form: In each case we say = 0 where the body would appear. This syntax
indicates our intention that there be no other definition of this virtual function. Why
did we define these functions this way?
To answer this question, let's begin by thinking about what the definitions would look
like if we tried to write them. In our design, Pic_base exists only to act as the common
base class for our concrete picture classes. We will create objects of these concrete
types as a result of executing one of the Picture operations, or in response to a user's
creating a Picture from a vector. None of these operations directly creates
or manipulates Pic_base objects. If there are never any Pic_base objects, then what
would it mean to take the height or width of a Pic_base object (as opposed to doing
so for an object of a type derived from Pic_base)? These operations are needed only
for the derived classes, in which there always will be a concrete picture. For a
Pic_base itself, there is nothing of which to take the height or width.
Instead of forcing us to concoct an arbitrary definition for these operations, the C++
language lets us say that there will be no definition for a given virtual function. As a
side effect of declining to implement the virtual function, we also promise that there
will never be objects of the associated type. There may still be objects of types derived
from this type, but there are no objects of its exact type.
The way that we specify that we don't intend to implement a virtual function is to say
= 0, as we did on height, width, and display. Doing so makes it a pure virtual
function. By giving a class even a single pure virtual function, we are also implicitly
specifying that there will never be objects of that class. Such classes are called abstract
base classes, because they exist only to capture an abstract interface for an
inheritance hierarchy. They are purely abstract: There are no objects of the base class
itself. Once we give a class any pure virtuals, the compiler will enforce our design by
preventing us from creating any objects of an abstract class.
15.1.3 The derived classes
As with virtual itself, the fact that a function is a pure virtual is inherited. If a
derived class defines all of its inherited pure virtual functions, it becomes a concrete
class, and we can create objects of that class. However, if the derived class fails to
define even a single pure virtual function that it inherits, then the abstract nature is
also inherited. In this case the derived class is itself abstract, and we will not be able to
create objects of the derived class. Because each of our derived classes is intended to
model a concrete class, we know that we have to redefine all of the virtuals in each of
the derived classes.
The only other things we have to think about right now are what data each of our
derived classes will contain, and the associated question of how we will construct objects
of each type. We designed these classes to model the structure of how a picture was
formed. The type of the picture object tells us how it was created: A String_Pic is
created from character data that a user supplied to us; a Frame_Pic results from
running frame on another Picture, and so on. In addition to knowing how an object
was created, we also need to store the object(s) from which it was created. For a
String_Pic, we'll need to remember the characters that the user gave us, which we
can do in a vector. We create a Frame_Pic by framing another Picture,
so we'll need to store the Picture that was framed. Similarly, we create HCat_Pics
and VCat_Pics by combining two other Pictures. These classes will store the
Pictures used in creating the resultant new object.
Before settling on a design that stores Pictures in the Pic_base derived classes, we
should think through the implications of this design a bit more deeply. Class Picture is
an interface class intended for use by our users. As such, it captures the interface to our
problem domain but not the implementation. Specifically, it does not have height,
width, or display operations. If we think a bit about how these functions might be
implemented, we'll see that we'll need access to the corresponding operations on the
Picture(s) stored in each of the derived types. For example, to calculate height of a
VCat_Pic, we need to add the heights of the two Pictures from which it was formed.
Similarly, we'll obtain the width by finding the maximum of the widths of the two
component Pictures.
An implication of storing a Picture in each of the derived classes is that we'll have to
give class Picture functions that duplicate the Pic_base operations. Doing so
obscures our initial design intent, which was that class Picture should be concerned
with interface not implementation. We can maintain our design by realizing that what we
need in the derived classes is not an interface object but an implementation object. This
realization implies that instead of storing a Picture, we should store a
Ptr. This design keeps a clean separation between interface and
implementation, while still maintaining our intention to reference count our
implementation objects to avoid unnecessary data duplication.
Although our design is clean, enough indirection is involved that a picture may help:
Here we assume that we are generating three Pictures. The first Picture represents
a String_Pic object that holds the data that we got from the user. The second one
represents a Frame_Pic object that we constructed by calling frame on the initial
Picture. Finally, we construct a Picture that represents the output of vcat run on
the two previous Pictures. Each Picture has a single data member, which is a
Ptr. That Ptr points to an object of the appropriate Pic_base derived
type. Each such object, in turn, contains either the vector that holds a copy of the data
we got from the user, or one or two Ptrs that point to the Pic_base objects used to
create the Picture. Not shown in this diagram are the reference counts associated with
the Ptr objects, because we assume that the Ptr class is doing its job, and we can
ignore the details of that job.
What's different from what we did in Chapter 5 is that only String_Pic contains any
characters. The others hold one or two Ptrs. Therefore, we won't copy any characters
when we create f or v. Instead, the Ptr will be yet another reference to the Ptrs in the
Pictures that are used in creating a new Picture, and the Ptr class will take care of
the reference counting for us. So, when we call frame(pic), the effect is to create a
new Frame_Pic object, and to point its Ptr at the same String_Pic that is stored in
pic. Similarly, the VCat_Pic contains two Ptrs pointing to the Frame_Pic and the
String_Pic respectively. We will not destroy any of these Pic_base objects; doing so
is the responsibility of the Ptr class. It will arrange to destroy each Pic_base object
when the last Ptr that refers to each object has gone away.
At this point, we should capture these design decisions in code. We know what data
each object will contain, and we know what our operations will be:
class Pic_base {
// no public interface
typedef std::vector::size_type ht_sz;
typedef std::string::size_type wd_sz;
// this class is an abstract base class
virtual wd_sz width() const = 0;
virtual ht_sz height() const = 0;
virtual void display(std::ostream&, ht_sz, bool) const = 0;
};
class Frame_Pic: public Pic_base {
// no public interface
Ptr p;
Frame_Pic(const Ptr& pic): p(pic) { }
wd_sz width() const;
ht_sz height() const;
void display(std::ostream&, ht_sz, bool) const;
};
Here we say that Frame_Pic inherits from Pic_base, and declare our intention to
define class-specific versions of each of the three virtuals from that base class. Thus,
Frame_Pic will not be an abstract class, and we will be able to create Frame_Pic
objects. It is worth noting that we have declared these virtuals in the private
section of the class. Doing so lets the compiler enforce our design decision that only
class Picture and operations on Pictures can access the Pic_base hierarchy. Of
course, because these virtuals are private, we may need to revisit the class
definition to include friend declarations for class Picture, or the associated
operations, as needed.
The Frame_Pic constructor needs only to copy the Ptr from the object that is being
framed, which it does in the constructor initializer. The constructor body is empty,
because there is no other work to do.
Continuing with our other derived classes, the concatenation classes will operate
similarly to Frame_Pic: Each class will need to remember its two constituent pictures.
How they were concatenated, vertically or horizontally, will be implicit in the type itself:
class VCat_Pic: public Pic_base {
Ptr top, bottom;
VCat_Pic(const Ptr& t, const Ptr& b):
top(t), bottom(b) { }
wd_sz width() const;
ht_sz height() const;
void display(std::ostream&, ht_sz, bool) const;
};
class HCat_Pic: public Pic_base {
Ptr left, right;
HCat_Pic(const Ptr& l, const Ptr& r):
left(l), right(r) { }
wd_sz width() const;
ht_sz height() const;
void display(std::ostream&, ht_sz, bool) const;
};
The String_Pic class differs slightly from the others in that it stores a copy of the
vector that contains the picture's data:
class String_Pic: public Pic_base {
std::vector data;
String_Pic(const std::vector& v): data(v) { }
wd_sz width() const;
ht_sz height() const;
void display(std::ostream&, ht_sz, bool) const;
};
We still copy the underlying characters from our user's vector parameter v into our
own member, which is called data. This is the only place in our entire program that
copies characters. Everywhere else, we copy only Ptr objects, which
copies pointers and manipulates reference counts.
15.1.4 Copy control
Perhaps the most interesting aspect of our design is what isn't here. There are no copy
constructors, assignment operators, or destructors. Why?
The reason is that the synthesized defaults work. The vector class takes care of
managing the space for the initial copy of the characters that our user gives us as part
of creating a new Picture. If we copy or assign two Pictures that refer to
String_Pics, or we destroy a Picture, then the Ptr operations will do the right
thing to manage the Picture objects, and arrange to delete the underlying
String_Pic at the right time. More generally, the Ptr class takes care of copying,
assigning, and destroying the Ptr members in the other Pic_base classes—and in
class Picture itself.
15.2 Implementation
At this point we have a pretty good design of both our interface and the implementation.
The Picture class and the associated operations on Pictures will manage the user
interface. The Picture constructor and the operations will create objects of one of the
types derived from Pic_base. We'll use Ptr to manage the underlying
space, thus avoiding extraneous copies of the data. It is now time to implement the
interface operations and each of the derived classes.
15.2.1 Implementing the user interface
We'll start by implementing the interface class and operations. What we know so far is
class Picture {
public:
Picture(const std::vector& =
std::vector());
private:
Ptr p;
};
Picture frame(const Picture&);
Picture hcat(const Picture&, const Picture&);
Picture vcat(const Picture&, const Picture&);
std::ostream& operator<<(std::ostream&, const Picture&);
Let's think first about the operations that create new Pictures. Each of these
operations creates an object of an appropriate class derived from Pic_base. That
object will copy the Ptr from the Picture(s) on which the operation executes. We will
bind a Picture to this newly created Pic_base object, and return that Picture. For
example, if p is a Picture, then frame(p) should create a new Frame_Pic that is
attached to the Pic_base from p. It will then generate a new Picture that is attached
to the new Frame_Pic. Let's start here:
Picture frame(const Picture& pic) {
Pic_base* ret = new Frame_Pic(pic.p);
// what do we return?
}
We start by defining a local pointer to Pic_base, which we initialize by creating a new
Frame_Pic that copies the underlying Ptr inside pic. There are now two problems.
The easy one is that the Frame_Pic constructor is private. As we saw in
§15.1.3/276, each of the Pic_base classes is a hidden class. We don't want users to
know about these classes, and so we defined only private operations to let the compiler
enforce this design decision. We can solve this problem by making the frame operation
a friend of class Frame_Pic.
The other problem is more subtle: We have created a new object of type Frame_Pic,
but what we need is an object of type Picture. More generally, we can imagine that
hcat, vcat, and other functions that we might subsequently write will generate objects
of other types derived from Pic_base, and that they will do so in contexts in which we
really want objects of type Picture. The point is that frame and related operations
return Pictures, not Pic_bases. Fortunately, we know from §12.2/213 that we can
convert one type to another if we provide an appropriate constructor. In this case, the
appropriate constructor is one that constructs a Picture from a Pic_base*:
class Picture {
Ptr p;
Picture(Pic_base* ptr): p(ptr) { }
// as before
};
Our constructor initializes p with the pointer to Pic_base that we were given.
