C++ Core Guidelines C++核心指南

C++ Core Guidelines C++核心指南

April 13, 2023

Editors:
编辑：

• Bjarne Stroustrup
• Herb Sutter

This is a living document under continuous improvement.
Had it been an open-source (code) project, this would have been release 0.8.
Copying, use, modification, and creation of derivative works from this project is licensed under an MIT-style license.
Contributing to this project requires agreeing to a Contributor License. See the accompanying LICENSE file for details.
We make this project available to “friendly users” to use, copy, modify, and derive from, hoping for constructive input.

Comments and suggestions for improvements are most welcome.
We plan to modify and extend this document as our understanding improves and the language and the set of available libraries improve.
When commenting, please note the introduction that outlines our aims and general approach.
The list of contributors is here.

Problems:
问题：

• The sets of rules have not been completely checked for completeness, consistency, or enforceability.
• Triple question marks (???) mark known missing information
• Update reference sections; many pre-C++11 sources are too old.
• For a more-or-less up-to-date to-do list see: To-do: Unclassified proto-rules

You can read an explanation of the scope and structure of this Guide or just jump straight in:
你可以阅读本指南范围和结构的解释，也可以直接进入

• In: Introduction 介绍
• P: Philosophy 哲学
• I: Interfaces 接口
• F: Functions 函数
• C: Classes and class hierarchies 类和类层次结构
• Enum: Enumerations 枚举
• R: Resource management 资源管理
• ES: Expressions and statements 表达式和语句
• Per: Performance 性能
• CP: Concurrency and parallelism 并发和并行
• E: Error handling 错误处理
• Con: Constants and immutability 常数和不变性
• T: Templates and generic programming 模板与泛型编程
• CPL: C-style programming C风格编程
• SF: Source files 源文件
• SL: The Standard Library 标准库

Supporting sections:
支持部分：

• A: Architectural ideas
• NR: Non-Rules and myths
• RF: References 参考
• Pro: Profiles 简介
• GSL: Guidelines support library 指南支持库
• NL: Naming and layout suggestions 命名和布局建议
• FAQ: Answers to frequently asked questions 常见问题解答
• Appendix A: Libraries 库
• Appendix B: Modernizing code 代码现代化
• Appendix C: Discussion 讨论
• Appendix D: Supporting tools 支持工具
• Glossary 词汇表
• To-do: Unclassified proto-rules 未分类的原型规则

You can sample rules for specific language features:
您可以选取特定语言功能的规则：

• assignment 赋值:
  regular types 正则类型 –
  prefer initialization 预初始化 –
  copy 拷贝 –
  move 移动 –
  other operations 其他操作 –
  default 默认
• class:
  data –
  invariant –
  members –
  helpers –
  concrete types –
  ctors, =, and dtors –
  hierarchy –
  operators 操作符
• concept:
  rules 规则 –
  in generic programming –
  template arguments 模板参数 –
  semantics
• constructor 构造函数:
  invariant –
  establish invariant –
  throw –
  default –
  not needed –
  explicit –
  delegating 委托 –
  virtual
• derived class 派生类:
  when to use –
  as interface –
  destructors –
  copy –
  getters and setters –
  multiple inheritance –
  overloading –
  slicing –
  dynamic_cast
• destructor 析构函数:
  and constructors –
  when needed? 何时需要 –
  must not fail
• exception 异常:
  errors –
  throw –
  for errors only –
  noexcept –
  minimize try –
  what if no exceptions?
• for:
  range-for and for –
  for and while –
  for-initializer –
  empty body –
  loop variable –
  loop variable type ???
• function 函数:
  naming 命名 –
  single operation 单一操作 –
  no throw –
  arguments 参数 –
  argument passing 参数传递 –
  multiple return values 多个返回值 –
  pointers 指针 –
  lambdas
• inline:
  small functions 小函数 –
  in headers
• initialization 初始化:
  always –
  prefer {} –
  lambdas –
  in-class initializers –
  class members –
  factory functions
• lambda expression:
  when to use 何时采用
• operator 操作符:
  conventional –
  avoid conversion operators –
  and lambdas
• public, private, and protected:
  information hiding –
  consistency –
  protected
• static_assert:
  compile-time checking 编译时检查 –
  and concepts
• struct:
  for organizing data –
  use if no invariant –
  no private members
• template:
  abstraction 抽象 –
  containers 容器 –
  concepts
• unsigned:
  and signed –
  bit manipulation 位操作
• virtual:
  interfaces 接口 –
  not virtual –
  destructor 析构函数 –
  never fail

You can look at design concepts used to express the rules:
您可以查看用于表达规则的设计概念：

• assertion: ???
• error: ???
• exception: exception guarantee (???)
• failure: ???
• invariant: ???
• leak: ???
• library: ???
• precondition: ???
• postcondition: ???
• resource: ???

Abstract 摘要

This document is a set of guidelines for using C++ well.
The aim of this document is to help people to use modern C++ effectively.
By “modern C++” we mean effective use of the ISO C++ standard (currently C++20, but almost all of our recommendations also apply to C++17, C++14 and C++11).
In other words, what would you like your code to look like in 5 years’ time, given that you can start now? In 10 years’ time?

The guidelines are focused on relatively high-level issues, such as interfaces, resource management, memory management, and concurrency.
Such rules affect application architecture and library design.
Following the rules will lead to code that is statically type safe, has no resource leaks, and catches many more programming logic errors than is common in code today.
And it will run fast – you can afford to do things right.

We are less concerned with low-level issues, such as naming conventions and indentation style.
However, no topic that can help a programmer is out of bounds.

Our initial set of rules emphasizes safety (of various forms) and simplicity.
They might very well be too strict.
We expect to have to introduce more exceptions to better accommodate real-world needs.
We also need more rules.

You will find some of the rules contrary to your expectations or even contrary to your experience.
If we haven’t suggested you change your coding style in any way, we have failed!
Please try to verify or disprove rules!
In particular, we’d really like to have some of our rules backed up with measurements or better examples.

You will find some of the rules obvious or even trivial.
Please remember that one purpose of a guideline is to help someone who is less experienced or coming from a different background or language to get up to speed.

Many of the rules are designed to be supported by an analysis tool.
Violations of rules will be flagged with references (or links) to the relevant rule.
We do not expect you to memorize all the rules before trying to write code.
One way of thinking about these guidelines is as a specification for tools that happens to be readable by humans.

The rules are meant for gradual introduction into a code base.
We plan to build tools for that and hope others will too.

Comments and suggestions for improvements are most welcome.
We plan to modify and extend this document as our understanding improves and the language and the set of available libraries improve.

In: Introduction 介绍

This is a set of core guidelines for modern C++ (currently C++20 and C++17) taking likely future enhancements and ISO Technical Specifications (TSs) into account.
The aim is to help C++ programmers to write simpler, more efficient, more maintainable code.

Introduction summary:

• In.target: Target readership
• In.aims: Aims
• In.not: Non-aims
• In.force: Enforcement
• In.struct: The structure of this document
• In.sec: Major sections

In.target: Target readership

All C++ programmers. This includes programmers who might consider C.

In.aims: Aims

The purpose of this document is to help developers to adopt modern C++ (currently C++20 and C++17) and to achieve a more uniform style across code bases.

We do not suffer the delusion that every one of these rules can be effectively applied to every code base. Upgrading old systems is hard. However, we do believe that a program that uses a rule is less error-prone and more maintainable than one that does not. Often, rules also lead to faster/easier initial development.
As far as we can tell, these rules lead to code that performs as well or better than older, more conventional techniques; they are meant to follow the zero-overhead principle (“what you don’t use, you don’t pay for” or “when you use an abstraction mechanism appropriately, you get at least as good performance as if you had handcoded using lower-level language constructs”).
Consider these rules ideals for new code, opportunities to exploit when working on older code, and try to approximate these ideals as closely as feasible.
Remember:

In.0: Don’t panic!

Take the time to understand the implications of a guideline rule on your program.

These guidelines are designed according to the “subset of superset” principle (Stroustrup05).
They do not simply define a subset of C++ to be used (for reliability, safety, performance, or whatever).
Instead, they strongly recommend the use of a few simple “extensions” (library components)
that make the use of the most error-prone features of C++ redundant, so that they can be banned (in our set of rules).

The rules emphasize static type safety and resource safety.
For that reason, they emphasize possibilities for range checking, for avoiding dereferencing nullptr, for avoiding dangling pointers, and the systematic use of exceptions (via RAII).
Partly to achieve that and partly to minimize obscure code as a source of errors, the rules also emphasize simplicity and the hiding of necessary complexity behind well-specified interfaces.

Many of the rules are prescriptive.
We are uncomfortable with rules that simply state “don’t do that!” without offering an alternative.
One consequence of that is that some rules can be supported only by heuristics, rather than precise and mechanically verifiable checks.
Other rules articulate general principles. For these more general rules, more detailed and specific rules provide partial checking.

These guidelines address the core of C++ and its use.
We expect that most large organizations, specific application areas, and even large projects will need further rules, possibly further restrictions, and further library support.
For example, hard-real-time programmers typically can’t use free store (dynamic memory) freely and will be restricted in their choice of libraries.
We encourage the development of such more specific rules as addenda to these core guidelines.
Build your ideal small foundation library and use that, rather than lowering your level of programming to glorified assembly code.

The rules are designed to allow gradual adoption.

Some rules aim to increase various forms of safety while others aim to reduce the likelihood of accidents, many do both.
The guidelines aimed at preventing accidents often ban perfectly legal C++.
However, when there are two ways of expressing an idea and one has shown itself a common source of errors and the other has not, we try to guide programmers towards the latter.

In.not: Non-aims

The rules are not intended to be minimal or orthogonal.
In particular, general rules can be simple, but unenforceable.
Also, it is often hard to understand the implications of a general rule.
More specialized rules are often easier to understand and to enforce, but without general rules, they would just be a long list of special cases.
We provide rules aimed at helping novices as well as rules supporting expert use.
Some rules can be completely enforced, but others are based on heuristics.

These rules are not meant to be read serially, like a book.
You can browse through them using the links.
However, their main intended use is to be targets for tools.
That is, a tool looks for violations and the tool returns links to violated rules.
The rules then provide reasons, examples of potential consequences of the violation, and suggested remedies.

These guidelines are not intended to be a substitute for a tutorial treatment of C++.
If you need a tutorial for some given level of experience, see the references.

This is not a guide on how to convert old C++ code to more modern code.
It is meant to articulate ideas for new code in a concrete fashion.
However, see the modernization section for some possible approaches to modernizing/rejuvenating/upgrading.
Importantly, the rules support gradual adoption: It is typically infeasible to completely convert a large code base all at once.

These guidelines are not meant to be complete or exact in every language-technical detail.
For the final word on language definition issues, including every exception to general rules and every feature, see the ISO C++ standard.

The rules are not intended to force you to write in an impoverished subset of C++.
They are emphatically not meant to define a, say, Java-like subset of C++.
They are not meant to define a single “one true C++” language.
We value expressiveness and uncompromised performance.

The rules are not value-neutral.
They are meant to make code simpler and more correct/safer than most existing C++ code, without loss of performance.
They are meant to inhibit perfectly valid C++ code that correlates with errors, spurious complexity, and poor performance.