Remember that class Ptr has a constructor that takes a T*, which in this case is a
Pic_base*. The initializer p(ptr) invokes this Ptr::Ptr(T*) constructor, passing it
ptr. Once we have this Picture constructor, we can complete the frame operation:
Picture frame(const Picture& pic)
{
return new Frame_Pic(pic.p);
}
We've eliminated the local Pic_base object because we don't need it. Instead, we
create a new Frame_Pic object, the address of which is automatically converted to a
Picture, which we return from the function. Completely understanding this little
function requires a good grasp of the subtleties involved when using automatic
conversions and copy constructors. The single statement in this function has the same
effect as
// create the new Frame_Pic
Pic_base* temp1 = new Frame_Pic(pic.p);
// construct a Picture from a Pic_base*
Picture temp2(temp1);
// return the Picture, which will invoke the Picture copy constructor
return temp2;
Like frame, the concatenation functions rely on our new Picture constructor:
Picture hcat(const Picture& l, const Picture& r)
{
return new HCat_Pic(l.p, r.p);
}
Picture vcat(const Picture& t, const Picture& b)
{
return new VCat_Pic(t.p, b.p);
}
In each case, we construct an object of the appropriate type, bind a Ptr to
it, construct a Picture from the Ptr, and return a copy of that Picture.
Of course, for these functions to compile, we'll need to add the appropriate friend
declarations to the HCat_Pic and VCat_Pic classes.
To construct a Picture from a vector, we adopt the same strategy that we
used for the other kinds of pictures:
Picture::Picture(const vector& v):
p(new String_Pic(v)) { }
Again, we create a new String_Pic object, but this time we use it directly to initialize
p, instead of returning it. Of course, we will have to remember to make Picture a
friend of String_Pic, so that it can access the String_Pic constructor.
It is important to realize how this constructor differs from the frame, hcat, and vcat
functions. Each of these other functions is defined to return a Picture, and in each one
we use a pointer to a class derived from Pic_base in the return statement.
Therefore, we implicitly used the Picture (Pic_base*) constructor to create a
Picture to return. In the Picture constructor that we've just written, we are still
creating a pointer to a class derived from Pic_base—in this case, class
String_Pic—but now we're using this pointer to initialize member p, which has type
Ptr. Doing so uses the Ptr(T*) constructor in class Pic_base, not the
Picture (Pic_base*) constructor, because we're constructing a Ptr, not
a Picture.
To complete the implementation of our interface functions, we must define the output
operator. This operation is also straightforward: We need to iterate through the
underlying Pic_base, and call display to write each line of the output:
ostream& operator<<(ostream& os, const Picture& picture)
{
const Pic_base::ht_sz ht = picture.p->height();
for (Pic_base::ht_sz i = 0; i != ht; ++i) {
picture.p->display(os, i, false);
os << endl;
}
return os;
};
We initialize ht by calling the (virtual) height function for the underlying
Pic_base, so that we do not have to recompute the height each time through the loop.
Remember that p is actually a Ptr, and that Ptr has overloaded -> to
implement references through the Ptr as references through the pointer that the Ptr
contains. We iterate ht times through the underlying Pic_base, each time calling the
(virtual) display function, asking it to write the current row. The third argument
(false) indicates that display need not pad the output. If we need padding to write
one of the interior Pictures, the inner display functions will indicate that padding is
needed. At this stage, we can't yet tell if padding is necessary. We write endl to end
each line of output, and when we're done, we return os.
As with the other operations that we have implemented, we will have to remember to
add a friend declaration to class Pic_base to allow operator<< to access its
display and height members.
15.2.2 The String_Pic class
Having completed the interface class and operations, we can turn our attention to the
derived classes. We'll start with String_Pic:
class String_Pic: public Pic_base {
friend class Picture;
std::vector data;
String_Pic(const std::vector& v): data(v) { }
ht_sz height() const { return data.size(); }
wd_sz width() const;
void display(std::ostream&, ht_sz, bool) const;
};
We have implemented the height function, but otherwise the String_Pic class is
unchanged from the one that we described in §15.1.3/277. The height function is
trivial: It forwards the request to the size member of vector.
To determine the width of a String_Pic, we need to look at each element in data to
see which is the longest:
Pic_base::wd_sz String_Pic::width() const {
Pic_base::wd_sz n = 0;
for (Pic_base::ht_sz i = 0; i != data.size(); ++i)
n = max(n, data[i].size());
return n;
}
Except for the type names, this function looks like the original width function from
§5.8/91. Because a String_Pic holds a vector, we should not be surprised
at this similarity.
The display function is more complicated. It has to iterate through the underlying
vector, writing the string associated with the requested row number.
What about padding? Note that this function might be called directly from the output
operator, as would happen if the Picture we were writing pointed to a String_Pic—
or it might be called indirectly, as part of writing a larger Picture of which this
String_Pic is a part. In the latter case, the display function may be asked to pad
the output to make each row fill the same size in the overall output. The amount of
padding will vary for each string, and will be whatever is needed to consume as many
spaces in the output as the number needed for the longest string. In other words,
we'll need to pad from the length of this string to the width() of this String_Pic.
A bit of forethought should convince us that we're likely to need to pad other pictures
too. For now, we'll assume that we have a pad function that takes an output stream and
the start and (one past) the end positions to pad with blanks. We'll implement this
function shortly.
Another complexity arises from the fact that the row number passed to display may
exceed the height of this String_Pic. One way in which this situation could happen
is if the String_Pic is part of a horizontally concatenated Picture in which one side
is shorter than the other. Our Pictures line up at the top border, but they may be of
different heights. Thus, we will need to check whether the row we're asked to write is in
range. With this analysis complete, we can now write the code:
void String_Pic::display(ostream& os, ht_sz row, bool do_pad) const
{
wd_sz start = 0;
// write the row if we're still in range
if (row < height()) {
os << data[row];
start = data[row].size();
}
// pad the output if necessary
if (do_pad)
pad(os, start, width());
}
We first check whether the row we were asked to write is in range—that is, whether row
is less than the height() of this String_Pic. If so, then we write it and set start to
indicate how many characters we wrote in the process. Regardless of whether we wrote
a row, we check whether we are supposed to pad the output. If so, we pad from start
to the overall width() of this String_Pic. If the row is out of range, then start is 0,
so we write an entire row of blanks.
15.2.3 Padding the output
We can now think about our padding function. Because we want to access this function
from each of the derived classes, we'll define the pad operation as a function member of
Pic_base that is both static and protected:
class Pic_base {
// as before
protected:
static void pad(std::ostream& os, wd_sz beg, wd_sz end) {
while (beg != end) {
os << " ";
++beg;
}
}
};
This function takes an ostream on which to write blanks, and two values that control
how many blanks to write. When a display function needs to call pad, it will pass the
current column number and one past the last column number that needs to be filled in
by the current display operation. The pad function will fill this range with blanks.
Note the use of the static keyword on the declaration of pad. As we saw in
§13.4/244, this use of static indicates that pad is a static member function. Such
functions differ from an ordinary member function in that they are not associated with
an object of the class type.
It may also be surprising that we can define a member function for an abstract base
class. After all, if there can be no objects of the base class, why should there be
member functions? However, remember that each derived object contains a base-class
part. Each derived class also inherits any member functions defined in the base. Thus, a
base-class function will execute on the base-class portion of a derived object. In this
particular case, the function that we are defining is a static member, so the question
of access to members of the base is moot. But it is important to realize that abstract
classes may define data members, and (ordinary) member functions, as well as static
ones. These functions will access the base-class objects that are part of derived objects.
Static members (both functions and static data members, which we can also define) are
useful in that they let us minimize the names that are defined globally. Our pad function
is a good example. We can imagine many abstractions that have the notion of padding.
In this book we talked about padding in the context of writing a formatted report of
student grades, as well as in the context of writing Pictures. If the Picture class
were to define pad as a global function, then we would not also be able to define a pad
function for Student_info, or vice versa. By making pad a static member, we allow
for the fact that other abstractions in our program might have the notion of padding. As
long as each class defines what pad means only in the context of the class, these
mutually independent notions of padding can coexist within our program.
15.2.4 The VCat_Pic class
Implementing the concatenation classes is not hard. We'll start with VCat_Pic:
class VCat_Pic: public Pic_base {
friend Picture vcat(const Picture&, const Picture&);
Ptr top, bottom;
VCat_Pic(const Ptr& t, const Ptr& b):
top(t), bottom(b) { }
wd_sz width() const
{ return std::max(top->width(), bottom->width()); }
ht_sz height() const
{ return top->height() + bottom->height(); }
void display(std::ostream&, ht_sz, bool) const;
};
We added the appropriate friend declaration for the vcat operation, and implemented
the height and width functions inline. If a picture is concatenated vertically, its
height is the sum of its two components' heights, and the width is the greater of the
two components' widths.
The display function is not much harder:
void VCat_Pic::display(ostream& os, ht_sz row, bool do_pad) const
{
wd_sz w = 0;
if (row < top->height()) {
// we are in the top subpicture
top->display(os, row, do_pad);
w = top->width();
} else if (row < height()) {
// we are in the bottom subpicture
bottom->display(os, row - top->height(), do_pad);
w = bottom->width();
}
if (do_pad)
pad(os, w, width());
}
First, we define a variable w, which will contain the width of the current row, in case we
need it for padding. Next, we check whether we're in the top component, by testing row
against the height() of the top picture. If we're in that range, then we invoke
display to write the top component, passing the bool that we were given to indicate
whether to pad the output. Remember, display is virtual, so this call will invoke
whatever the appropriate display function is for the kind of Pic_base to which top
actually refers. Once we've written the given row, we remember its width in w.
If we're not in top, we might be in bottom. If we get to this else test, then we know
that row is greater than top->height(), so we now check whether row is within the
overall range of this picture. If so, it must be in the bottom picture. As we did for top,
we call display on bottom to write the picture, offsetting the row number to adjust
for having already written as many rows as are in top. Having written the row from the
bottom picture, we remember its width. If we're out of range, w remains 0.
When we're done writing the row, we check whether padding is needed. If so, we pad
from the width that we remembered in w to the full width of our own picture.
15.2.5 The HCat_Pic class
Not surprisingly, the HCat_Pic class looks a lot like VCat_Pic:
class HCat_Pic: public Pic_base {
friend Picture hcat(const Picture&, const Picture&);
Ptr left, right;
HCat_Pic(const Ptr& l, const Ptr& r):
left(l), right(r) { }
wd_sz width() const { return left->width() + right->width(); }
ht_sz height() const
{ return std::max(left->height(), right->height()); }
void display(std::ostream&, ht_sz, bool) const;
};
Because we're concatenating two pictures side by side, this time the width is the sum
of the components' widths, and the height is the greater of the two heights. Here,
the display function is simpler than the corresponding one from VCat_Pic, because
we delegate managing whether the row is in range to the component pictures:
void HCat_Pic::display(ostream& os, ht_sz row, bool do_pad) const
{
left->display(os, row, do_pad || row < right->height());
right->display(os, row, do_pad);
}
First we write the requested row from left by calling display, asking it to write the
given row. We pad this row if we were asked to pad our own output, or if we're on a row
that is within the range of the right-hand picture (in which case we must pad each row
of the left-hand picture to ensure that the corresponding row of the right-hand picture
begins at the right place in the output line). If the row is out of range for left, then the
display function executed on left will deal with that problem. Similarly, we delegate
writing the requested row from right to the display function on right. This time we
pass along the do_pad value that we were given, because there is no reason to force
padding on the right-hand side.
15.2.6 The Frame_Pic class
The only derived class we have left to implement is Frame_Pic:
class Frame_Pic: public Pic_base {
friend Picture frame(const Picture&);
Ptr p;
Frame_Pic(const Ptr& pic): p(pic) { }
wd_sz width() const { return p->width() + 4; }
ht_sz height() const { return p->height() + 4; }
void display(std::ostream&, ht_sz, bool) const;
};
The height and width operations forward their calculations to the picture that was
framed. We add 4 to these values, to account for the borders and the space that
separates the border from the interior, framed picture. The display function is tedious
but not hard:
void Frame_Pic::display(ostream& os, ht_sz row, bool do_pad) const
{
if (row >= height()) {
// out of range
if (do_pad)
pad(os, 0, width());
} else {
if (row == 0 || row == height() - 1) {
// top or bottom row
os << string(width(), '*');
} else if (row == 1 || row == height() - 2) {
// second from top or bottom row
os << "*";
pad(os, 1, width() - 1);
os << "*";
} else {
// interior row
os << "* ";
p->display(os, row - 2, true);
os << " *";
}
}
}
First we check whether the requested row is in range; if not and we're being asked to
pad, we do so, filling the entire row with blanks. If we're in range, there are three
cases: We're writing the top or bottom border, we're writing the mostly blank line that
separates the border from the interior picture, or we're writing a row from the interior
picture.
We know that we're dealing with the top or bottom border if the row number is 0 or
height() - 1. In this case, we write a row that consists entirely of asterisks to form
the border. If we're one row in from the border, then we want to write an asterisk,
followed by the appropriate number of blanks, followed by another asterisk. Finally, we
might be in an interior row of the picture. In this case, we want to write that row of the
border, which is an asterisk followed by a blank, and then the interior picture, followed
by another blank and asterisk for the right-hand border. We write the interior picture by
calling display, offsetting the row value to account for the border that we have already
written. In the call to display, we indicate that the interior picture should pad the
output so that when we write the right-hand border, it will be straight.
15.2.7 Don't forget friends
The only work that remains is to add the appropriate friend declarations to Picture
and Pic_base. We've already noted that we need to add a friend declaration to class
Picture for each of the Picture operations. After all, these operations all use the Ptr
inside Picture, and need permission to access that member. What may be less obvious
is the collection of friends that we need to add to class Pic_base:
// forward declaration, described in §15.3/288
class Picture;
class Pic_base {
friend std::ostream& operator<<(std::ostream&, const Picture&);
friend class Frame_Pic;
friend class HCat_Pic;
friend class VCat_Pic;
friend class String_Pic;
// no public interface
typedef std::vector::size_type ht_sz;
typedef std::string::size_type wd_sz;
// this class is an abstract base class
virtual wd_sz width() const = 0;
virtual ht_sz height() const = 0;
virtual void display(std::ostream&, ht_sz, bool) const = 0;
protected:
static void pad(std::ostream& os, wd_sz, wd_sz);
};
class Picture {
friend std::ostream& operator<<(std::ostream&, const Picture&);
friend Picture frame(const Picture&);
friend Picture hcat(const Picture&, const Picture&);
friend Picture vcat(const Picture&, const Picture&);
public:
Picture(const std::vector& =
std::vector());
private:
Picture(Pic_base* ptr): p(ptr) { }
Ptr p;
};
// operations on Pictures
Picture frame(const Picture&);
Picture hcat(const Picture&, const Picture&);
Picture vcat(const Picture&, const Picture&);
std::ostream& operator<<(std::ostream&, const Picture&);
The first friend declaration in Pic_base should be easy to understand. The output
operator invokes both the height() and display() functions, so it must be granted
access to these members. What may be more surprising is the friend declarations for
the classes that inherit from Pic_base. Don't they have access to the members of
Pic_base through inheritance? Yes they do, in principle, but except for pad, all of the
members of Pic_base are private. Why didn't we just make these other members
protected, as we did with the pad function? The answer is that it wouldn't have solved
the problem.
A member of a derived class (such as Frame_Pic) can access the protected members
of the base-class parts of objects of its own class (such as Frame_Pic), or of other
classes derived from it, but it cannot access the protected members of base-class
objects that stand alone—that is, that are not part of a derived-class object. Therefore,
member functions of class Frame_Pic, which is derived from class Pic_base, can
access protected members of the Pic_base parts of Frame_Pic objects, or objects
of classes derived from Frame_Pic, but they cannot access protected members of
stand-alone Pic_base objects directly.
One might think that this restriction would be irrelevant to our program. After all, class
Pic_base is an abstract base class, so there can be no stand-alone objects of that
class. However, the access rules apply to any attempt to access a member of what
appears to be a stand-alone Pic_base object, even if at run time the object is of a
derived class. For example, consider the height function in class Frame_Pic:
ht_sz Frame_Pic::height() const { return p->height() + 4; }
This function uses the expression p->height(), which implicitly calls the operator->
member of class Ptr (§14.3/263) to obtain a pointer. This pointer has type
Pic_base*; we dereference it to access the height member of the corresponding
object. Because the pointer's type is Pic_base*, the compiler will check protection as if
we were trying to access a member of a Pic_base object, even though the actual
object will be of a type derived from Pic_base. Therefore, even if we made height a
protected member, we would still have to include friend declarations to allow this
access. Each of the derived classes in our hierarchy turns out to require friend
declaration for similar reasons.
This rule may be surprising, but its logic is straightforward: If the language granted
derived objects access to the protected members of a base-class object, then it would
be trivial to subvert the protection mechanisms. If we needed access to a protected
member of a class, we could define a new class that inherited from the class that we
wanted to access. Then we could define the operation that needed access to the
protected member as a member of that newly derived class. By doing so, we could
override the original class designer's protection strategy. For this reason, protected
access is restricted to members of the base-class part of a derived-class object, and
does not allow direct access to the members of base-class objects.
15.3 Details
Abstract base classes have one or more pure virtual functions:
class Node {
virtual Node* clone() const = 0;
};
says that clone is a pure virtual function, and, by implication, that Node is an
abstract base class. It is not possible to create objects of an abstract class. A class can
be made abstract through inheritance: If the class fails to redefine even a single
inherited pure virtual, then the derived class is also abstract.
Forward declarations: The requirement to define names before using them (§0.8/6)
causes trouble in writing families of classes that refer to one another. To avoid this
trouble, you can declare just the name of the class by writing
class class-name;
thereby saying that class-name names a class, but not describing the class itself.
We used such a forward declaration for class Picture in §15.2.7/286. The Picture
class contains a member of type Ptr, and the Pic_base class has a
friend declaration for operator<< that uses the type const Picture&. Therefore,
these two classes refer to each other.
Such mutual type dependencies can yield programs that are impossible to implement.
For example:
class Yang; // forward declaration
class Yin { Yang y; };
class Yang {
Yin y;
};
Here, we have said that every Yin object contains a Yang object, which contains a Yin
object, and so on. Implementing such types would require infinite memory.
The mutual dependency in our picture classes does not cause such problems, because
class Picture does not contain a member of type Pic_base directly. Instead, it has a
member of type Ptr, which contains a Pic_base*. Using pointers in this
way avoids infinitely nested objects.
Moreover, in the case of a pointer (or reference), the compiler does not actually need to
know the details of the type until operations are invoked through the pointer (or
reference). Because the declaration of operator<< uses the const Picture& type
only to declare a parameter type, the compiler needs to know only that the name
Picture names a type. The details of that type aren't needed until we define
operator<<.
Exercises
15-0. Compile, execute, and test the programs in this chapter.
15-1. Test your system by writing a program that executes
Picture p = // some initial starting picture
Picture q = frame(p);
Picture r = hcat(p, q) ;
Picture s = vcat(q, r);
cout << frame(hcat(s, vcat(r, q))) << endl;
15-2. Reimplement the Frame_Pic class so that the frame uses three different
characters: one for the corners, another for the top and bottom borders, and a third for
the side borders.
15-3. Give users the option to specify what characters to use for these border
characters.
15-4. Add an operation to reframe a Picture, which changes the frame characters.
The operation should change all of the frames in the interior picture.
15-5. Reimplement HCat_Pic so that when pictures of a different size are
concatenated, the shorter one is centered in the space consumed by the longer one.
That is, if we horizontally concatenate two pictures, one of which is four lines long and
the other is two lines long, the first and last rows of the output picture will be blank on
the side of the shorter picture. What can we now conclude about the necessity of the
tests between row and 0.
15-6. The Vec and str classes that we developed in Chapters 11 and 12 are powerful
enough to be used to implement Pictures. Reimplement the material in this chapter to
use Vec instead of vector, and test your implementation.
16
Where do we go from here?
We have come to the end of the main part of our book. This ending may seem
premature: There are still significant parts of the language, and large parts of the
library, that we have not described. However, we have chosen to stop at this point for
two important reasons.
The first reason is that we have already presented tools that you can use to solve a wide
range of programming problems. We believe that your best strategy at this point is to
practice using these tools on your own problems before you learn about any more new
tools. It might even be a good idea to begin by rereading this entire book, doing all the
exercises that you didn't do the first time around.
If you are looking for ideas about programming style or technique, we recommend our
previous book, Ruminations on C++ (Addison-Wesley, 1997), which contains a mixture
of stylistic essays and programming examples.
The second reason is that once you have written enough programs that use the material
that we have covered so far, you will no longer need the detailed tutorial style that we
have adopted in this book. Instead, you will have gained enough C++ programming
experience that you will be able to take advantage of books that contain more detailed
information, and less explanation, than this one.
16.1 Use the abstractions you have
There is an old story about a visitor who has become lost in New York, with tickets in
hand to a piano recital. Stopping a passerby, the visitor asks, "Excuse me. Can you tell
me how to get to Carnegie Hall?" The answer: "Practice!"
It is important to understand—thoroughly—how to use the abstractions that you have
available before you try to learn about new ones. The abstractions that you have include
the ones from the standard library, and others that you may have had to create as you
solve programming problems. By combining ideas from the standard library, which we
can apply to a wide variety of problems, with ideas that solve problems in a particular
application domain, we can write useful programs with surprisingly little effort. In
particular, if we design our own abstractions well, we should be able to use them to
solve problems that we had not considered when we designed them.