The rules are not precise to the point where a person (or machine) can follow them without thinking.
The enforcement parts try to be that, but we would rather leave a rule or a definition a bit vague
and open to interpretation than specify something precisely and wrong.
Sometimes, precision comes only with time and experience.
Design is not (yet) a form of Math.

The rules are not perfect.
A rule can do harm by prohibiting something that is useful in a given situation.
A rule can do harm by failing to prohibit something that enables a serious error in a given situation.
A rule can do a lot of harm by being vague, ambiguous, unenforceable, or by enabling every solution to a problem.
It is impossible to completely meet the “do no harm” criteria.
Instead, our aim is the less ambitious: “Do the most good for most programmers”;
if you cannot live with a rule, object to it, ignore it, but don’t water it down until it becomes meaningless.
Also, suggest an improvement.

In.force: Enforcement

Rules with no enforcement are unmanageable for large code bases.
Enforcement of all rules is possible only for a small weak set of rules or for a specific user community.

• But we want lots of rules, and we want rules that everybody can use.
• But different people have different needs.
• But people don’t like to read lots of rules.
• But people can’t remember many rules.

So, we need subsetting to meet a variety of needs.

• But arbitrary subsetting leads to chaos.

We want guidelines that help a lot of people, make code more uniform, and strongly encourage people to modernize their code.
We want to encourage best practices, rather than leave all to individual choices and management pressures.
The ideal is to use all rules; that gives the greatest benefits.

This adds up to quite a few dilemmas.
We try to resolve those using tools.
Each rule has an Enforcement section listing ideas for enforcement.
Enforcement might be done by code review, by static analysis, by compiler, or by run-time checks.
Wherever possible, we prefer “mechanical” checking (humans are slow, inaccurate, and bore easily) and static checking.
Run-time checks are suggested only rarely where no alternative exists; we do not want to introduce “distributed bloat”.
Where appropriate, we label a rule (in the Enforcement sections) with the name of groups of related rules (called “profiles”).
A rule can be part of several profiles, or none.
For a start, we have a few profiles corresponding to common needs (desires, ideals):

• type: No type violations (reinterpreting a T as a U through casts, unions, or varargs)
• bounds: No bounds violations (accessing beyond the range of an array)
• lifetime: No leaks (failing to delete or multiple delete) and no access to invalid objects (dereferencing nullptr, using a dangling reference).

The profiles are intended to be used by tools, but also serve as an aid to the human reader.
We do not limit our comment in the Enforcement sections to things we know how to enforce; some comments are mere wishes that might inspire some tool builder.

Tools that implement these rules shall respect the following syntax to explicitly suppress a rule:

[[gsl::suppress(tag)]]

and optionally with a message (following usual C++11 standard attribute syntax):

[[gsl::suppress(tag, justification: "message")]]

where

• tag is the anchor name of the item where the Enforcement rule appears (e.g., for C.134 it is “Rh-public”), the
  name of a profile group-of-rules (“type”, “bounds”, or “lifetime”),
  or a specific rule in a profile (type.4, or bounds.2)

• "message" is a string literal

In.struct: The structure of this document

Each rule (guideline, suggestion) can have several parts:

• The rule itself – e.g., no naked new
• A rule reference number – e.g., C.7 (the 7th rule related to classes).
  Since the major sections are not inherently ordered, we use letters as the first part of a rule reference “number”.
  We leave gaps in the numbering to minimize “disruption” when we add or remove rules.
• Reasons (rationales) – because programmers find it hard to follow rules they don’t understand
• Examples – because rules are hard to understand in the abstract; can be positive or negative
• Alternatives – for “don’t do this” rules
• Exceptions – we prefer simple general rules. However, many rules apply widely, but not universally, so exceptions must be listed
• Enforcement – ideas about how the rule might be checked “mechanically”
• See alsos – references to related rules and/or further discussion (in this document or elsewhere)
• Notes (comments) – something that needs saying that doesn’t fit the other classifications
• Discussion – references to more extensive rationale and/or examples placed outside the main lists of rules

Some rules are hard to check mechanically, but they all meet the minimal criteria that an expert programmer can spot many violations without too much trouble.
We hope that “mechanical” tools will improve with time to approximate what such an expert programmer notices.
Also, we assume that the rules will be refined over time to make them more precise and checkable.

A rule is aimed at being simple, rather than carefully phrased to mention every alternative and special case.
Such information is found in the Alternative paragraphs and the Discussion sections.
If you don’t understand a rule or disagree with it, please visit its Discussion.
If you feel that a discussion is missing or incomplete, enter an Issue
explaining your concerns and possibly a corresponding PR.

Examples are written to illustrate rules.

• Examples are not intended to be production quality or to cover all tutorial dimensions.
  For example, many examples are language-technical and use names like f, base, and x.
• We try to ensure that “good” examples follow the Core Guidelines.
• Comments are often illustrating rules where they would be unnecessary and/or distracting in “real code.”
• We assume knowledge of the standard library. For example, we use plain vector rather than std::vector.

This is not a language manual.
It is meant to be helpful, rather than complete, fully accurate on technical details, or a guide to existing code.
Recommended information sources can be found in the references.

In.sec: Major sections

• In: Introduction
• P: Philosophy
• I: Interfaces
• F: Functions
• C: Classes and class hierarchies
• Enum: Enumerations
• R: Resource management
• ES: Expressions and statements
• Per: Performance
• CP: Concurrency and parallelism
• E: Error handling
• Con: Constants and immutability
• T: Templates and generic programming
• CPL: C-style programming
• SF: Source files
• SL: The Standard Library

Supporting sections:

• A: Architectural ideas
• NR: Non-Rules and myths
• RF: References
• Pro: Profiles
• GSL: Guidelines support library
• NL: Naming and layout suggestions
• FAQ: Answers to frequently asked questions
• Appendix A: Libraries
• Appendix B: Modernizing code
• Appendix C: Discussion
• Appendix D: Supporting tools
• Glossary
• To-do: Unclassified proto-rules

These sections are not orthogonal.

Each section (e.g., “P” for “Philosophy”) and each subsection (e.g., “C.hier” for “Class Hierarchies (OOP)”) have an abbreviation for ease of searching and reference.
The main section abbreviations are also used in rule numbers (e.g., “C.11” for “Make concrete types regular”).

P: Philosophy 哲学

The rules in this section are very general.

Philosophy rules summary:

• P.1: Express ideas directly in code
• P.2: Write in ISO Standard C++
• P.3: Express intent
• P.4: Ideally, a program should be statically type safe
• P.5: Prefer compile-time checking to run-time checking
• P.6: What cannot be checked at compile time should be checkable at run time
• P.7: Catch run-time errors early
• P.8: Don’t leak any resources
• P.9: Don’t waste time or space
• P.10: Prefer immutable data to mutable data
• P.11: Encapsulate messy constructs, rather than spreading through the code
• P.12: Use supporting tools as appropriate
• P.13: Use support libraries as appropriate

Philosophical rules are generally not mechanically checkable.
However, individual rules reflecting these philosophical themes are.
Without a philosophical basis, the more concrete/specific/checkable rules lack rationale.

P.1: Express ideas directly in code

Reason

Compilers don’t read comments (or design documents) and neither do many programmers (consistently).
What is expressed in code has defined semantics and can (in principle) be checked by compilers and other tools.

Example

class Date {
public:
    Month month() const;  // do
    int month();          // don't
    // ...
};

The first declaration of month is explicit about returning a Month and about not modifying the state of the Date object.
The second version leaves the reader guessing and opens more possibilities for uncaught bugs.

Example, bad

This loop is a restricted form of std::find:

void f(vector& v)
{
    string val;
    cin >> val;
    // ...
    int index = -1;                    // bad, plus should use gsl::index
    for (int i = 0; i < v.size(); ++i) {
        if (v[i] == val) {
            index = i;
            break;
        }
    }
    // ...
}

Example, good

A much clearer expression of intent would be:

void f(vector& v)
{
    string val;
    cin >> val;
    // ...
    auto p = find(begin(v), end(v), val);  // better
    // ...
}

A well-designed library expresses intent (what is to be done, rather than just how something is being done) far better than direct use of language features.

A C++ programmer should know the basics of the standard library, and use it where appropriate.
Any programmer should know the basics of the foundation libraries of the project being worked on, and use them appropriately.
Any programmer using these guidelines should know the guidelines support library, and use it appropriately.

Example

change_speed(double s);   // bad: what does s signify?
// ...
change_speed(2.3);

A better approach is to be explicit about the meaning of the double (new speed or delta on old speed?) and the unit used:

change_speed(Speed s);    // better: the meaning of s is specified
// ...
change_speed(2.3);        // error: no unit
change_speed(23_m / 10s);  // meters per second

We could have accepted a plain (unit-less) double as a delta, but that would have been error-prone.
If we wanted both absolute speed and deltas, we would have defined a Delta type.

Enforcement

Very hard in general.

• use const consistently (check if member functions modify their object; check if functions modify arguments passed by pointer or reference)
• flag uses of casts (casts neuter the type system)
• detect code that mimics the standard library (hard)

P.2: Write in ISO Standard C++

Reason

This is a set of guidelines for writing ISO Standard C++.

Note

There are environments where extensions are necessary, e.g., to access system resources.
In such cases, localize the use of necessary extensions and control their use with non-core Coding Guidelines. If possible, build interfaces that encapsulate the extensions so they can be turned off or compiled away on systems that do not support those extensions.

Extensions often do not have rigorously defined semantics. Even extensions that
are common and implemented by multiple compilers might have slightly different
behaviors and edge case behavior as a direct result of not having a rigorous
standard definition. With sufficient use of any such extension, expected
portability will be impacted.

Note

Using valid ISO C++ does not guarantee portability (let alone correctness).
Avoid dependence on undefined behavior (e.g., undefined order of evaluation)
and be aware of constructs with implementation defined meaning (e.g., sizeof(int)).

Note

There are environments where restrictions on use of standard C++ language or library features are necessary, e.g., to avoid dynamic memory allocation as required by aircraft control software standards.
In such cases, control their (dis)use with an extension of these Coding Guidelines customized to the specific environment.

Enforcement

Use an up-to-date C++ compiler (currently C++20 or C++17) with a set of options that do not accept extensions.

P.3: Express intent

Reason

Unless the intent of some code is stated (e.g., in names or comments), it is impossible to tell whether the code does what it is supposed to do.

Example

gsl::index i = 0;
while (i < v.size()) {
    // ... do something with v[i] ...
}

The intent of “just” looping over the elements of v is not expressed here. The implementation detail of an index is exposed (so that it might be misused), and i outlives the scope of the loop, which might or might not be intended. The reader cannot know from just this section of code.

Better:

for (const auto& x : v) { /* do something with the value of x */ }

Now, there is no explicit mention of the iteration mechanism, and the loop operates on a reference to const elements so that accidental modification cannot happen. If modification is desired, say so:

for (auto& x : v) { /* modify x */ }

For more details about for-statements, see ES.71.
Sometimes better still, use a named algorithm. This example uses the for_each from the Ranges TS because it directly expresses the intent:

for_each(v, [](int x) { /* do something with the value of x */ });
for_each(par, v, [](int x) { /* do something with the value of x */ });

The last variant makes it clear that we are not interested in the order in which the elements of v are handled.

A programmer should be familiar with

• The guidelines support library
• The ISO C++ Standard Library
• Whatever foundation libraries are used for the current project(s)

Note

Alternative formulation: Say what should be done, rather than just how it should be done.