We can find an example of this ideal in the classes that we wrote in Chapter 13 to store
student grades and in Chapter 15 to generate character pictures. We have used the
character-picture classes in a variety of forms for years. In contrast, we wrote the
student-record classes from scratch for this book. Only when we were thinking about
what to say in this chapter did we realize that we could combine these two abstractions
in a particularly nice way.
The combination uses character pictures to write a histogram of students' grades. The
point, of course, is that such a visual display lets us see anomalies much more quickly
than does a mere table of numbers. The basic idea is to convert each final grade into a
string of = symbols whose length is proportional to the grade. For example, with
appropriate input, we might generate the following output:
********************************
* *
* James =============== *
* Kevin ================ *
* Lynn ================= *
* MaryKate ================ *
* Pat ============ *
* Paul ================== *
* Rhia ================ *
* Sarah =================== *
* *
********************************
From this histogram, it is immediately obvious that Pat is having trouble keeping up with
the course.
What is nice about this example is how small it is, and how directly the solution mirrors
the problem:
Picture histogram(const vector& students) {
Picture names;
Picture grades;
// for each student
for (vector::const_iterator it = students.begin();
it != students.end(); ++it) {
// create vertically concatenated pictures of the names and grades
names = vcat(names, vector(1, it->name()));
grades = vcat(grades,
vector(1, " " + string(it->grade() / 5, '=')));
}
// horizontally concatenate the name and grade pictures to combine them
return hcat(names, grades);
}
Our histogram function takes a (const reference to a) vector of Student_info
objects, each of which represents a student. From these records, we will create two
Pictures, one of which, names, contains all the students' names; the other, grades,
contains a row that corresponds to each student's final grade. When we've processed
every student, we horizontally concatenate the two pictures, lining up each student with
the corresponding grade. Because each picture is conceptually a rectangle, this
horizontal concatenation automatically accounts for the different lengths of the students'
names.
The main program builds up the vector, in familiar fashion, by reading a file of student
records. When it's done, it calls histogram to generate the Picture, frames it, and
then uses the output operator to write it:
int main()
{
vector students;
Student_info s;
// read the names and grades
while (s.read(cin))
students.push_back(s);
// put the students in alphabetical order
sort(students.begin(), students.end(), Student_info::compare);
// write the names and histograms
cout << frame(histogram(students)) << endl;
return 0;
}
The most important new idea in this example is that it contains no new ideas! What
made it easy was being so familiar with the ideas that we have already covered that we
can combine them in ways that we had not anticipated. This kind of familiarity comes
only with practice.

16.2 Learn more
Eventually, you are going to need to know more details about the language and library.
For that matter, our ideal of learning nothing new until you understand thoroughly
everything that we have presented so far—like most ideals—is probably not entirely
attainable in practice. At some point, you are going to encounter a question about C++
that this book does not answer, so you will need to turn elsewhere for advice.
For this purpose, two books stand out. The first, Bjarne Stroustrup's The C++
Programming Language, Third Edition (Addison-Wesley, 1998), is the most complete
single source of information about C++. It covers the entire language and library, and
tells you everything you need to know—and probably more—about all aspects of both.
The second, Matthew Austern's Generic Programming and the STL (Addison-Wesley,
1999), discusses the standard library's algorithms and data structures in much more
detail than either our book or Stroustrup's. Moreover, it is authoritative, because
although Austern was not the original author or implementer of this part of the
library—that honor goes to Alex Stepanov—he has been working closely with Stepanov
for the past several years, and is one of the main driving forces behind the further
evolution of that library.
Together, this book, Ruminations on C++, and the books by Stroustrup and Austern
comprise well over 2,000 pages. We are, therefore, reluctant to recommend any
additional reading. However, if your appetite is truly voracious, we suggest that you visit
http://www.accu.org. There, you will find reviews of more than 2,000 books, many of
which are related to C++. They also have the good taste to give their highest
recommendations to Ruminations on C++ and the books by Stroustrup and Austern.
As you look for reading material, keep in mind that books on the shelf do not make you
a better programmer. Ultimately, the only way to improve your programming is to write
programs. Have fun!
Exercises
16-0. Compile, execute, and test the programs in this chapter.
16-1. Write a self-reproducing program. Such a program is one that does no input, and
that, when run, writes a copy of its own source text on the standard output stream.
Appendix A
Language details
This appendix serves two purposes: It includes some additional low-level details, and it
summarizes in one place the expressions and statements of the language, including
some that we have not used elsewhere in this book. The low-level material relates
primarily to the complexities of C++'s declarator syntax, and the details of the built-in
arithmetic types, both of which the language inherits from the C programming language.
These details are not necessary for understanding the programs in this book, and indeed
are not necessary to writing good C++ programs. However, there are many programs
that do require knowledge of these details, so it may be useful to review them.
In this appendix we describe syntax as follows: constant-width symbols stand for
themselves, italic words stand for syntactic categories,... means zero or more repetitions
of the item that immediately precedes it, and phrases enclosed in italic brackets [ ] are
optional. Moreover, we use italic curly braces { } for grouping, and | to show
alternatives. For example,
declaration-stmt: decl-specifiers [ declarator [ initializer ]] [, declarator [ initializer means that a declaration-stmt consists of decl-specifiers , followed by zero or
more declarators , each optionally followed by an initializer , followed by a
semicolon.
A.1 Declarations
Declarations can be hard to understand, especially if they declare several names with
different types or deal with functions that return pointers to functions. For example, in
§10.1.1/171, we saw that
int* p, q;
defines p as an object of type "pointer to int " and q as an object of type int , and in
§10.1.2/173, we saw that
double (*get_analysis_ptr())(const vector&);
declares get_analysis_ptr as a function, with no arguments, that returns a pointer
to a function with a const vector& argument that returns double
. You can clarify such declarations by rewriting them, for example, as
int* p;
int q;
and
// define analysis_fp as a name for the type of a function that takes a
// const vector& argument and returns a double
typedef double (*analysis_fp)(const vector&);
analysis_fp get_analysis_ptr();
Unfortunately, that strategy won't save you from having to read confusing declarations
in other people's programs.
In general, a declaration looks like
declaration-stmt: decl-specifiers [ declarator [ initializer ]] [, declarator [ initializer It declares a name for each of its declarators . These names are meaningful from
where they are declared to the end of the declaration's scope. Some declarations are
also definitions. Names may be declared multiple times but must be defined only once.
A declaration is a definition if it allocates storage or defines a class or function body.
C++ inherits its declaration syntax from C. The key to understanding declarations is to
realize that each declaration consists of two parts: a sequence of decl-specifiers
that collectively specify a type and other attributes of what is being declared, followed
zero or more declarators (each of which can optionally have an associated
initializer ). Each declarator ascribes to a name a type that depends on the
specifiers and on the form of the declarator.
The first step in understanding any declaration is to locate the boundary between the
specifiers and the declarators. Doing so is surprisingly simple: All the specifiers are
keywords or names of types, so the specifiers end just before the first symbol that isn't
one of those. For example, in
const char * const * const * cp;
the first symbol that is neither a keyword nor the name of a type is * , so the specifiers
are const char , and the (only) declarator is * const * const * cp .
As another example, consider the arcane declaration from §10.1.2/173:
double (*get_analysis_ptr())(const vector&);
The boundary here is easy to find: double names a type, and the open parenthesis that
follows it is neither a keyword nor the name of a type. Therefore, the declspecifiers
part is just double , and the declarator is the rest of the declaration, not
including the semicolon.
A.1.1 Specifiers
We can divide the decl-specifiers into three broad categories: type specifiers,
storage-class specifiers, and miscellaneous specifiers:
decl-specifiers: {type-specifier | storage-class-specifier | other-decl-specifier} ...
However, this division serves only to aid understanding, because there is no
corresponding division in declarations themselves: decl-specifiers can appear in
any order.
Type specifiers determine the type that underlies any declaration. We discuss the builtin
types themselves in §A.2/299.
type-specifier: char | wchar_t | bool | short | int | long | signed
unsigned | float | double | void | type-name | const | volatile
type-name: class-name | enum-name | typedef-name
The const specifier says that objects of the type may not be modified, volatile tells
the compiler that the variable may change in ways outside the definition of the language
and that aggressive optimizations should be avoided.
Note that const can appear both as part of the specifiers, thus modifying the type, and
as part of the declarator, specifying a const pointer. There is never any ambiguity
because a const that is part of a declarator always follows a * .
Storage-class specifiers determine the location and lifetime of a variable:
storage-class-specifiers: register | static | extern | mutable
The register specifier suggests that the compiler should try to optimize performance
by putting the object into a register if possible.
Ordinarily, local variables are destroyed on exit from the block in which they were
declared; static variables are preserved across entry and exit from a scope.
The extern specifier indicates that the current declaration is not a definition, implying
that there is a corresponding definition elsewhere.
The mutable storage class is used only for class data-members , and allows those
data-members to be modified even if they are members of const objects.
Other declaration specifiers define properties that are not related to types:
other-decl-specifier: friend | inline | virtual | typedef
The friend specifier (§12.3.2/216 and §13.4.2/246) overrides protection.
inline is a specifier for function definitions and is a hint to the compiler to lay the code
down inline if possible. The definition of the function must be in scope when the call is to
be expanded, so it is usually a good idea to put the body of an inline function into the
same header that declares the function.
The virtual specifier (§13.2.1/234) may be used only with member functions, and
denotes a function for which calls can be dynamically bound.
The typedef specifier (§3.2.2/43) defines a synonym for a type.
A.1.2 Declarators
A declaration declares one entity for each declarator, giving that entity a name, and
implicitly giving the entity the storage class, type, and other attributes as specified by
the specifiers. The specifiers and declarator together determine whether the name
names an object, array, pointer, reference, or function. For example,
int *x, f();
declares that x is a pointer to int and f is a function returning int . It is the
declarators *x and f() that make the distinction between the types of x and f .
declarator: [ * [ const ] | & ]... direct-declarator
direct-declarator: declarator-id | ( declarator ) |
direct-declarator ( parameter-declaration-list ) |
direct-declarator [ constant-expression ]
A declarator-id is an identifier , possibly qualified:
declarator-id: [ nested-name-specifier ] identifier
nested-name-specifier: { class-or-namespace-name ::}...
If a declarator is a direct-declarator that consists only of a declarator-id , then
it specifies that the identifier being declared has the properties implied by the declspecifiers
, without further modification. For example, in the declaration
int n;
the declarator is n , which is a direct-declarator that consists only of a
declarator-id , so by implication, n has type int .