Note

Some language constructs express intent better than others.

Example

If two ints are meant to be the coordinates of a 2D point, say so:

draw_line(int, int, int, int);  // obscure: (x1,y1,x2,y2)? (x,y,h,w)? ...?
                                // need to look up documentation to know

draw_line(Point, Point);        // clearer

Enforcement

Look for common patterns for which there are better alternatives

• simple for loops vs. range-for loops
• f(T*, int) interfaces vs. f(span) interfaces
• loop variables in too large a scope
• naked new and delete
• functions with many parameters of built-in types

There is a huge scope for cleverness and semi-automated program transformation.

P.4: Ideally, a program should be statically type safe

Reason

Ideally, a program would be completely statically (compile-time) type safe.
Unfortunately, that is not possible. Problem areas:

• unions
• casts
• array decay
• range errors
• narrowing conversions

Note

These areas are sources of serious problems (e.g., crashes and security violations).
We try to provide alternative techniques.

Enforcement

We can ban, restrain, or detect the individual problem categories separately, as required and feasible for individual programs.
Always suggest an alternative.
For example:

• unions – use variant (in C++17)
• casts – minimize their use; templates can help
• array decay – use span (from the GSL)
• range errors – use span
• narrowing conversions – minimize their use and use narrow or narrow_cast (from the GSL) where they are necessary

P.5: Prefer compile-time checking to run-time checking

Reason

Code clarity and performance.
You don’t need to write error handlers for errors caught at compile time.

Example

// Int is an alias used for integers
int bits = 0;         // don't: avoidable code
for (Int i = 1; i; i <<= 1)
    ++bits;
if (bits < 32)
    cerr << "Int too small\n";

This example fails to achieve what it is trying to achieve (because overflow is undefined) and should be replaced with a simple static_assert:

// Int is an alias used for integers
static_assert(sizeof(Int) >= 4);    // do: compile-time check

Or better still just use the type system and replace Int with int32_t.

Example

void read(int* p, int n);   // read max n integers into *p

int a[100];
read(a, 1000);    // bad, off the end

better

void read(span r); // read into the range of integers r

int a[100];
read(a);        // better: let the compiler figure out the number of elements

Alternative formulation: Don’t postpone to run time what can be done well at compile time.

Enforcement

• Look for pointer arguments.
• Look for run-time checks for range violations.

P.6: What cannot be checked at compile time should be checkable at run time

Reason

Leaving hard-to-detect errors in a program is asking for crashes and bad results.

Note

Ideally, we catch all errors (that are not errors in the programmer’s logic) at either compile time or run time. It is impossible to catch all errors at compile time and often not affordable to catch all remaining errors at run time. However, we should endeavor to write programs that in principle can be checked, given sufficient resources (analysis programs, run-time checks, machine resources, time).

Example, bad

// separately compiled, possibly dynamically loaded
extern void f(int* p);

void g(int n)
{
    // bad: the number of elements is not passed to f()
    f(new int[n]);
}

Here, a crucial bit of information (the number of elements) has been so thoroughly “obscured” that static analysis is probably rendered infeasible and dynamic checking can be very difficult when f() is part of an ABI so that we cannot “instrument” that pointer. We could embed helpful information into the free store, but that requires global changes to a system and maybe to the compiler. What we have here is a design that makes error detection very hard.

Example, bad

We can of course pass the number of elements along with the pointer:

// separately compiled, possibly dynamically loaded
extern void f2(int* p, int n);

void g2(int n)
{
    f2(new int[n], m);  // bad: a wrong number of elements can be passed to f()
}

Passing the number of elements as an argument is better (and far more common) than just passing the pointer and relying on some (unstated) convention for knowing or discovering the number of elements. However (as shown), a simple typo can introduce a serious error. The connection between the two arguments of f2() is conventional, rather than explicit.

Also, it is implicit that f2() is supposed to delete its argument (or did the caller make a second mistake?).

Example, bad

The standard library resource management pointers fail to pass the size when they point to an object:

// separately compiled, possibly dynamically loaded
// NB: this assumes the calling code is ABI-compatible, using a
// compatible C++ compiler and the same stdlib implementation
extern void f3(unique_ptr, int n);

void g3(int n)
{
    f3(make_unique(n), m);    // bad: pass ownership and size separately
}

Example

We need to pass the pointer and the number of elements as an integral object:

extern void f4(vector&);   // separately compiled, possibly dynamically loaded
extern void f4(span);      // separately compiled, possibly dynamically loaded
                                // NB: this assumes the calling code is ABI-compatible, using a
                                // compatible C++ compiler and the same stdlib implementation

void g3(int n)
{
    vector v(n);
    f4(v);                     // pass a reference, retain ownership
    f4(span{v});          // pass a view, retain ownership
}

This design carries the number of elements along as an integral part of an object, so that errors are unlikely and dynamic (run-time) checking is always feasible, if not always affordable.

Example

How do we transfer both ownership and all information needed for validating use?

vector f5(int n)    // OK: move
{
    vector v(n);
    // ... initialize v ...
    return v;
}

unique_ptr f6(int n)    // bad: loses n
{
    auto p = make_unique(n);
    // ... initialize *p ...
    return p;
}

owner f7(int n)    // bad: loses n and we might forget to delete
{
    owner p = new int[n];
    // ... initialize *p ...
    return p;
}

Example

• ???
• show how possible checks are avoided by interfaces that pass polymorphic base classes around, when they actually know what they need?
  Or strings as “free-style” options

Enforcement

• Flag (pointer, count)-style interfaces (this will flag a lot of examples that can’t be fixed for compatibility reasons)
• ???

P.7: Catch run-time errors early

Reason

Avoid “mysterious” crashes.
Avoid errors leading to (possibly unrecognized) wrong results.

Example

void increment1(int* p, int n)    // bad: error-prone
{
    for (int i = 0; i < n; ++i) ++p[i];
}

void use1(int m)
{
    const int n = 10;
    int a[n] = {};
    // ...
    increment1(a, m);   // maybe typo, maybe m <= n is supposed
                        // but assume that m == 20
    // ...
}

Here we made a small error in use1 that will lead to corrupted data or a crash.
The (pointer, count)-style interface leaves increment1() with no realistic way of defending itself against out-of-range errors.
If we could check subscripts for out of range access, then the error would not be discovered until p[10] was accessed.
We could check earlier and improve the code:

void increment2(span p)
{
    for (int& x : p) ++x;
}

void use2(int m)
{
    const int n = 10;
    int a[n] = {};
    // ...
    increment2({a, m});    // maybe typo, maybe m <= n is supposed
    // ...
}

Now, m <= n can be checked at the point of call (early) rather than later.
If all we had was a typo so that we meant to use n as the bound, the code could be further simplified (eliminating the possibility of an error):

void use3(int m)
{
    const int n = 10;
    int a[n] = {};
    // ...
    increment2(a);   // the number of elements of a need not be repeated
    // ...
}

Example, bad

Don’t repeatedly check the same value. Don’t pass structured data as strings:

Date read_date(istream& is);    // read date from istream

Date extract_date(const string& s);    // extract date from string

void user1(const string& date)    // manipulate date
{
    auto d = extract_date(date);
    // ...
}

void user2()
{
    Date d = read_date(cin);
    // ...
    user1(d.to_string());
    // ...
}

The date is validated twice (by the Date constructor) and passed as a character string (unstructured data).

Example

Excess checking can be costly.
There are cases where checking early is inefficient because you might never need the value, or might only need part of the value that is more easily checked than the whole. Similarly, don’t add validity checks that change the asymptotic behavior of your interface (e.g., don’t add a O(n) check to an interface with an average complexity of O(1)).

class Jet {    // Physics says: e * e < x * x + y * y + z * z
    float x;
    float y;
    float z;
    float e;
public:
    Jet(float x, float y, float z, float e)
        :x(x), y(y), z(z), e(e)
    {
        // Should I check here that the values are physically meaningful?
    }

    float m() const
    {
        // Should I handle the degenerate case here?
        return sqrt(x * x + y * y + z * z - e * e);
    }

    ???
};

The physical law for a jet (e * e < x * x + y * y + z * z) is not an invariant because of the possibility for measurement errors.

???

Enforcement

• Look at pointers and arrays: Do range-checking early and not repeatedly
• Look at conversions: Eliminate or mark narrowing conversions
• Look for unchecked values coming from input
• Look for structured data (objects of classes with invariants) being converted into strings
• ???

P.8: Don’t leak any resources

Reason

Even a slow growth in resources will, over time, exhaust the availability of those resources.
This is particularly important for long-running programs, but is an essential piece of responsible programming behavior.

Example, bad

void f(char* name)
{
    FILE* input = fopen(name, "r");
    // ...
    if (something) return;   // bad: if something == true, a file handle is leaked
    // ...
    fclose(input);
}

Prefer RAII:

void f(char* name)
{
    ifstream input {name};
    // ...
    if (something) return;   // OK: no leak
    // ...
}

See also: The resource management section

Note

A leak is colloquially “anything that isn’t cleaned up.”
The more important classification is “anything that can no longer be cleaned up.”
For example, allocating an object on the heap and then losing the last pointer that points to that allocation.
This rule should not be taken as requiring that allocations within long-lived objects must be returned during program shutdown.
For example, relying on system guaranteed cleanup such as file closing and memory deallocation upon process shutdown can simplify code.
However, relying on abstractions that implicitly clean up can be as simple, and often safer.

Note

Enforcing the lifetime safety profile eliminates leaks.
When combined with resource safety provided by RAII, it eliminates the need for “garbage collection” (by generating no garbage).
Combine this with enforcement of the type and bounds profiles and you get complete type- and resource-safety, guaranteed by tools.

Enforcement

• Look at pointers: Classify them into non-owners (the default) and owners.
  Where feasible, replace owners with standard-library resource handles (as in the example above).
  Alternatively, mark an owner as such using owner from the GSL.
• Look for naked new and delete
• Look for known resource allocating functions returning raw pointers (such as fopen, malloc, and strdup)

P.9: Don’t waste time or space

Reason

This is C++.

Note

Time and space that you spend well to achieve a goal (e.g., speed of development, resource safety, or simplification of testing) is not wasted.
“Another benefit of striving for efficiency is that the process forces you to understand the problem in more depth.” - Alex Stepanov

Example, bad

struct X {
    char ch;
    int i;
    string s;
    char ch2;

    X& operator=(const X& a);
    X(const X&);
};

X waste(const char* p)
{
    if (!p) throw Nullptr_error{};
    int n = strlen(p);
    auto buf = new char[n];
    if (!buf) throw Allocation_error{};
    for (int i = 0; i < n; ++i) buf[i] = p[i];
    // ... manipulate buffer ...
    X x;
    x.ch = 'a';
    x.s = string(n);    // give x.s space for *p
    for (gsl::index i = 0; i < x.s.size(); ++i) x.s[i] = buf[i];  // copy buf into x.s
    delete[] buf;
    return x;
}

void driver()
{
    X x = waste("Typical argument");
    // ...
}

Yes, this is a caricature, but we have seen every individual mistake in production code, and worse.
Note that the layout of X guarantees that at least 6 bytes (and most likely more) are wasted.
The spurious definition of copy operations disables move semantics so that the return operation is slow
(please note that the Return Value Optimization, RVO, is not guaranteed here).
The use of new and delete for buf is redundant; if we really needed a local string, we should use a local string.
There are several more performance bugs and gratuitous complication.

Example, bad

void lower(zstring s)
{
    for (int i = 0; i < strlen(s); ++i) s[i] = tolower(s[i]);
}

This is actually an example from production code.
We can see that in our condition we have i < strlen(s). This expression will be evaluated on every iteration of the loop, which means that strlen must walk through string every loop to discover its length. While the string contents are changing, it’s assumed that tolower will not affect the length of the string, so it’s better to cache the length outside the loop and not incur that cost each iteration.

Note

An individual example of waste is rarely significant, and where it is significant, it is typically easily eliminated by an expert.
However, waste spread liberally across a code base can easily be significant and experts are not always as available as we would like.
The aim of this rule (and the more specific rules that support it) is to eliminate most waste related to the use of C++ before it happens.
After that, we can look at waste related to algorithms and requirements, but that is beyond the scope of these guidelines.

Enforcement

Many more specific rules aim at the overall goals of simplicity and elimination of gratuitous waste.

• Flag an unused return value from a user-defined non-defaulted postfix operator++ or operator-- function. Prefer using the prefix form instead. (Note: “User-defined non-defaulted” is intended to reduce noise. Review this enforcement if it’s still too noisy in practice.)

P.10: Prefer immutable data to mutable data

Reason

It is easier to reason about constants than about variables.
Something immutable cannot change unexpectedly.
Sometimes immutability enables better optimization.
You can’t have a data race on a constant.

See Con: Constants and immutability

P.11: Encapsulate messy constructs, rather than spreading through the code

Reason

Messy code is more likely to hide bugs and harder to write.
A good interface is easier and safer to use.
Messy, low-level code breeds more such code.

Example

int sz = 100;
int* p = (int*) malloc(sizeof(int) * sz);
int count = 0;
// ...
for (;;) {
    // ... read an int into x, exit loop if end of file is reached ...
    // ... check that x is valid ...
    if (count == sz)
        p = (int*) realloc(p, sizeof(int) * sz * 2);
    p[count++] = x;
    // ...
}

This is low-level, verbose, and error-prone.
For example, we “forgot” to test for memory exhaustion.
Instead, we could use vector:

vector v;
v.reserve(100);
// ...
for (int x; cin >> x; ) {
    // ... check that x is valid ...
    v.push_back(x);
}

Note

The standards library and the GSL are examples of this philosophy.
For example, instead of messing with the arrays, unions, cast, tricky lifetime issues, gsl::owner, etc.,
that are needed to implement key abstractions, such as vector, span, lock_guard, and future, we use the libraries
designed and implemented by people with more time and expertise than we usually have.
Similarly, we can and should design and implement more specialized libraries, rather than leaving the users (often ourselves)
with the challenge of repeatedly getting low-level code well.
This is a variant of the subset of superset principle that underlies these guidelines.

Enforcement

• Look for “messy code” such as complex pointer manipulation and casting outside the implementation of abstractions.

P.12: Use supporting tools as appropriate

Reason

There are many things that are done better “by machine”.
Computers don’t tire or get bored by repetitive tasks.
We typically have better things to do than repeatedly do routine tasks.

Example

Run a static analyzer to verify that your code follows the guidelines you want it to follow.

Note

See

• Static analysis tools
• Concurrency tools
• Testing tools

There are many other kinds of tools, such as source code repositories, build tools, etc.,
but those are beyond the scope of these guidelines.

Note

Be careful not to become dependent on over-elaborate or over-specialized tool chains.
Those can make your otherwise portable code non-portable.

P.13: Use support libraries as appropriate

Reason

Using a well-designed, well-documented, and well-supported library saves time and effort;
its quality and documentation are likely to be greater than what you could do
if the majority of your time must be spent on an implementation.
The cost (time, effort, money, etc.) of a library can be shared over many users.
A widely used library is more likely to be kept up-to-date and ported to new systems than an individual application.
Knowledge of a widely-used library can save time on other/future projects.
So, if a suitable library exists for your application domain, use it.

Example

std::sort(begin(v), end(v), std::greater<>());

Unless you are an expert in sorting algorithms and have plenty of time,
this is more likely to be correct and to run faster than anything you write for a specific application.
You need a reason not to use the standard library (or whatever foundational libraries your application uses) rather than a reason to use it.

Note

By default use

• The ISO C++ Standard Library
• The Guidelines Support Library

Note

If no well-designed, well-documented, and well-supported library exists for an important domain,
maybe you should design and implement it, and then use it.

I: Interfaces 接口

An interface is a contract between two parts of a program. Precisely stating what is expected of a supplier of a service and a user of that service is essential.
Having good (easy-to-understand, encouraging efficient use, not error-prone, supporting testing, etc.) interfaces is probably the most important single aspect of code organization.

Interface rule summary:

• I.1: Make interfaces explicit
• I.2: Avoid non-const global variables
• I.3: Avoid singletons
• I.4: Make interfaces precisely and strongly typed
• I.5: State preconditions (if any)
• I.6: Prefer Expects() for expressing preconditions
• I.7: State postconditions
• I.8: Prefer Ensures() for expressing postconditions
• I.9: If an interface is a template, document its parameters using concepts
• I.10: Use exceptions to signal a failure to perform a required task
• I.11: Never transfer ownership by a raw pointer (T*) or reference (T&)
• I.12: Declare a pointer that must not be null as not_null
• I.13: Do not pass an array as a single pointer
• I.22: Avoid complex initialization of global objects
• I.23: Keep the number of function arguments low
• I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning
• I.25: Prefer empty abstract classes as interfaces to class hierarchies
• I.26: If you want a cross-compiler ABI, use a C-style subset
• I.27: For stable library ABI, consider the Pimpl idiom
• I.30: Encapsulate rule violations

See also:

• F: Functions
• C.concrete: Concrete types
• C.hier: Class hierarchies
• C.over: Overloading and overloaded operators
• C.con: Containers and other resource handles
• E: Error handling
• T: Templates and generic programming

I.1: Make interfaces explicit

Reason

Correctness. Assumptions not stated in an interface are easily overlooked and hard to test.

Example, bad

Controlling the behavior of a function through a global (namespace scope) variable (a call mode) is implicit and potentially confusing. For example:

int round(double d)
{
    return (round_up) ? ceil(d) : d;    // don't: "invisible" dependency
}

It will not be obvious to a caller that the meaning of two calls of round(7.2) might give different results.

Exception

Sometimes we control the details of a set of operations by an environment variable, e.g., normal vs. verbose output or debug vs. optimized.
The use of a non-local control is potentially confusing, but controls only implementation details of otherwise fixed semantics.

Example, bad

Reporting through non-local variables (e.g., errno) is easily ignored. For example:

// don't: no test of fprintf's return value
fprintf(connection, "logging: %d %d %d\n", x, y, s);

What if the connection goes down so that no logging output is produced? See I.???.

Alternative: Throw an exception. An exception cannot be ignored.

Alternative formulation: Avoid passing information across an interface through non-local or implicit state.
Note that non-const member functions pass information to other member functions through their object’s state.

Alternative formulation: An interface should be a function or a set of functions.
Functions can be function templates and sets of functions can be classes or class templates.

Enforcement

• (Simple) A function should not make control-flow decisions based on the values of variables declared at namespace scope.
• (Simple) A function should not write to variables declared at namespace scope.

I.2: Avoid non-const global variables

Reason

Non-const global variables hide dependencies and make the dependencies subject to unpredictable changes.

Example

struct Data {
    // ... lots of stuff ...
} data;            // non-const data

void compute()     // don't
{
    // ... use data ...
}

void output()     // don't
{
    // ... use data ...
}

Who else might modify data?

Warning: The initialization of global objects is not totally ordered.
If you use a global object initialize it with a constant.
Note that it is possible to get undefined initialization order even for const objects.

Exception

A global object is often better than a singleton.

Note

Global constants are useful.

Note

The rule against global variables applies to namespace scope variables as well.

Alternative: If you use global (more generally namespace scope) data to avoid copying, consider passing the data as an object by reference to const.
Another solution is to define the data as the state of some object and the operations as member functions.

Warning: Beware of data races: If one thread can access non-local data (or data passed by reference) while another thread executes the callee, we can have a data race.
Every pointer or reference to mutable data is a potential data race.

Using global pointers or references to access and change non-const, and otherwise non-global,
data isn’t a better alternative to non-const global variables since that doesn’t solve the issues of hidden dependencies or potential race conditions.

Note

You cannot have a race condition on immutable data.

References: See the rules for calling functions.

Note

The rule is “avoid”, not “don’t use.” Of course there will be (rare) exceptions, such as cin, cout, and cerr.

Enforcement

(Simple) Report all non-const variables declared at namespace scope and global pointers/references to non-const data.

I.3: Avoid singletons

Reason

Singletons are basically complicated global objects in disguise.

Example

class Singleton {
    // ... lots of stuff to ensure that only one Singleton object is created,
    // that it is initialized properly, etc.
};

There are many variants of the singleton idea.
That’s part of the problem.

Note

If you don’t want a global object to change, declare it const or constexpr.

Exception

You can use the simplest “singleton” (so simple that it is often not considered a singleton) to get initialization on first use, if any:

X& myX()
{
    static X my_x {3};
    return my_x;
}

This is one of the most effective solutions to problems related to initialization order.
In a multi-threaded environment, the initialization of the static object does not introduce a race condition
(unless you carelessly access a shared object from within its constructor).

Note that the initialization of a local static does not imply a race condition.
However, if the destruction of X involves an operation that needs to be synchronized we must use a less simple solution.
For example:

X& myX()
{
    static auto p = new X {3};
    return *p;  // potential leak
}

Now someone must delete that object in some suitably thread-safe way.
That’s error-prone, so we don’t use that technique unless

• myX is in multi-threaded code,
• that X object needs to be destroyed (e.g., because it releases a resource), and
• X’s destructor’s code needs to be synchronized.

If you, as many do, define a singleton as a class for which only one object is created, functions like myX are not singletons, and this useful technique is not an exception to the no-singleton rule.

Enforcement

Very hard in general.

• Look for classes with names that include singleton.
• Look for classes for which only a single object is created (by counting objects or by examining constructors).
• If a class X has a public static function that contains a function-local static of the class’ type X and returns a pointer or reference to it, ban that.

I.4: Make interfaces precisely and strongly typed

Reason

Types are the simplest and best documentation, improve legibility due to their well-defined meaning, and are checked at compile time.
Also, precisely typed code is often optimized better.

Example, don’t

Consider:

void pass(void* data);    // weak and under qualified type void* is suspicious

Callers are unsure what types are allowed and if the data may
be mutated as const is not specified. Note all pointer types
implicitly convert to void*, so it is easy for callers to provide this value.

The callee must static_cast data to an unverified type to use it.
That is error-prone and verbose.

Only use const void* for passing in data in designs that are indescribable in C++. Consider using a variant or a pointer to base instead.

Alternative: Often, a template parameter can eliminate the void* turning it into a T* or T&.
For generic code these Ts can be general or concept constrained template parameters.

Example, bad

Consider:

draw_rect(100, 200, 100, 500); // what do the numbers specify?

draw_rect(p.x, p.y, 10, 20); // what units are 10 and 20 in?

It is clear that the caller is describing a rectangle, but it is unclear what parts they relate to. Also, an int can carry arbitrary forms of information, including values of many units, so we must guess about the meaning of the four ints. Most likely, the first two are an x,y coordinate pair, but what are the last two?