If a declarator has one of the other possible forms, then you can determine the type of
the identifier as follows: First, let T be the type implied by the decl-specifiers, ignoring
non- type properties such as friend or static , and let D be the declarator. Then
repeat the following steps until you have reduced D to a declarator-id , at which
point T is the type you seek:
1. If D has the form (D1 ), then replace D with D1 .
If D has the form * D1 or * const D1 , replace T with "pointer to T " or "constant
pointer to T " (not "pointer to constant T "), depending on whether the const is
present. Then replace D with D1 .
2.
If D has the form D1 (parameter-declaration-list), replace T with "function returning
T " with arguments as defined by the parameter-declaration-list, and replace D with
D1 .
3.
If D has the form D1 [constant-expression ], then replace T with "array of T " that
has the number of elements given by the constant-expression, and replace D with
D1 .
4.
Finally, if the declarator has the form & D1 , then replace T with "reference to T "
and D with D1 .
5.
As an example, consider the declaration
int *f();
We start with T and D being int and *f() , so D has the form *D1 , where D1 is f() .
You might think that D could have either of the forms D1() or *D1 . However, if D had
form D1() , then D1 would have to be *f , and D1 would also have to be a directdeclarator
(because the grammar at the beginning of this section allows only a directdeclarator
to precede ()). If we look at the definition of direct-declarator, however, we
see that it cannot contain a * . Therefore, D can only be *f() , which has the form *D1
, where D is f() .
Now that we have determined that D1 is f() , we know that we must replace T with
"pointer to T , " which is "pointer to int , " and replace D with f() .
We have not yet reduced D to a declarator-id, so we must repeat the process. This time,
D1 can only be f, so we replace T with "function returning T," which is "function
returning pointer to int with no arguments," and we replace D with f .
At this point we have reduced D to a declarator-id, so we're done. We have determined
that the declaration
int *f();
declares f to have type "function with no arguments returning pointer to int . " As
another example, the declaration
int* p, q;
has two declarators, *p and q . For each declarator, T is int . For the first declarator, D
is *p , so we transform T to "pointer to int ," and D to p . The declaration, therefore,
gives p the type "pointer to int ."
We analyze the second declarator independently, with T again being int and D being q
. At this point it should be obvious that the declaration gives q the type int .
Finally, let's analyze the arcane example from §10.1.2/173:
double (*get_analysis_ptr())(const vector&);
The analysis proceeds in the following five stages:
T: double D: (*get_analysis_ptr())(const
vector&)
1.
T: function returning double with const vector& argument
D: (*get_analysis_ptr())
2.
3. T: function returning double ... (as before) D: *get_analysis_ptr()
4. T: pointer to function returning double ... D: get_analysis_ptr()
T: function returning pointer to function returning double ... D:
get_analysis_ptr
5.
In other words, we learn that get_analysis_ptr is a function that returns a pointer
to a function that returns a double result, and takes a const
vector& as its argument. We leave unwinding const
vector& as an exercise. Fortunately, few function declarations are
this confusing; most of them look like
declarator: declarator-id ( parameter-declaration-list )
By far the most common difficult case is a function that returns a pointer to function.
A.2 Types
Types pervade C++ programs. Every object, expression, and function has a type, and
the type of an entity determines its behavior. With a single exception (the type of an
object within an inheritance hierarchy that is accessed through a pointer or reference),
the type of every entity is known at compile time.
In C++, types can be thought of as ways of structuring and accessing memory as well
as ways of defining operations that can be performed on objects of the type. That is,
types specify both properties of data and operations on that data.
Although this book concentrates on using and building higher-level data structures, it is
important to understand the primitive types used to build them. These primitive types
represent common abstractions that are close to the hardware, such as numbers
(integral and floating point), characters (including wide characters for international
character sets), truth values, and machine addresses (pointers, references, and arrays).
Literals, also often called constants, represent integer, floating-point, Boolean,
character, or string values. This section reviews and expands on facilities related to the
built-in types.
A.2.1 Integral types
C++ inherits from C a bewildering variety of integral types, including integers, Boolean,
and character types. Because C++ is intended to be able to run efficiently on a wide
variety of hardware, it leaves many of the details of its fundamental types up to the
implementation rather than defining those types precisely.
A.2.1.1 Integer
There are three distinct signed integer types and three distinct unsigned integer types:
short int int long int
unsigned short int unsigned int unsigned long int
The short and long types can be abbreviated by dropping the keyword int. The
keywords, if there are more than one, can appear in any order.
Each of these types is capable of representing any integer within an implementationdefined
range. Each type but the first must offer a range at least as generous as that of
the type that precedes it. The ranges for short int and int must be at least ±32767
(±(215 -1)), and the range for long int must be at least ±2147483647 (±(231 -1)).
Every signed integral type has a corresponding unsigned type. Every unsigned type
represents integers modulo 2n, where n depends on the type and the implementation.
Analogous to the signed types, the n that corresponds to every unsigned type except
unsigned char must be at least as large as the n for the preceding type. Moreover,
every unsigned type must be capable of holding every non-negative value in the range
of the corresponding signed type, and each signed type is required to have the same
internal representation as the corresponding unsigned type for the values that they have
in common. It follows from these requirements that the four unsigned types must have
an extra bit that corresponds to signed types' sign bits, meaning that the unsigned types
must correspond to values of n that are at least 8, 16, 16, and 32 respectively.
Compilers are allowed to use either one's- or two's-complement representation for the
signed types.
The standard library defines a type called size_t that is a synonym for one of the
unsigned types. It is guaranteed to be large enough to hold the size of the largest
possible object, including arrays. Its type is defined in the system header .
Integer literals: An integer literal is a sequence of digits, optionally preceded by a base
indicator, and optionally followed by a size indicator. Pedantically speaking, integer
literals do not have signs, so that -3 is an expression, not a literal.
If the literal begins with 0x or 0X, then the integer is represented in hexadecimal, and
the "digits" can include any of AaBbCcDdEeFf as well as the usual decimal digits. If the
literal begins with a 0 that is not followed by x or X, then the integer is represented in
octal, and the "digits" can include only 01234567.
The size indicator is u, l, ul, or lu, in either upper- or lowercase. If it is lu or ul,
then the literal has type unsigned long. If it is u, then the literal has type unsigned
if the value will fit; otherwise, it has type unsigned long. If it is l, then the literal has
type long if the value will fit; otherwise, it has type unsigned long.
If there is a base indicator but no size indicator, then the type is the first of int,
unsigned, long, and unsigned long into which the value will fit. If there is neither
a size nor a base indicator, then the type is int if the value will fit, and long otherwise.
These rules imply that the type of an integer literal often depends on the
implementation. Fortunately, integer literals in well-written programs tend to be small,
so these details don't matter most of the time. Nevertheless, we have mentioned them
in case you need to refer back to them.
A.2.1.2 Boolean
Expressions that are treated as conditions have type bool. The possible values of type
bool are true and false. It is possible to use a number or a pointer as a truth value.
In such contexts, zero is considered false, and any other value is considered true.
When a bool value is used as a number, false is treated as 0 and true as 1.
Boolean literals: The only Boolean literals are true and false, each of which has type
bool, with the obvious meaning.
A.2.1.3 Character
In C++, characters are just tiny integers. In particular, they can be used in arithmetic
expressions in the same way as integers.
As is the case with integers, characters can be signed or not, so there are distinct
signed char and unsigned char types. Every implementation's range for signed
char is required to be able to contain every character in the machine's basic character
set, as well as being at least ±127 (±(27 -1)).
In addition, there is a plain char type, which, although it is a distinct type, is required
to have the same representation as one of the other two types. It is up to the
implementation to determine which of those types it should be. As usual, the choice is
intended to reflect which representation is most natural for the machine.
There is also a "wide character" type, called wchar_t, which must contain at least 16
bits, and is intended to be used for representing characters in languages such as
Japanese, which have many more characters than the Latin alphabet provides. The
wchar_t type is required to behave the same way as one of the other integral types.
The particular other type involved depends on the implementation and is normally
chosen to yield the most efficient representation.
Character literals: A character literal, of which 'a' is an example, is typically a single
character surrounded by single quotes. It has type char, which, as we know from
§A.2.1.3/301, is really a kind of integer. Every implementation defines a correspondence
between characters, such as a, and their integral values. Most programs have no reason
to depend on that correspondence, because programmers can write literals such as 'a'
to mean "the integer that corresponds to the character a." Because the correspondence
varies from one implementation to another, programmers should not depend on
arithmetic properties of characters. For example, there is no assurance that 'a'+1 is
equal to 'b'. Digits, however, are guaranteed to have contiguous values. For example,
'3' + 1 is always equal to '4' (but not necessarily equal to 4).
String literals, A string literal, of which "Hello, world! " is an example, is a
sequence of zero or more characters surrounded by double quotes. The type of a string
literal is const char*. The compiler inserts a null character at the end of every string
literal.
Two string literals that are separated only by whitespace are automatically concatenated
to form a longer string literal. This behavior allows string literals that span more than a
single line to be written more conveniently.
A.2.1.4 Character representations
We said that a character literal is typically a single character surrounded by single
quotes, and that a string literal is typically a sequence of characters surrounded by
double quotes. The reason for the "typically" is that there are some exceptions to the
general rule. These exceptions apply equally to character literals and string literals:
To represent a quote of the same kind that began the literal, you must precede it
by another backslash, as in '\'', or "the \ quotes\"", to make it clear that
the quote does not end the literal. For convenience, you can precede the other kind
of quote by a backslash as well, as in '\"'.
To represent a backslash, you must precede it by a backslash, as in '\\', so that
the compiler will not think that the backslash is there to give special meaning to the
character following it.
There are rules having to do with international character sets, which are beyond the
scope of this book, but which affect the meaning of programs with two or more
consecutive question marks in their text. To make it possible to avoid consecutive
question marks, C++ allows \? to represent a question mark, so that you can write
literals such as "What?\?" without consecutive question marks.
A number of control characters, which affect output in various ways, have printable
representations inside literals: newline (\n), horizontal tab (\t), vertical tab (\v),
backspace (\b), carriage return (\r), form feed (\f), and alert (\a). Their actual
effect when written on an output device depends on the implementation.
If you really need a character with a particular internal representation, you can
represent it as \x followed by hexadecimal digits (in upper- or lowercase), or by \
followed by up to three octal digits. So, for example, '\x20' and '\40' both
represent the character whose internal representation is decimal 32 (20 in hex, 40
in octal). On implementations based on the ASCII character set, this character is
the same as ' '. The most common use of this representation—and the only one you
will see in many programs—is that '\0' is the character whose value is zero.
A.2.2 Floating point
C++ has three floating-point types, called float, double, and long double in order
of nondecreasing precision. The implementation is allowed to implement float with the
same precision as double, or double with the same precision as long double. Every
implementation is required to offer at least six significant (decimal) digits in the float
type and ten in the double and long double types. Most implementations offer only
six significant digits for float and fifteen for double.