Comments and parameter names can help, but we could be explicit:

void draw_rectangle(Point top_left, Point bottom_right);
void draw_rectangle(Point top_left, Size height_width);

draw_rectangle(p, Point{10, 20});  // two corners
draw_rectangle(p, Size{10, 20});   // one corner and a (height, width) pair

Obviously, we cannot catch all errors through the static type system
(e.g., the fact that a first argument is supposed to be a top-left point is left to convention (naming and comments)).

Example, bad

Consider:

set_settings(true, false, 42); // what do the numbers specify?

The parameter types and their values do not communicate what settings are being specified or what those values mean.

This design is more explicit, safe and legible:

alarm_settings s{};
s.enabled = true;
s.displayMode = alarm_settings::mode::spinning_light;
s.frequency = alarm_settings::every_10_seconds;
set_settings(s);

For the case of a set of boolean values consider using a flags enum; a pattern that expresses a set of boolean values.

enable_lamp_options(lamp_option::on | lamp_option::animate_state_transitions);

Example, bad

In the following example, it is not clear from the interface what time_to_blink means: Seconds? Milliseconds?

void blink_led(int time_to_blink) // bad -- the unit is ambiguous
{
    // ...
    // do something with time_to_blink
    // ...
}

void use()
{
    blink_led(2);
}

Example, good

std::chrono::duration types helps making the unit of time duration explicit.

void blink_led(milliseconds time_to_blink) // good -- the unit is explicit
{
    // ...
    // do something with time_to_blink
    // ...
}

void use()
{
    blink_led(1500ms);
}

The function can also be written in such a way that it will accept any time duration unit.

template
void blink_led(duration time_to_blink) // good -- accepts any unit
{
    // assuming that millisecond is the smallest relevant unit
    auto milliseconds_to_blink = duration_cast(time_to_blink);
    // ...
    // do something with milliseconds_to_blink
    // ...
}

void use()
{
    blink_led(2s);
    blink_led(1500ms);
}

Enforcement

• (Simple) Report the use of void* as a parameter or return type.
• (Simple) Report the use of more than one bool parameter.
• (Hard to do well) Look for functions that use too many primitive type arguments.

I.5: State preconditions (if any)

Reason

Arguments have meaning that might constrain their proper use in the callee.

Example

Consider:

double sqrt(double x);

Here x must be non-negative. The type system cannot (easily and naturally) express that, so we must use other means. For example:

double sqrt(double x); // x must be non-negative

Some preconditions can be expressed as assertions. For example:

double sqrt(double x) { Expects(x >= 0); /* ... */ }

Ideally, that Expects(x >= 0) should be part of the interface of sqrt() but that’s not easily done. For now, we place it in the definition (function body).

References: Expects() is described in GSL.

Note

Prefer a formal specification of requirements, such as Expects(p);.
If that is infeasible, use English text in comments, such as // the sequence [p:q) is ordered using <.

Note

Most member functions have as a precondition that some class invariant holds.
That invariant is established by a constructor and must be reestablished upon exit by every member function called from outside the class.
We don’t need to mention it for each member function.

Enforcement

(Not enforceable)

See also: The rules for passing pointers. ???

I.6: Prefer Expects() for expressing preconditions

Reason

To make it clear that the condition is a precondition and to enable tool use.

Example

int area(int height, int width)
{
    Expects(height > 0 && width > 0);            // good
    if (height <= 0 || width <= 0) my_error();   // obscure
    // ...
}

Note

Preconditions can be stated in many ways, including comments, if-statements, and assert().
This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and might have the wrong semantics (do you always want to abort in debug mode and check nothing in productions runs?).

Note

Preconditions should be part of the interface rather than part of the implementation,
but we don’t yet have the language facilities to do that.
Once language support becomes available (e.g., see the contract proposal) we will adopt the standard version of preconditions, postconditions, and assertions.

Note

Expects() can also be used to check a condition in the middle of an algorithm.

Note

No, using unsigned is not a good way to sidestep the problem of ensuring that a value is non-negative.

Enforcement

(Not enforceable) Finding the variety of ways preconditions can be asserted is not feasible. Warning about those that can be easily identified (assert()) has questionable value in the absence of a language facility.

I.7: State postconditions

Reason

To detect misunderstandings about the result and possibly catch erroneous implementations.

Example, bad

Consider:

int area(int height, int width) { return height * width; }  // bad

Here, we (incautiously) left out the precondition specification, so it is not explicit that height and width must be positive.
We also left out the postcondition specification, so it is not obvious that the algorithm (height * width) is wrong for areas larger than the largest integer.
Overflow can happen.
Consider using:

int area(int height, int width)
{
    auto res = height * width;
    Ensures(res > 0);
    return res;
}

Example, bad

Consider a famous security bug:

void f()    // problematic
{
    char buffer[MAX];
    // ...
    memset(buffer, 0, sizeof(buffer));
}

There was no postcondition stating that the buffer should be cleared and the optimizer eliminated the apparently redundant memset() call:

void f()    // better
{
    char buffer[MAX];
    // ...
    memset(buffer, 0, sizeof(buffer));
    Ensures(buffer[0] == 0);
}

Note

Postconditions are often informally stated in a comment that states the purpose of a function; Ensures() can be used to make this more systematic, visible, and checkable.

Note

Postconditions are especially important when they relate to something that is not directly reflected in a returned result, such as a state of a data structure used.

Example

Consider a function that manipulates a Record, using a mutex to avoid race conditions:

mutex m;

void manipulate(Record& r)    // don't
{
    m.lock();
    // ... no m.unlock() ...
}

Here, we “forgot” to state that the mutex should be released, so we don’t know if the failure to ensure release of the mutex was a bug or a feature.
Stating the postcondition would have made it clear:

void manipulate(Record& r)    // postcondition: m is unlocked upon exit
{
    m.lock();
    // ... no m.unlock() ...
}

The bug is now obvious (but only to a human reading comments).

Better still, use RAII to ensure that the postcondition (“the lock must be released”) is enforced in code:

void manipulate(Record& r)    // best
{
    lock_guard _ {m};
    // ...
}

Note

Ideally, postconditions are stated in the interface/declaration so that users can easily see them.
Only postconditions related to the users can be stated in the interface.
Postconditions related only to internal state belongs in the definition/implementation.

Enforcement

(Not enforceable) This is a philosophical guideline that is infeasible to check
directly in the general case. Domain specific checkers (like lock-holding
checkers) exist for many toolchains.

I.8: Prefer Ensures() for expressing postconditions

Reason

To make it clear that the condition is a postcondition and to enable tool use.

Example

void f()
{
    char buffer[MAX];
    // ...
    memset(buffer, 0, MAX);
    Ensures(buffer[0] == 0);
}

Note

Postconditions can be stated in many ways, including comments, if-statements, and assert().
This can make them hard to distinguish from ordinary code, hard to update, hard to manipulate by tools, and might have the wrong semantics.

Alternative: Postconditions of the form “this resource must be released” are best expressed by RAII.

Note

Ideally, that Ensures should be part of the interface, but that’s not easily done.
For now, we place it in the definition (function body).
Once language support becomes available (e.g., see the contract proposal) we will adopt the standard version of preconditions, postconditions, and assertions.

Enforcement

(Not enforceable) Finding the variety of ways postconditions can be asserted is not feasible. Warning about those that can be easily identified (assert()) has questionable value in the absence of a language facility.

I.9: If an interface is a template, document its parameters using concepts

Reason

Make the interface precisely specified and compile-time checkable in the (not so distant) future.

Example

Use the C++20 style of requirements specification. For example:

template
  requires input_iterator && equality_comparable_with, Val>
Iter find(Iter first, Iter last, Val v)
{
    // ...
}

See also: Generic programming and concepts.

Enforcement

Warn if any non-variadic template parameter is not constrained by a concept (in its declaration or mentioned in a requires clause).

I.10: Use exceptions to signal a failure to perform a required task

Reason

It should not be possible to ignore an error because that could leave the system or a computation in an undefined (or unexpected) state.
This is a major source of errors.

Example

int printf(const char* ...);    // bad: return negative number if output fails

template
// good: throw system_error if unable to start the new thread
explicit thread(F&& f, Args&&... args);

Note

What is an error?

An error means that the function cannot achieve its advertised purpose (including establishing postconditions).
Calling code that ignores an error could lead to wrong results or undefined systems state.
For example, not being able to connect to a remote server is not by itself an error:
the server can refuse a connection for all kinds of reasons, so the natural thing is to return a result that the caller should always check.
However, if failing to make a connection is considered an error, then a failure should throw an exception.

Exception

Many traditional interface functions (e.g., UNIX signal handlers) use error codes (e.g., errno) to report what are really status codes, rather than errors. You don’t have a good alternative to using such, so calling these does not violate the rule.

Alternative

If you can’t use exceptions (e.g., because your code is full of old-style raw-pointer use or because there are hard-real-time constraints), consider using a style that returns a pair of values:

int val;
int error_code;
tie(val, error_code) = do_something();
if (error_code) {
    // ... handle the error or exit ...
}
// ... use val ...

This style unfortunately leads to uninitialized variables.
Since C++17 the “structured bindings” feature can be used to initialize variables directly from the return value:

auto [val, error_code] = do_something();
if (error_code) {
    // ... handle the error or exit ...
}
// ... use val ...

Note

We don’t consider “performance” a valid reason not to use exceptions.

• Often, explicit error checking and handling consume as much time and space as exception handling.
• Often, cleaner code yields better performance with exceptions (simplifying the tracing of paths through the program and their optimization).
• A good rule for performance critical code is to move checking outside the critical part of the code.
• In the longer term, more regular code gets better optimized.
• Always carefully measure before making performance claims.

See also: I.5 and I.7 for reporting precondition and postcondition violations.

Enforcement

• (Not enforceable) This is a philosophical guideline that is infeasible to check directly.
• Look for errno.

I.11: Never transfer ownership by a raw pointer (T*) or reference (T&)

Reason

If there is any doubt whether the caller or the callee owns an object, leaks or premature destruction will occur.

Example

Consider:

X* compute(args)    // don't
{
    X* res = new X{};
    // ...
    return res;
}

Who deletes the returned X? The problem would be harder to spot if compute returned a reference.
Consider returning the result by value (use move semantics if the result is large):

vector compute(args)  // good
{
    vector res(10000);
    // ...
    return res;
}

Alternative: Pass ownership using a “smart pointer”, such as unique_ptr (for exclusive ownership) and shared_ptr (for shared ownership).
However, that is less elegant and often less efficient than returning the object itself,
so use smart pointers only if reference semantics are needed.

Alternative: Sometimes older code can’t be modified because of ABI compatibility requirements or lack of resources.
In that case, mark owning pointers using owner from the guidelines support library:

owner compute(args)    // It is now clear that ownership is transferred
{
    owner res = new X{};
    // ...
    return res;
}

This tells analysis tools that res is an owner.
That is, its value must be deleted or transferred to another owner, as is done here by the return.

owner is used similarly in the implementation of resource handles.

Note

Every object passed as a raw pointer (or iterator) is assumed to be owned by the
caller, so that its lifetime is handled by the caller. Viewed another way:
ownership transferring APIs are relatively rare compared to pointer-passing APIs,
so the default is “no ownership transfer.”