Floating-point literals: A floating-point literal is a nonempty sequence of digits with an
exponent at the end, a decimal point somewhere in it, or both. Like integer literals,
floating-point literals do not begin with a sign; -3.1 is an expression, not a literal. The
decimal point may be at the beginning, middle, or end of the sequence of digits, or it
may be omitted entirely if there is an exponent. The exponent is e or E, followed by an
optional sign, and one or more digits. The exponent is always interpreted in decimal.
For example, 312E5 and 31200000. represent the same number, but 31200000 is an
integer literal, not a floating-point literal. As another example, 1.2E-3 represents the
same number as .0012 or, for that matter, 0.000012e+2.
Floating-point literals normally have type double. If you want a literal to have type
float, you can append f or F; if you want it to have type long double, you can
append l or L.
A.2.3 Constant expressions
A constant-expression is an expression of integral type with a value that is known at
compile time. The operands in a constant-expression can contain only literals,
enumerators, const variables, or static data members of integral type that are
initialized from constant-expressions or sizeof expressions. The expressions may not
contain functions, class objects, pointers, or references and are not allowed to use
assignment, increment, decrement, function-call, or comma operators.
A constant-expression can appear wherever a constant is is expected. Examples include
dimensions in array declarations (§10.1.3/174), labels in switch statements
(§A.4/309), and initializers for enumerators (§A.2.5/305).
A.2.4 Conversions
Conversions happen as needed to bring the operands of each operator to a common
type. When there is a choice, conversions that preserve information are preferred over
those that lose information. Moreover, conversion to unsigned types is preferred to
conversion to signed types, all arithmetic on short integers or characters implies
conversion to int or longer, and floating-point arithmetic implies conversion to double
or longer.
The simplest conversions are promotions. Promotions allow values of a smaller type
(e.g., char) to be promoted to a larger, related type (e.g., int); they preserve the sign
of the initial value. Integral promotions convert values of char, signed char,
unsigned char, short, and unsigned short to int, if the values will fit, and to
unsigned int otherwise. Wide characters and enumeration types (§A.2.5/305) are
promoted to the smallest int that can represent all the values in the underlying type.
The types int, unsigned int, long, and unsigned long are tried in order, bool
is promoted to int, and float is promoted to double.
Conversion of an integral type to a floating-point type preserves as much precision as
can be represented on the machine hardware.
Converting a larger signed value (e.g., long) to a shorter value (e.g., short) is
implementation defined. Converting a larger unsigned value to a shorter value is modulo
2n, where n is the number of bits in the shorter type. Converting a floating-point value
to an integral type truncates by discarding the fractional part. The behavior is undefined
if the truncated value doesn't fit.
Pointers, integral types, and floating-point values can be converted to bool. If the value
is 0 then the resulting bool is false, otherwise, it is true, bools can be converted to
the other types; true is converted as a value of 1 and false as 0.
A constant-expression (§A.2.3/303) that evaluates to 0 may be converted to a pointer.
Any pointer can be converted to a void*. Also, a pointer to nonconst can be converted
to a pointer to const—similarly for a nonconst reference. A pointer or reference to an
object of a type that was publicly derived may be converted to a pointer or reference to
any of its base classes.
Arithmetic conversions: Because the operands can be integral or floating point, and
signed or unsigned, it can be a little tricky to figure out the type of the result of an
arithmetic operation. The rules for doing so are called the usual arithmetic
conversions, and work as follows:
If either operand is floating point, the result is floating point, with the precision of
the most precise operand.
Otherwise, if either operand is unsigned long, then so is the result.
Otherwise, if one operand is long int and the other is unsigned (any unsigned
type but unsigned long, which would force the result to be unsigned long by
the previous rule), then the result depends on the implementation: If the range of
long int contains the range of unsigned int, then the result is long int;
otherwise, it is unsigned int.
Otherwise, if either operand is long int, the result is long int.
Otherwise, the operands must both be signed integers of int type or shorter, and
the result is int.
One consequence of these rules is that the result of an arithmetic operation is never
short or char, either signed or unsigned. All arithmetic is done only on int or wider
types.
A.2.5 Enumerated types
An enumerated type specifies a set of integral values. Objects of the type may take on
only values specified by the type:
enum enum-name {
enumerator [ , enumerator ] ...
};
enum-names are type-names and can be used where a type-name is expected.
Variables of the enum-name type can have values only from the list of enumerators:
enumerator: identifier [ = constant-expression ]
Unless specified, the values of enumerated types correspond to consecutive integers,
starting from zero. It is also possible to state explicit values for the enumerators. The
initializers must have integral (§A.2.1/300) type, and a value that the compiler can
determine during compilation (§A.2.3/303). If a value of an enumerated type is used in
a context that requires an integer, the value will be converted automatically.
A.2.6 Overloading
More than one function can have the same name, provided that the functions differ in
type or number of parameters.
Calling an overloaded function implies a compile-time check to determine which of the
set of overloaded functions should be called. The function to call is determined by
comparing the actual arguments at the call with the types of the formal parameters. The
function that best matches the actual arguments is selected. To be the best match, a
function must be a better match than the others on one or more arguments and no
worse in any argument.
The best match for any particular argument is defined as follows:
An exact match (the argument types are identical) is best.
A match using promotions (§A.2.4/303) is better than a match using built-in
conversions, which is better than a match using conversions defined by a class type
(§12.2/213 and §12.5/222).
It is an error if there is more than one function that best matches the call.
A.3 Expressions
C++ inherits a rich expression syntax from C, to which it adds operator overloading
(§A.3.1/308). Operator overloading allows program authors to define the argument and
return types and meaning of operators, but not their precedence, valence (number of
operands), or associativity, nor the meaning of built-in operators on operands of built-in
types. In this section we will describe—with a few additions—the operators as they apply
to built-in types.
Every operator yields a value, the type of which depends on the types of its operands. In
general, if the operands have the same type, that is the type of the result. Otherwise,
standard conversions are performed to bring the types to a common type (§A.2.4/303).
An lvalue is a value that denotes a nontemporary object, hence it has an address.
Certain operations are valid only for lvalues, and some operations yield lvalues. Every
expression yields a value; some expressions also yield lvalues.
Only three operators guarantee the order of evaluation for their operands:
&& The right operand is evaluated only if the left operand is true .
|| The right operand is evaluated only if the left operand is false .
? : Only one expression after the condition will be evaluated. The expression after
the ? is evaluated if the condition is true ; otherwise, the expression after the : is
evaluated. The result is the expression that was evaluated; it is an lvalue if both
expressions were lvalues of the same type.
For the other operators, aside from precedence rules, order of evaluation is not
guaranteed. That is, the compiler is permitted to evaluate operands in any order.
Parentheses can be used to override the default precedence, but explicit temporaries are
required to control the order of evaluation completely.
Each operator has a specified precedence and associativity. In the following table, we
summarize all the operators in order by precedence. When several operators are
grouped together, they share the same precedence and associativity. Each grouping
introduces a new level of precedence. This table expands the one first presented in
Chapter 2 and includes all the operators:
C::m
The member m from class C
N::m
The member m from namespace N .
::m
The name m at global scope.
terminal Identifier or literal constant; identifiers are lvalues, literals are not
x[y]
The element in object x indexed by y . Yields an lvalue.
x->y
The member y of the object pointed to by x . Yields an lvalue.
x.y
The member y of object x . Yields an lvalue if x is an lvalue.
f(args )
Call function f passing args as argument(s).
x++
Increments the lvalue x . Yields the original value of x .
x--
Decrements the lvalue x . Yields the original value of x .
*x
Dereferences the pointer x . Yields the object pointed to as an lvalue.
&x
The address of the object x . Yields a pointer.
-x
Unary minus. May be applied only to expressions of numeric type.
!x
Logical negation. If x is zero, then !x is true , otherwise false .
~x
Ones complement of x . x must be an integral type.
++x
Increments the lvalue x . Yields the incremented value as an lvalue.
--x
Decrements the lvalue x . Yields the decremented value as an lvalue.
sizeof(e)
The number of bytes, as a size_t , consumed by expression e .
sizeof(T)
The number of bytes, as a size_t , consumed by objects of type T .
T(args )
Constructs a T object from args.
new T
Allocates a new, default-initialized object of type T .
new T(args )
Allocates a new object of type T initialized by args.
new T[n]
Allocates an array of n default-initialized objects of type T .
delete p
Frees object pointed to by p .
delete [] p
Frees the array of objects pointed to by p .
x * y
Product of x and y .
x / y
Quotient of x and y . If both operands are integers,
the implementation chooses whether to round toward 0 or -8.
x % y
x - ((x / y) * y) .
x + y
Sum of x and y , if both operands are numeric.
If one operand is a pointer and the other is integral,
yields a pointer to a position y elements after x .
x - y
Result of subtracting y from x if operands are numeric.
If x and y are pointers, yields the distance, in elements, between them.
x >> y
For integral x and y, x shifted right by y bits; y must be non-negative.
If x is an istream , reads from x into y and returns lvalue x .
x << y
For integral x and y, x shifted left by y bits; y must be non-negative.
If x is an ostream , writes y into x and returns lvalue x .
x relop y
Relational operators yield a bool indicating the truth of the relation.
The operators (<, >, <= , and >= ) have their obvious meanings.
If x and y are pointers, they must point to the same object or array.
x == y
Yields a bool indicating whether x equals y .
x != y
Yields a bool indicating whether x is not equal to y .
x & y
Bitwise and. x and y must be integral.
x ^ y
Bitwise exclusive or. x and y must be integral.
x | y
Bitwise or. x and y must be integral.
x && y
Yields a bool indicating whether both x and y are true .
Evaluates y only if x is true .
x || y
Yields a bool indicating whether either x or y is true .
Evaluates y only if x is false .
x = y
Assigns the value of y to x . Yields x as its (lvalue) result.
x op= y
Compound assignment operators. Equivalent to x = x op y ,
where op is an arithmetic, bitwise, or shift operator.
x ? y1 : y2
Yields y1 if x is true ; y2 otherwise.
Only one of y1 or y2 is evaluated.
y1 and y2 must be of the same type.
If y1 and y2 are lvalues, the result is an lvalue.
The operator is right-associative.
throw x
Signal an error by throwing value x .
The type of x determines which handler will catch the error.
x , y
Evaluates x , discards the result, then evaluates y . Yields y .
A.3.1 Operators
Most of the built-in operators may be overloaded. The throw, scope, dot, and conditional
operator (the ? : operator) may not be overloaded. All of the other operators may be.
§11.2.4/192 describes how to define an overloaded operator.
The postfix increment/decrement operator is distinguished from the prefix version by
being defined as taking a dummy, unused parameter. That is, to overload the postfix
operators, we write
class Number {
public:
Number operator++(int) { /* function-body */ }
Number operator--(int) { /* function-body */ }
};
The most commonly overloaded operators include the assignment and index operator,
the shift operators used to do input-output with ostream s and istream s, and the
operators used to implement iterators summarized in §B.2.5/317.