See also: Argument passing, use of smart pointer arguments, and value return.

Enforcement

• (Simple) Warn on delete of a raw pointer that is not an owner. Suggest use of standard-library resource handle or use of owner.
• (Simple) Warn on failure to either reset or explicitly delete an owner pointer on every code path.
• (Simple) Warn if the return value of new or a function call with an owner return value is assigned to a raw pointer or non-owner reference.

I.12: Declare a pointer that must not be null as not_null

Reason

To help avoid dereferencing nullptr errors.
To improve performance by avoiding redundant checks for nullptr.

Example

int length(const char* p);            // it is not clear whether length(nullptr) is valid

length(nullptr);                      // OK?

int length(not_null p);  // better: we can assume that p cannot be nullptr

int length(const char* p);            // we must assume that p can be nullptr

By stating the intent in source, implementers and tools can provide better diagnostics, such as finding some classes of errors through static analysis, and perform optimizations, such as removing branches and null tests.

Note

not_null is defined in the guidelines support library.

Note

The assumption that the pointer to char pointed to a C-style string (a zero-terminated string of characters) was still implicit, and a potential source of confusion and errors. Use czstring in preference to const char*.

// we can assume that p cannot be nullptr
// we can assume that p points to a zero-terminated array of characters
int length(not_null p);

Note: length() is, of course, std::strlen() in disguise.

Enforcement

• (Simple) ((Foundation)) If a function checks a pointer parameter against nullptr before access, on all control-flow paths, then warn it should be declared not_null.
• (Complex) If a function with pointer return value ensures it is not nullptr on all return paths, then warn the return type should be declared not_null.

I.13: Do not pass an array as a single pointer

Reason

(pointer, size)-style interfaces are error-prone. Also, a plain pointer (to array) must rely on some convention to allow the callee to determine the size.

Example

Consider:

void copy_n(const T* p, T* q, int n); // copy from [p:p+n) to [q:q+n)

What if there are fewer than n elements in the array pointed to by q? Then, we overwrite some probably unrelated memory.
What if there are fewer than n elements in the array pointed to by p? Then, we read some probably unrelated memory.
Either is undefined behavior and a potentially very nasty bug.

Alternative

Consider using explicit spans:

void copy(span r, span r2); // copy r to r2

Example, bad

Consider:

void draw(Shape* p, int n);  // poor interface; poor code
Circle arr[10];
// ...
draw(arr, 10);

Passing 10 as the n argument might be a mistake: the most common convention is to assume [0:n) but that is nowhere stated. Worse is that the call of draw() compiled at all: there was an implicit conversion from array to pointer (array decay) and then another implicit conversion from Circle to Shape. There is no way that draw() can safely iterate through that array: it has no way of knowing the size of the elements.

Alternative: Use a support class that ensures that the number of elements is correct and prevents dangerous implicit conversions. For example:

void draw2(span);
Circle arr[10];
// ...
draw2(span(arr));  // deduce the number of elements
draw2(arr);    // deduce the element type and array size

void draw3(span);
draw3(arr);    // error: cannot convert Circle[10] to span

This draw2() passes the same amount of information to draw(), but makes the fact that it is supposed to be a range of Circles explicit. See ???.

Exception

Use zstring and czstring to represent C-style, zero-terminated strings.
But when doing so, use std::string_view or span from the GSL to prevent range errors.

Enforcement

• (Simple) ((Bounds)) Warn for any expression that would rely on implicit conversion of an array type to a pointer type. Allow exception for zstring/czstring pointer types.
• (Simple) ((Bounds)) Warn for any arithmetic operation on an expression of pointer type that results in a value of pointer type. Allow exception for zstring/czstring pointer types.

I.22: Avoid complex initialization of global objects

Reason

Complex initialization can lead to undefined order of execution.

Example

// file1.c

extern const X x;

const Y y = f(x);   // read x; write y

// file2.c

extern const Y y;

const X x = g(y);   // read y; write x

Since x and y are in different translation units the order of calls to f() and g() is undefined;
one will access an uninitialized const.
This shows that the order-of-initialization problem for global (namespace scope) objects is not limited to global variables.

Note

Order of initialization problems become particularly difficult to handle in concurrent code.
It is usually best to avoid global (namespace scope) objects altogether.

Enforcement

• Flag initializers of globals that call non-constexpr functions
• Flag initializers of globals that access extern objects

I.23: Keep the number of function arguments low

Reason

Having many arguments opens opportunities for confusion. Passing lots of arguments is often costly compared to alternatives.

Discussion

The two most common reasons why functions have too many parameters are:

1. Missing an abstraction.
   There is an abstraction missing, so that a compound value is being
   passed as individual elements instead of as a single object that enforces an invariant.
   This not only expands the parameter list, but it leads to errors because the component values
   are no longer protected by an enforced invariant.

2. Violating “one function, one responsibility.”
   The function is trying to do more than one job and should probably be refactored.

Example

The standard-library merge() is at the limit of what we can comfortably handle:

template
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
                     InputIterator2 first2, InputIterator2 last2,
                     OutputIterator result, Compare comp);

Note that this is because of problem 1 above – missing abstraction. Instead of passing a range (abstraction), STL passed iterator pairs (unencapsulated component values).

Here, we have four template arguments and six function arguments.
To simplify the most frequent and simplest uses, the comparison argument can be defaulted to <:

template
OutputIterator merge(InputIterator1 first1, InputIterator1 last1,
                     InputIterator2 first2, InputIterator2 last2,
                     OutputIterator result);

This doesn’t reduce the total complexity, but it reduces the surface complexity presented to many users.
To really reduce the number of arguments, we need to bundle the arguments into higher-level abstractions:

template
OutputIterator merge(InputRange1 r1, InputRange2 r2, OutputIterator result);

Grouping arguments into “bundles” is a general technique to reduce the number of arguments and to increase the opportunities for checking.

Alternatively, we could use a standard library concept to define the notion of three types that must be usable for merging:

template
  requires mergeable
Out merge(In1 r1, In2 r2, Out result);

Example

The safety Profiles recommend replacing

void f(int* some_ints, int some_ints_length);  // BAD: C style, unsafe

with

void f(gsl::span some_ints);              // GOOD: safe, bounds-checked

Here, using an abstraction has safety and robustness benefits, and naturally also reduces the number of parameters.

Note

How many parameters are too many? Try to use fewer than four (4) parameters.
There are functions that are best expressed with four individual parameters, but not many.

Alternative: Use better abstraction: Group arguments into meaningful objects and pass the objects (by value or by reference).

Alternative: Use default arguments or overloads to allow the most common forms of calls to be done with fewer arguments.

Enforcement

• Warn when a function declares two iterators (including pointers) of the same type instead of a range or a view.
• (Not enforceable) This is a philosophical guideline that is infeasible to check directly.

I.24: Avoid adjacent parameters that can be invoked by the same arguments in either order with different meaning

Reason

Adjacent arguments of the same type are easily swapped by mistake.

Example, bad

Consider:

void copy_n(T* p, T* q, int n);  // copy from [p:p + n) to [q:q + n)

This is a nasty variant of a K&R C-style interface. It is easy to reverse the “to” and “from” arguments.

Use const for the “from” argument:

void copy_n(const T* p, T* q, int n);  // copy from [p:p + n) to [q:q + n)

Exception

If the order of the parameters is not important, there is no problem:

int max(int a, int b);

Alternative

Don’t pass arrays as pointers, pass an object representing a range (e.g., a span):

void copy_n(span p, span q);  // copy from p to q

Alternative

Define a struct as the parameter type and name the fields for those parameters accordingly:

struct SystemParams {
    string config_file;
    string output_path;
    seconds timeout;
};
void initialize(SystemParams p);

This tends to make invocations of this clear to future readers, as the parameters
are often filled in by name at the call site.

Note

Only the interface’s designer can adequately address the source of violations of this guideline.

Enforcement strategy

(Simple) Warn if two consecutive parameters share the same type

We are still looking for a less-simple enforcement.

I.25: Prefer empty abstract classes as interfaces to class hierarchies

Reason

Abstract classes that are empty (have no non-static member data) are more likely to be stable than base classes with state.

Example, bad

You just knew that Shape would turn up somewhere

class Shape {  // bad: interface class loaded with data
public:
    Point center() const { return c; }
    virtual void draw() const;
    virtual void rotate(int);
    // ...
private:
    Point c;
    vector outline;
    Color col;
};

This will force every derived class to compute a center – even if that’s non-trivial and the center is never used. Similarly, not every Shape has a Color, and many Shapes are best represented without an outline defined as a sequence of Points. Using an abstract class is better:

class Shape {    // better: Shape is a pure interface
public:
    virtual Point center() const = 0;   // pure virtual functions
    virtual void draw() const = 0;
    virtual void rotate(int) = 0;
    // ...
    // ... no data members ...
    // ...
    virtual ~Shape() = default;
};

Enforcement

(Simple) Warn if a pointer/reference to a class C is assigned to a pointer/reference to a base of C and the base class contains data members.

I.26: If you want a cross-compiler ABI, use a C-style subset

Reason

Different compilers implement different binary layouts for classes, exception handling, function names, and other implementation details.

Exception

Common ABIs are emerging on some platforms freeing you from the more draconian restrictions.

Note

If you use a single compiler, you can use full C++ in interfaces. That might require recompilation after an upgrade to a new compiler version.

Enforcement

(Not enforceable) It is difficult to reliably identify where an interface forms part of an ABI.

I.27: For stable library ABI, consider the Pimpl idiom

Reason

Because private data members participate in class layout and private member functions participate in overload resolution, changes to those
implementation details require recompilation of all users of a class that uses them. A non-polymorphic interface class holding a pointer to
implementation (Pimpl) can isolate the users of a class from changes in its implementation at the cost of an indirection.

Example

interface (widget.h)

class widget {
    class impl;
    std::unique_ptr pimpl;
public:
    void draw(); // public API that will be forwarded to the implementation
    widget(int); // defined in the implementation file
    ~widget();   // defined in the implementation file, where impl is a complete type
    widget(widget&&) noexcept; // defined in the implementation file
    widget(const widget&) = delete;
    widget& operator=(widget&&) noexcept; // defined in the implementation file
    widget& operator=(const widget&) = delete;
};

implementation (widget.cpp)

class widget::impl {
    int n; // private data
public:
    void draw(const widget& w) { /* ... */ }
    impl(int n) : n(n) {}
};
void widget::draw() { pimpl->draw(*this); }
widget::widget(int n) : pimpl{std::make_unique(n)} {}
widget::widget(widget&&) noexcept = default;
widget::~widget() = default;
widget& widget::operator=(widget&&) noexcept = default;

Notes

See GOTW #100 and cppreference for the trade-offs and additional implementation details associated with this idiom.

Enforcement

(Not enforceable) It is difficult to reliably identify where an interface forms part of an ABI.

I.30: Encapsulate rule violations

Reason

To keep code simple and safe.
Sometimes, ugly, unsafe, or error-prone techniques are necessary for logical or performance reasons.
If so, keep them local, rather than “infecting” interfaces so that larger groups of programmers have to be aware of the
subtleties.
Implementation complexity should, if at all possible, not leak through interfaces into user code.

Example

Consider a program that, depending on some form of input (e.g., arguments to main), should consume input
from a file, from the command line, or from standard input.
We might write

bool owned;
owner inp;
switch (source) {
case std_in:        owned = false; inp = &cin;                       break;
case command_line:  owned = true;  inp = new istringstream{argv[2]}; break;
case file:          owned = true;  inp = new ifstream{argv[2]};      break;
}
istream& in = *inp;

This violated the rule against uninitialized variables,
the rule against ignoring ownership,
and the rule against magic constants.
In particular, someone has to remember to somewhere write

if (owned) delete inp;

We could handle this particular example by using unique_ptr with a special deleter that does nothing for cin,
but that’s complicated for novices (who can easily encounter this problem) and the example is an example of a more general
problem where a property that we would like to consider static (here, ownership) needs infrequently be addressed
at run time.
The common, most frequent, and safest examples can be handled statically, so we don’t want to add cost and complexity to those.
But we must also cope with the uncommon, less-safe, and necessarily more expensive cases.
Such examples are discussed in [Str15].

So, we write a class

class Istream { [[gsl::suppress(lifetime)]]
public:
    enum Opt { from_line = 1 };
    Istream() { }
    Istream(zstring p) : owned{true}, inp{new ifstream{p}} {}            // read from file
    Istream(zstring p, Opt) : owned{true}, inp{new istringstream{p}} {}  // read from command line
    ~Istream() { if (owned) delete inp; }
    operator istream&() { return *inp; }
private:
    bool owned = false;
    istream* inp = &cin;
};

Now, the dynamic nature of istream ownership has been encapsulated.
Presumably, a bit of checking for potential errors would be added in real code.

Enforcement

• Hard, it is hard to decide what rule-breaking code is essential
• Flag rule suppression that enable rule-violations to cross interfaces

F: Functions 函数

A function specifies an action or a computation that takes the system from one consistent state to the next. It is the fundamental building block of programs.

It should be possible to name a function meaningfully, to specify the requirements of its argument, and clearly state the relationship between the arguments and the result. An implementation is not a specification. Try to think about what a function does as well as about how it does it.
Functions are the most critical part in most interfaces, so see the interface rules.

Function rule summary:

Function definition rules:

• F.1: “Package” meaningful operations as carefully named functions
• F.2: A function should perform a single logical operation
• F.3: Keep functions short and simple
• F.4: If a function might have to be evaluated at compile time, declare it constexpr
• F.5: If a function is very small and time-critical, declare it inline
• F.6: If your function must not throw, declare it noexcept
• F.7: For general use, take T* or T& arguments rather than smart pointers
• F.8: Prefer pure functions
• F.9: Unused parameters should be unnamed
• F.10: If an operation can be reused, give it a name
• F.11: Use an unnamed lambda if you need a simple function object in one place only

Parameter passing expression rules:

• F.15: Prefer simple and conventional ways of passing information
• F.16: For “in” parameters, pass cheaply-copied types by value and others by reference to const
• F.17: For “in-out” parameters, pass by reference to non-const
• F.18: For “will-move-from” parameters, pass by X&& and std::move the parameter
• F.19: For “forward” parameters, pass by TP&& and only std::forward the parameter
• F.20: For “out” output values, prefer return values to output parameters
• F.21: To return multiple “out” values, prefer returning a struct or tuple
• F.60: Prefer T* over T& when “no argument” is a valid option

Parameter passing semantic rules:

• F.22: Use T* or owner to designate a single object
• F.23: Use a not_null to indicate that “null” is not a valid value
• F.24: Use a span or a span_p to designate a half-open sequence
• F.25: Use a zstring or a not_null to designate a C-style string
• F.26: Use a unique_ptr to transfer ownership where a pointer is needed
• F.27: Use a shared_ptr to share ownership

Value return semantic rules:

• F.42: Return a T* to indicate a position (only)
• F.43: Never (directly or indirectly) return a pointer or a reference to a local object
• F.44: Return a T& when copy is undesirable and “returning no object” isn’t needed
• F.45: Don’t return a T&&
• F.46: int is the return type for main()
• F.47: Return T& from assignment operators
• F.48: Don’t return std::move(local)
• F.49: Don’t return const T

Other function rules:

• F.50: Use a lambda when a function won’t do (to capture local variables, or to write a local function)
• F.51: Where there is a choice, prefer default arguments over overloading
• F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms
• F.53: Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread
• F.54: When writing a lambda that captures this or any class data member, don’t use [=] default capture
• F.55: Don’t use va_arg arguments
• F.56: Avoid unnecessary condition nesting

Functions have strong similarities to lambdas and function objects.

See also: C.lambdas: Function objects and lambdas

F.def: Function definitions

A function definition is a function declaration that also specifies the function’s implementation, the function body.

F.1: “Package” meaningful operations as carefully named functions

Reason

Factoring out common code makes code more readable, more likely to be reused, and limit errors from complex code.
If something is a well-specified action, separate it out from its surrounding code and give it a name.

Example, don’t

void read_and_print(istream& is)    // read and print an int
{
    int x;
    if (is >> x)
        cout << "the int is " << x << '\n';
    else
        cerr << "no int on input\n";
}

Almost everything is wrong with read_and_print.
It reads, it writes (to a fixed ostream), it writes error messages (to a fixed ostream), it handles only ints.
There is nothing to reuse, logically separate operations are intermingled and local variables are in scope after the end of their logical use.
For a tiny example, this looks OK, but if the input operation, the output operation, and the error handling had been more complicated the tangled
mess could become hard to understand.

Note

If you write a non-trivial lambda that potentially can be used in more than one place, give it a name by assigning it to a (usually non-local) variable.

Example

sort(a, b, [](T x, T y) { return x.rank() < y.rank() && x.value() < y.value(); });

Naming that lambda breaks up the expression into its logical parts and provides a strong hint to the meaning of the lambda.

auto lessT = [](T x, T y) { return x.rank() < y.rank() && x.value() < y.value(); };

sort(a, b, lessT);

The shortest code is not always the best for performance or maintainability.

Exception

Loop bodies, including lambdas used as loop bodies, rarely need to be named.
However, large loop bodies (e.g., dozens of lines or dozens of pages) can be a problem.
The rule Keep functions short and simple implies “Keep loop bodies short.”
Similarly, lambdas used as callback arguments are sometimes non-trivial, yet unlikely to be reusable.

Enforcement

• See Keep functions short and simple
• Flag identical and very similar lambdas used in different places.

F.2: A function should perform a single logical operation

Reason

A function that performs a single operation is simpler to understand, test, and reuse.

Example

Consider:

void read_and_print()    // bad
{
    int x;
    cin >> x;
    // check for errors
    cout << x << "\n";
}

This is a monolith that is tied to a specific input and will never find another (different) use. Instead, break functions up into suitable logical parts and parameterize:

int read(istream& is)    // better
{
    int x;
    is >> x;
    // check for errors
    return x;
}

void print(ostream& os, int x)
{
    os << x << "\n";
}

These can now be combined where needed:

void read_and_print()
{
    auto x = read(cin);
    print(cout, x);
}

If there was a need, we could further templatize read() and print() on the data type, the I/O mechanism, the response to errors, etc. Example:

auto read = [](auto& input, auto& value)    // better
{
    input >> value;
    // check for errors
};

auto print(auto& output, const auto& value)
{
    output << value << "\n";
}

Enforcement

• Consider functions with more than one “out” parameter suspicious. Use return values instead, including tuple for multiple return values.
• Consider “large” functions that don’t fit on one editor screen suspicious. Consider factoring such a function into smaller well-named suboperations.
• Consider functions with 7 or more parameters suspicious.

F.3: Keep functions short and simple

Reason

Large functions are hard to read, more likely to contain complex code, and more likely to have variables in larger than minimal scopes.
Functions with complex control structures are more likely to be long and more likely to hide logical errors

Example

Consider:

double simple_func(double val, int flag1, int flag2)
    // simple_func: takes a value and calculates the expected ASIC output,
    // given the two mode flags.
{
    double intermediate;
    if (flag1 > 0) {
        intermediate = func1(val);
        if (flag2 % 2)
             intermediate = sqrt(intermediate);
    }
    else if (flag1 == -1) {
        intermediate = func1(-val);
        if (flag2 % 2)
             intermediate = sqrt(-intermediate);
        flag1 = -flag1;
    }
    if (abs(flag2) > 10) {
        intermediate = func2(intermediate);
    }
    switch (flag2 / 10) {
    case 1: if (flag1 == -1) return finalize(intermediate, 1.171);
            break;
    case 2: return finalize(intermediate, 13.1);
    default: break;
    }
    return finalize(intermediate, 0.);
}

This is too complex.
How would you know if all possible alternatives have been correctly handled?
Yes, it breaks other rules also.

We can refactor:

double func1_muon(double val, int flag)
{
    // ???
}

double func1_tau(double val, int flag1, int flag2)
{
    // ???
}

double simple_func(double val, int flag1, int flag2)
    // simple_func: takes a value and calculates the expected ASIC output,
    // given the two mode flags.
{
    if (flag1 > 0)
        return func1_muon(val, flag2);
    if (flag1 == -1)
        // handled by func1_tau: flag1 = -flag1;
        return func1_tau(-val, flag1, flag2);
    return 0.;
}

Note

“It doesn’t fit on a screen” is often a good practical definition of “far too large.”
One-to-five-line functions should be considered normal.

Note

Break large functions up into smaller cohesive and named functions.
Small simple functions are easily inlined where the cost of a function call is significant.

Enforcement

• Flag functions that do not “fit on a screen.”
  How big is a screen? Try 60 lines by 140 characters; that’s roughly the maximum that’s comfortable for a book page.
• Flag functions that are too complex. How complex is too complex?
  You could use cyclomatic complexity. Try “more than 10 logical paths through.” Count a simple switch as one path.

F.4: If a function might have to be evaluated at compile time, declare it constexpr

Reason

constexpr is needed to tell the compiler to allow compile-time evaluation.

Example

The (in)famous factorial:

constexpr int fac(int n)
{
    constexpr int max_exp = 17;      // constexpr enables max_exp to be used in Expects
    Expects(0 <= n && n < max_exp);  // prevent silliness and overflow
    int x = 1;
    for (int i = 2; i <= n; ++i) x *= i;
    return x;
}

This is C++14.
For C++11, use a recursive formulation of fac().

Note

constexpr does not guarantee compile-time evaluation;
it just guarantees that the function can be evaluated at compile time for constant expression arguments if the programmer requires it or the compiler decides to do so to optimize.

constexpr int min(int x, int y) { return x < y ? x : y; }

void test(int v)
{
    int m1 = min(-1, 2);            // probably compile-time evaluation
    constexpr int m2 = min(-1, 2);  // compile-time evaluation
    int m3 = min(-1, v);            // run-time evaluation
    constexpr int m4 = min(-1, v);  // error: cannot evaluate at compile time
}

Note

Don’t try to make all functions constexpr.
Most computation is best done at run time.

Note

Any API that might eventually depend on high-level run-time configuration or
business logic should not be made constexpr. Such customization can not be
evaluated by the compiler, and any constexpr functions that depended upon
that API would have to be refactored or drop constexpr.

Enforcement

Impossible and unnecessary.
The compiler gives an error if a non-constexpr function is called where a constant is required.

F.5: If a function is very small and time-critical, declare it inline

Reason

Some optimizers are good at inlining without hints from the programmer, but don’t rely on it.
Measure! Over the last 40 years or so, we have been promised compilers that can inline better than humans without hints from humans.
We are still waiting.
Specifying inline (explicitly, or implicitly when writing member functions inside a class definition) encourages the compiler to do a better job.