A.4 Statements
Like most programming languages, C++ distinguishes between declarations,
expressions, and statements. In appropriate contexts, declarations and statements can
be nested within other declarations and statements, but neither can be nested inside an
expression. Every statement ultimately appears inside the definition of a function, where
it forms part of what happens when that function is called.
Unless otherwise specified, the statements that constitute a function are executed in the
order in which they appear. Exceptions include loops; calls to functions; the goto,
break, and continue statements; and the try and throw statements associated with
exception handling.
Statements are written in free form. Beginning a new line in midstatement does not
affect the statement's meaning. Most statements end with semicolons—the main
exception is the block (which begins with { and ends with }).
;
Null statement; has no effect when executed.
e;
Expression statement; evaluates e for its side effects.
{ }
Statement block; executes statements in the block in sequence. Declarations in the
block persist (only) until closing brace.
if (condition) statement1
Evaluates condition and executes statementl if condition is true.
if (condition) statement1 else statement2
Evaluates the condition and executes statement1 if condition is true;
otherwise executes statement2. Each else is associated with the nearest unmatched
if.
while (condition) statement
Tests condition and executes statement so long as condition is true.
do statement while (condition) ;
Executes statement and then tests condition. Continues executing statement
until condition is false,
for (init-stmt condition; expression) statement
Executes init-stmt once on entry to the loop and then tests condition. If
condition is true, executes statement and then executes expression. Continues
testing condition, followed by statement and expression, until condition is
false. If init-stmt is a declaration, then the scope of the variable is the for
statement itself.
switch (expression) statement
In practice, statement is almost always a block that includes labeled statements with
labels of the form
case value:
where each value must be a distinct integral constant expression (§A.2.3/303). In
addition, the label
default:
may appear, but no more than once. Executing a switch statement evaluates
expression and jumps to the case label whose value matches it. If there is no
match, control passes to the default: label, if any, or to the point immediately after
the entire switch statement. Because case labels are just labels, control will flow from
one to the next unless the programmer takes explicit action to prevent it from doing
so. The usual such action is to use a break statement before each case label after the
first.
break;
Jumps to the point immediately after the end of the nearest enclosing while, for,
do, or switch statement.
continue;
Jumps back to the beginning of the next iteration (including the test) in the nearest
enclosing for, while, or do statement.
goto label;
Behaves similarly to such statements in other languages. The target of a goto is a
label, which is an identifier followed by a colon. Labels can have the same names as
other entities without ambiguity. The scope of a label is the entire function in which it
appears, which implies that it is possible to jump from outside a block to inside it.
However, such a jump cannot bypass the initialization of a variable.
try { statements } catch (parameter-1) { statements-1 }
[ catch (parameter-2) {statements-2}]...
Executes code in statements that might throw an exception, which should be
handled by the one or more catch clauses that follow. The catch clause handles
exceptions whose thrown value is of similar type to the type of parameter-n by
executing code in statements-n. Similar here means that the thrown value has the
same type as the parameter or a type derived from the parameter's type. If the catch
has the form catch (...), then the clause catches any otherwise uncaught
exception. If there is no appropriate catch that matches the type of the exception,
then the exception propagates out of the function to the nearest enclosing try. If
there is no appropriate try, then the program terminates.
throw expression ;
Terminates the program or transfers control to a catch clause of a try statement
whose execution is in progress. Passes expression whose type determines which
catch clause can handle the exception. If no appropriate try statement is currently
being executed, the program terminates. Exceptions are often class objects, and are
usually thrown in one function and caught in another.
Appendix B
Library summary
The standard library is a major contribution to the standardization of C++. Throughout
this book, we have relied on the library to write succinct, idiomatic C++ programs. This
chapter reviews the library facilities that we used, and describes some generally useful
facilities that we did not have occasion to use. Each section presents a library class or
family of related classes by showing and explaining how to use the relevant facilities.
In general, the standard library introduces names into namespace std. Programs that
use standard-library facilities must therefore either prefix such names by std:: or
make them generally available with using-declarations. In this appendix, we will not
explicitly mention the need to do so. Accordingly, for example, we shall refer to cout
rather than to std::cout.
Our examples generally assume the following meanings for the names shown:
n
A variable or expression that yields a value of any of the integral types
t
A value of type T
s
A string value
cp
A pointer to the initial element of a null-terminated character array
c
A char value
p
A predicate, which is a function that returns bool or a value convertible to bool
os
An output stream
is
An input stream
strm
An input or output stream
b
An iterator that denotes the beginning of a sequence
e
An iterator that denotes (one past) the end of a sequence
d
An iterator that denotes a destination
it
An iterator that denotes an element
B.1 Input-output
Objects of classes istream, ostream, ifstream, and ofstream denote sequential
streams, with an object being bound to a single stream at any one time. Objects of
these types cannot be copied or assigned; therefore, the only way to pass a stream to or
from a function is through a pointer or a reference.
Fundamentals
#include
Declares input-output classes and associated operations.
cout
cerr
clog
Objects of type ostream bound to the standard output (cout) and error (cerr,
clog) streams. Output to cout and clog is buffered by default; output to cerr is
unbuffered by default.
cin
An object of type istream bound to the standard input stream.
Reading and writing
is >> t
Conventionally, reads a value from is into t after skipping whitespace. The input must
be in a form suitable for conversion to the type of t, which must be a nonconst lvalue.
Unsuitable input causes the request to fail, leaving is in failure state until a call is
made to is.clear(). The library defines the input operator for the built-in types and
string; class authors are encouraged to follow suit.
os << t
Conventionally, sends the value of t to os in a format appropriate to the type of t. The
library defines << for built-in types and string; class authors are encouraged to
follow suit.
is.get(c)
Reads the next character, even if it is a whitespace character, from is into c.
is.unget()
Backs up the stream is by one character. Useful when we want to read until we hit a
particular character, and then want to leave that character on the stream for
subsequent processing. Only one character of pushback memory is guaranteed.
Iterators
#include
Declares input and output stream iterators.
istream_iterator in(is);
Defines in as an input iterator that reads values of type T from is.
ostream_iterator out(os, const char* sep = "");
Defines out as an output iterator that writes values of type T on the stream os, writing
sep as separator value after each element. By default, the separator is a null string,
but it can be a string literal (§10.2/176) or a pointer to a null-terminated array of
characters.
File streams
#include
Declares facilities for input-output to streams that are attached to files.
ifstream is(cp);
Defines is and attaches it to the file named (in an implementation- dependent
manner) by cp. Class if stream is derived from istream.
ofstream os(cp);
Defines os and attaches it to the file named by cp. Class ofstream is derived from
ostream.
Controlling output format
#include
Defines the streamsize type, which is a signed integral type that is appropriate to
represent the sizes of input-output buffers.
os.width()
os.width(n)
Returns the width (as a streamsize) previously associated with os. Sets the width to
n if given. The next item written on that stream will be padded on the left to the
stream's width, after which the width will be reset to 0.
os.precision()
os.precision(n)
Returns the precision (as a streamsize) previously associated with os. Sets the
precision to n if given. Subsequent floating-point values written on that stream will
appear with the given number of significant digits.
Manipulators
#include
Declares manipulators other than endl, which is declared in .
os << endl
Ends the current output line and flushes the stream associated with os.
os << flush
Flushes the stream associated with os.
os << setprecision(n)
os << setw(n)
Equivalent to os.precision(n) and os.width(n) respectively.
Errors and end-of-file
strm.bad()
Returns a bool indicating whether the last operation on strm failed as a result of
invalid data.
strm.clear()
Attempts to reset strm so that it can be used after an invalid operation. Throws
ios::failure if strm cannot be reset.
strm.eof()
Returns a bool indicating whether strm has hit end-of-file.
strm.fail()
Returns a bool indicating whether the last operation on strm failed as a result of
hardware or other system-level problems.
strm.good()
Returns a bool indicating whether the last operation on strm succeeded.
B.2 Containers and iterators
This book covers the sequential containers vector and list, the associative container
map, and class string, which shares many container properties. All containers that
provide a particular operation use similar interfaces for the operation. We summarize
the common operations, followed by operations peculiar to specific containers.
Programmers who want to use sequential containers should use vector, unless there is
a reason to do otherwise. The most common such reason is a desire to insert or delete
many elements other than at the end of the container, an operation that the list class
supports much more efficiently than the vector class does.
B.2.1 Common container operations
All containers and the string class offer the following interface:
container::iterator
An iterator type associated with container, objects of which type may be used to
change the values stored in the container.
container::const_iterator
An iterator type associated with container, objects of which type may be used to
read, but not to modify, the values stored in the container.
container::reverse_iterator
container::const_reverse_iterator
Iterator types that access the container's elements in reverse order.
container::size_type
An unsigned integral type with room for the size of the largest container.
container::value_type
The type of the container elements.
c.begin()
c.end()
Iterators that denote the first element, if any, and one past the last element of c. Both
of these functions yield values that have c's const_iterator or iterator type,
depending on whether c is const.
c.rbegin()
c.rend()
Iterators (of C's const_reverse_iterator or reverse_iterator type, depending
on whether c is const) that access c's elements in reverse order.
container c;
Defines c as an empty container, with c.size() == 0.
container c2(c);
Defines c2 as a container, with c2.size() == c.size(). Each element of c2 is a
copy of the corresponding element of c.
c = c2
Replaces c's elements with copies of c2's elements. Returns c as an lvalue.
c.size()
The number of elements in c.
c.empty()
Returns true if c is empty, false otherwise.
c.clear()
Empties the container c. Equivalent to c.erase(c.begin(), c.end()). After the
Operation completes, c.size() == 0. Returns void.
B.2.2 Sequential containers
In addition to the common container operations, string and the sequential containers
(vector and list) also support the following operations:
container c(n, t);
Defines c to have n elements, each of which is a copy of t.
container c(b, e);
Defines c and initializes it with a copy of the elements in the sequence denoted by the
input iterators b and e.
c.insert(it, t)
c.insert(it, n, t)
c.insert(it, b, e)
Inserts elements into c immediately before it. If c is a vector or string, the
operation invalidates all iterators that refer to or after the insertion point and may
cause reallocation, invalidating all iterators into c. Note that for vector and string,
this operation may be slow if it is far from the end. The first form of insert inserts a
copy of t, and returns an iterator that refers to the newly inserted element. The
second form inserts n copies of t, and returns void. The third form inserts copies of
the elements in the sequence denoted by the input iterators b and e, and returns
void. The iterators b and e must not refer to elements of c.
c.erase(it)
c.erase(b, e)
Removes the element denoted by it, or the elements in the range [b, e), from c,
invalidating all iterators referring to erased elements. If c is a vector or string,
then all iterators referring to elements after the erasure are also invalidated. Returns
an iterator that refers to the position immediately after the erasure. Note that erase
on a vector or string is slow if the erasure is far from the end of the container.
c.assign(b, e)
Replaces c's elements with the elements in the sequence denoted by the input
iterators b and e.
c.front()
Returns a reference to the first element of c. Undefined if c is empty.
c.back()
Returns a reference to the last element of c. Undefined if c is empty.
c.push_back(t)
Appends a copy of t to c, increasing the size of c by one. Returns void.
c.pop_back()
Removes the last element from c. Returns void. Undefined if c is empty.
inserter(c, it)
Returns an output iterator that inserts values into c starting immediately before the
position denoted by it. Declared in .
back_inserter(c)
Returns an output iterator that can append new values to the end of c by calling
c.push_back. Declared in .
B.2.3 Additional sequential operations
Some operations are supported only on those containers for which the operations can be
done efficiently. These include the following:
c[n]
A reference to the nth element of c, where the initial element has position 0. The
reference is const if c is const, and nonconst otherwise. Undefined if n is out of
range. Valid only for vector and string.
c.push_front(t)
Inserts a copy of t at the beginning of c, increasing the size of c by one. Returns
void. Not valid for string or vector.
c.pop_front()
Removes the first element from c. Returns void. Undefined if c is empty. Valid only
for list.
front_inserter(c)
Returns an output iterator that can insert new values at the front of c by calling
c.push_front. Declared in .
B.2.4 Associative containers
Associative containers are optimized for fast access based on a key. In addition to the
general container operations outlined in §13.2.1/314, associative containers also provide
the following:
container::key_type
The type of the container's key. An associative container with keys of type K and
elements of type V has a value_type of pair, not V.
container c(cmp);
Defines c as an empty associative container that uses the predicate cmp to order the
elements.
container c(b, e, cmp);
Defines c as an associative container, initialized with a copy of the values in the
sequence denoted by the input iterators b and e, that uses cmp to order the elements.
c.insert(b, e)
Inserts elements into c from the sequence denoted by the input iterators b and e. The
map container inserts only those elements whose keys are not already in c.
c.erase(it)
Removes the element denoted by the iterator it from c. Returns void.
c.erase(b, e)
Removes elements in the range [b, e) from c. Returns void.
c.erase(k)
Removes all elements with key k from c. Returns the number removed.
c.find(k)
Returns an iterator referring to the element with key equal to k. Returns c.end() if
no such element exists.
B.2.5 Iterators
The standard library relies heavily on iterators to make its algorithms data-structure
independent. Iterators are an abstraction of pointers, in that they provide operations
that allow access to container elements analogous to what pointers allow on array
elements.
The standard algorithms are written to assume that iterators meet requirements that
the library classifies into iterator categories. Every library algorithm that uses iterators
of a particular category can work with every library- or user-defined class that provides
iterators that fall into that category.
Output: It is possible to use the iterator to advance through the container one
element at a time, and to write each element visited once and only once. Example:
Class ostream_iterator is an output iterator; and the copy algorithm requires
only the output-iterator properties for its third argument.
Input: It is possible to use the iterator to advance through the container one
element at a time, and to read each element as often as needed before advancing
to the next element. Example: Class istream_iterator is an input iterator, and
the copy algorithm requires only input-iterator properties for its first two
arguments.
Forward: It is possible to use the iterator to advance through the container one
element at a time, to revisit elements to which previously remembered iterators
refer, and to read or write each element as often as needed. Example: replace is
an algorithm that requires forward-iterator properties.
Bidirectional: It is possible to use the iterator to move through the container one
element at a time, either forward or backward. Example: list and map provide
bidirectional iterators, and reverse is an algorithm that requires bidirectional
iterators.
Random access: It is possible to move through the container using all the
operations supported by pointers. Example: vector, string, and built-in arrays
support random-access iterators. The sort algorithm requires random-access
iterators.
All iterator categories support testing for (in)equality. Random-access iterators support
all the relational operations.
Iterator categories can be thought of as cumulative, in the sense that every forward
iterator is also an input iterator and an output iterator, every bidirectional iterator is also
a forward iterator, and every random-access iterator is also a bidirectional iterator.
Thus, any algorithm that accepts any iterator type as an argument will accept a randomaccess
iterator. Class ostream_iterator and the insert iterator adaptors provide
output iterators, and thus can be used only by algorithms that require only outputiterator
operations.
All iterators support the following operations:
++p
p++
Advances p to the next position in the container. ++p returns p as an lvalue after
advancing it; p++ returns a copy of p's previous value.
*p
The element to which p refers. For output iterators, *p may be used only as the left
operand of =, and each distinct value of p may be used in this way only once. For input
iterators, *p may be used only for reading; and the act of incrementing p invalidates
all copies that might have been made of p's previous value. For all other iterator types,
*p yields a reference to the value stored in the container element to which p refers,
and p remains valid as long as the element to which p refers continues to exist.
p == p2
Yields true if p is equal to p2; false otherwise.
p != p2
Yields true if p is not equal to p2; false otherwise.
All iterators other than output iterators also support
p->x
Equivalent to (*p).x
Bidirectional and random-access iterators also support decrement operations:
--p
p--
Advances p backward to refer to the previous element, --p returns p as an lvalue
after advancing it; p-- returns a copy of p's previous value.
Random-access iterators provide all of the "pointer" operations, including the following:
p + n
If n >= 0, then the result is an iterator that refers to a point n positions beyond p.
The operation is undefined if fewer than n - 1 elements follow the element denoted
by p. If n < 0, then the result is an iterator that refers to the element -n positions
before the element denoted by p. The operation is undefined unless this element is
within range of the container.
n + p
Equivalent to p + n.
p - n
Equivalent to p + (-n).
p2 - p
Defined only if p and p2 refer to positions in the same container. If p2 >= p, yields
the number of elements in the range [p, p2). Otherwise, yields the negation of the
number of elements in the range [p2, p). The result has typeptrdiff_t
(§10.1.4/175).
p[n]
Equivalent to *(p + n).
p < p2
true if p denotes an earlier position in the container than that denoted by p2.
Undefined if p and p2 do not refer to positions in the same container.
p <= p2
Equivalent to (p < p2) || (p == p2).
p > p2
Equivalent to p2 < p.
p >= p2
Equivalent to p2 <= p.
B.2.6 vector
The vector class provides dynamically allocated, type-independent arrays, and
supports random-access iterators. In addition to the common sequential-container
operations (§B.2.1/314 and §B.2.2/315), vector also supports the following:
#include
Declares the vector class and associated operations.
v.reserve(n)
Reallocates v so that it can grow to accommodate at least n elements without further
reallocation.
v.resize(n)
Reallocates v to hold n elements. Invalidates all iterators referring to elements of v.
Preserves the first n elements. If the new size is less than the old, excess elements are
destroyed. If the new size is greater than the old, new elements are value-initialized
(§9.5/164).
B.2.7 list
The list class provides dynamically allocated, type-independent, doubly linked lists,
and supports bidirectional iterators (unlike vector, which supports random-access
iterators). In addition to the general operations on sequential containers (§B.2.1/314
and §B.2.2/315), list also supports the following:
#include
Declares the list class and associated operations.
l.splice(it, l2)
Inserts all the elements of l2 into l immediately before the position denoted by it,
and removes those elements from l2. Invalidates all iterators and references into l2.
After completion, l.size() is the sum of the original sizes of l and l2, and
l2.size() == 0. Returns void.
l.splice(it, l2, it2)
l.splice(it, l2, b, e)
Inserts the element denoted by it2, or the elements in the sequence denoted by [b,
e), into l immediately before the position denoted by it, and removes those elements
from l2. The element denoted by it2, or the elements in the range [b, e), must be
entirely within l2. Invalidates iterators and references to the spliced elements. Returns
void.
l.remove(t)
l.remove_if(p)
Removes from l all elements with value equal to t, or for which the predicate p yields
true. Returns void.
l.sort(cmp)
l.sort()
Sorts l using <, or the predicate cmp if supplied, to compare elements.
B.2.8 string
The string class provides variable-length character strings and random-access
iterators to access their characters. Although strings aren't true containers, they
support the container operations shown previously (§B.2.1/314 and §B.2.2/315) and can
be used with the algorithms (§B.3/321). In addition, the string class also supports the
following:
#include
Declares the string class and associated operations.
string s(cp);
Defines s as a string initialized to a copy of the characters denoted by cp.
os << s
Writes the characters in s onto os. Returns a reference to os.
is >> s
Reads a word from is into s, obliterating s's previous contents. Returns a reference to
is. Words are delimited by a whitespace (space, tab, newline).
getline(is, s)
Reads the input stream is up to and including the next newline, and stores the
characters read, excluding the newline, in s, obliterating s's previous contents.
Returns a reference to is.
s += s2
Appends s2 to s and returns a reference to s.
s + s2
Returns the result of concatenating s and s2.
s relop s2
Returns a bool indicating whether the relational operation is true. The string
library defines all the relational and equality operators: <, <=, >, >=, ==, and !=.
Two strings are equal if their respective elements are equal. If one string is a
prefix of the other, then the shorter is less than the longer. Otherwise, the result is
determined by comparing the first pair of respective characters at which the strings
differ.
s.substr(n, n2)
Returns a new string that holds n2 characters copied from s, starting at position n.
Undefined if n > s.size(). Copies characters in the range from n to the end of s if
n + n2 is greater than s.size().
s.c_str()
Yields a const pointer to a null-terminated character array that contains a copy of the
characters in s. The array persists only until the next nonconst member function is
called on s.
s.data()
Like c_str, but the array is not null terminated
s.copy(cp, n)
Copies up to the first n characters, without null termination, from s into a usersupplied
character array denoted by cp. It is the caller's responsibility to ensure that
there is room for at least n characters.
B.2.9 pair
Class pair provides an abstraction for a pair of values of type K and V
respectively. The operations on pair include the following:
#include
Declares the pair class and associated operations.
x.first
The first element of the pair named x.
x.second
The second element of the pair named x.
pair x(k, v);
Defines x as a new pair composed of elements with types K and V, and values k and
v, so that x. first has type K and x. second has type V. Note that to declare a pair
explicitly, you must know the types of its members.
make_pair(k, v)
Generates a new pair with element values k and v. Note that it is not
necessary to know the types of k and v to use this form.
B.2.10 map
Class map provides dynamically allocated, type-independent associative arrays. It uses
pair as an auxiliary class to store the (name, value) pairs that are the map's elements.
Iterators are bidirectional. Each map holds values of type V associated with keys of type
const K. Accordingly, the value stored in an element of a map can be changed, but the
key cannot be changed. In addition to the general container operations (§B.2.1/314)
and those on associative containers (§B.2.4/316), map also supports the following:
#include