Example

inline string cat(const string& s, const string& s2) { return s + s2; }

Exception

Do not put an inline function in what is meant to be a stable interface unless you are certain that it will not change.
An inline function is part of the ABI.

Note

constexpr implies inline.

Note

Member functions defined in-class are inline by default.

Exception

Function templates (including member functions of class templates A::function() and member function templates A::function()) are normally defined in headers and therefore inline.

Enforcement

Flag inline functions that are more than three statements and could have been declared out of line (such as class member functions).

F.6: If your function must not throw, declare it noexcept

Reason

If an exception is not supposed to be thrown, the program cannot be assumed to cope with the error and should be terminated as soon as possible. Declaring a function noexcept helps optimizers by reducing the number of alternative execution paths. It also speeds up the exit after failure.

Example

Put noexcept on every function written completely in C or in any other language without exceptions.
The C++ Standard Library does that implicitly for all functions in the C Standard Library.

Note

constexpr functions can throw when evaluated at run time, so you might need conditional noexcept for some of those.

Example

You can use noexcept even on functions that can throw:

vector collect(istream& is) noexcept
{
    vector res;
    for (string s; is >> s;)
        res.push_back(s);
    return res;
}

If collect() runs out of memory, the program crashes.
Unless the program is crafted to survive memory exhaustion, that might be just the right thing to do;
terminate() might generate suitable error log information (but after memory runs out it is hard to do anything clever).

Note

You must be aware of the execution environment that your code is running when
deciding whether to tag a function noexcept, especially because of the issue
of throwing and allocation. Code that is intended to be perfectly general (like
the standard library and other utility code of that sort) needs to support
environments where a bad_alloc exception could be handled meaningfully.
However, most programs and execution environments cannot meaningfully
handle a failure to allocate, and aborting the program is the cleanest and
simplest response to an allocation failure in those cases. If you know that
your application code cannot respond to an allocation failure, it could be
appropriate to add noexcept even on functions that allocate.

Put another way: In most programs, most functions can throw (e.g., because they
use new, call functions that do, or use library functions that reports failure
by throwing), so don’t just sprinkle noexcept all over the place without
considering whether the possible exceptions can be handled.

noexcept is most useful (and most clearly correct) for frequently used,
low-level functions.

Note

Destructors, swap functions, move operations, and default constructors should never throw.
See also C.44.

Enforcement

• Flag functions that are not noexcept, yet cannot throw.
• Flag throwing swap, move, destructors, and default constructors.

F.7: For general use, take T* or T& arguments rather than smart pointers

Reason

Passing a smart pointer transfers or shares ownership and should only be used when ownership semantics are intended.
A function that does not manipulate lifetime should take raw pointers or references instead.

Passing by smart pointer restricts the use of a function to callers that use smart pointers.
A function that needs a widget should be able to accept any widget object, not just ones whose lifetimes are managed by a particular kind of smart pointer.

Passing a shared smart pointer (e.g., std::shared_ptr) implies a run-time cost.

Example

// accepts any int*
void f(int*);

// can only accept ints for which you want to transfer ownership
void g(unique_ptr);

// can only accept ints for which you are willing to share ownership
void g(shared_ptr);

// doesn't change ownership, but requires a particular ownership of the caller
void h(const unique_ptr&);

// accepts any int
void h(int&);

Example, bad

// callee
void f(shared_ptr& w)
{
    // ...
    use(*w); // only use of w -- the lifetime is not used at all
    // ...
};

// caller
shared_ptr my_widget = /* ... */;
f(my_widget);

widget stack_widget;
f(stack_widget); // error

Example, good

// callee
void f(widget& w)
{
    // ...
    use(w);
    // ...
};

// caller
shared_ptr my_widget = /* ... */;
f(*my_widget);

widget stack_widget;
f(stack_widget); // ok -- now this works

Note

We can catch many common cases of dangling pointers statically (see lifetime safety profile). Function arguments naturally live for the lifetime of the function call, and so have fewer lifetime problems.

Enforcement

• (Simple) Warn if a function takes a parameter of a smart pointer type (that overloads operator-> or operator*) that is copyable but the function only calls any of: operator*, operator-> or get().
  Suggest using a T* or T& instead.
• Flag a parameter of a smart pointer type (a type that overloads operator-> or operator*) that is copyable/movable but never copied/moved from in the function body, and that is never modified, and that is not passed along to another function that could do so. That means the ownership semantics are not used.
  Suggest using a T* or T& instead.

See also:

• Prefer T* over T& when “no argument” is a valid option
• Smart pointer rule summary

F.8: Prefer pure functions

Reason

Pure functions are easier to reason about, sometimes easier to optimize (and even parallelize), and sometimes can be memoized.

Example

template
auto square(T t) { return t * t; }

Enforcement

Not possible.

F.9: Unused parameters should be unnamed

Reason

Readability.
Suppression of unused parameter warnings.

Example

widget* find(const set& s, const widget& w, Hint);   // once upon a time, a hint was used

Note

Allowing parameters to be unnamed was introduced in the early 1980s to address this problem.

If parameters are conditionally unused, declare them with the [[maybe_unused]] attribute.
For example:

template 
Value* find(const set& s, const Value& v, [[maybe_unused]] Hint h)
{
    if constexpr (sizeof(Value) > CacheSize)
    {
        // a hint is used only if Value is of a certain size
    }
}

Enforcement

Flag named unused parameters.

F.10: If an operation can be reused, give it a name

Reason

Documentation, readability, opportunity for reuse.

Example

struct Rec {
    string name;
    string addr;
    int id;         // unique identifier
};

bool same(const Rec& a, const Rec& b)
{
    return a.id == b.id;
}

vector find_id(const string& name);    // find all records for "name"

auto x = find_if(vr.begin(), vr.end(),
    [&](Rec& r) {
        if (r.name.size() != n.size()) return false; // name to compare to is in n
        for (int i = 0; i < r.name.size(); ++i)
            if (tolower(r.name[i]) != tolower(n[i])) return false;
        return true;
    }
);

There is a useful function lurking here (case insensitive string comparison), as there often is when lambda arguments get large.

bool compare_insensitive(const string& a, const string& b)
{
    if (a.size() != b.size()) return false;
    for (int i = 0; i < a.size(); ++i) if (tolower(a[i]) != tolower(b[i])) return false;
    return true;
}

auto x = find_if(vr.begin(), vr.end(),
    [&](Rec& r) { return compare_insensitive(r.name, n); }
);

Or maybe (if you prefer to avoid the implicit name binding to n):

auto cmp_to_n = [&n](const string& a) { return compare_insensitive(a, n); };

auto x = find_if(vr.begin(), vr.end(),
    [](const Rec& r) { return cmp_to_n(r.name); }
);

Note

whether functions, lambdas, or operators.

Exception

• Lambdas logically used only locally, such as an argument to for_each and similar control flow algorithms.
• Lambdas as initializers

Enforcement

• (hard) flag similar lambdas
• ???

F.11: Use an unnamed lambda if you need a simple function object in one place only

Reason

That makes the code concise and gives better locality than alternatives.

Example

auto earlyUsersEnd = std::remove_if(users.begin(), users.end(),
                                    [](const User &a) { return a.id > 100; });

Exception

Naming a lambda can be useful for clarity even if it is used only once.

Enforcement

• Look for identical and near identical lambdas (to be replaced with named functions or named lambdas).

F.call: Parameter passing

There are a variety of ways to pass parameters to a function and to return values.

F.15: Prefer simple and conventional ways of passing information

Reason

Using “unusual and clever” techniques causes surprises, slows understanding by other programmers, and encourages bugs.
If you really feel the need for an optimization beyond the common techniques, measure to ensure that it really is an improvement, and document/comment because the improvement might not be portable.

The following tables summarize the advice in the following Guidelines, F.16-21.

Normal parameter passing:

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Advanced parameter passing:

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Use the advanced techniques only after demonstrating need, and document that need in a comment.

For passing sequences of characters see String.

Exception

To express shared ownership using shared_ptr types, rather than following guidelines F.16-21,
follow R.34, R.35, and R.36.

F.16: For “in” parameters, pass cheaply-copied types by value and others by reference to const

Reason

Both let the caller know that a function will not modify the argument, and both allow initialization by rvalues.

What is “cheap to copy” depends on the machine architecture, but two or three words (doubles, pointers, references) are usually best passed by value.
When copying is cheap, nothing beats the simplicity and safety of copying, and for small objects (up to two or three words) it is also faster than passing by reference because it does not require an extra indirection to access from the function.

Example

void f1(const string& s);  // OK: pass by reference to const; always cheap

void f2(string s);         // bad: potentially expensive

void f3(int x);            // OK: Unbeatable

void f4(const int& x);     // bad: overhead on access in f4()

For advanced uses (only), where you really need to optimize for rvalues passed to “input-only” parameters:

• If the function is going to unconditionally move from the argument, take it by &&. See F.18.
• If the function is going to keep a copy of the argument, in addition to passing by const& (for lvalues),
  add an overload that passes the parameter by && (for rvalues) and in the body std::moves it to its destination. Essentially this overloads a “will-move-from”; see F.18.
• In special cases, such as multiple “input + copy” parameters, consider using perfect forwarding. See F.19.

Example

int multiply(int, int); // just input ints, pass by value

// suffix is input-only but not as cheap as an int, pass by const&
string& concatenate(string&, const string& suffix);

void sink(unique_ptr);  // input only, and moves ownership of the widget

Avoid “esoteric techniques” such as passing arguments as T&& “for efficiency”.
Most rumors about performance advantages from passing by && are false or brittle (but see F.18 and F.19).

Notes

A reference can be assumed to refer to a valid object (language rule).
There is no (legitimate) “null reference.”
If you need the notion of an optional value, use a pointer, std::optional, or a special value used to denote “no value.”

Enforcement

• (Simple) ((Foundation)) Warn when a parameter being passed by value has a size greater than 2 * sizeof(void*).
  Suggest using a reference to const instead.
• (Simple) ((Foundation)) Warn when a parameter passed by reference to const has a size less or equal than 2 * sizeof(void*). Suggest passing by value instead.
• (Simple) ((Foundation)) Warn when a parameter passed by reference to const is moved.

Exception

To express shared ownership using shared_ptr types, follow R.34 or R.36,
depending on whether or not the function unconditionally takes a reference to the argument.

F.17: For “in-out” parameters, pass by reference to non-const

Reason

This makes it clear to callers that the object is assumed to be modified.

Example

void update(Record& r);  // assume that update writes to r

Note

Some user-defined and standard library types, such as span or the iterators
are cheap to copy and may be passed by value, while doing so has
mutable (in-out) reference semantics:

void increment_all(span a)
{
  for (auto&& e : a)
    ++e;
}

Note

A T& argument can pass information into a function as well as out of it.
Thus T& could be an in-out-parameter. That can in itself be a problem and a source of errors:

void f(string& s)
{
    s = "New York";  // non-obvious error
}

void g()
{
    string buffer = ".................................";
    f(buffer);
    // ...
}

Here, the writer of g() is supplying a buffer for f() to fill, but f() simply replaces it (at a somewhat higher cost than a simple copy of the characters).
A bad logic error can happen if the writer of g() incorrectly assumes the size of the buffer.

Enforcement

• (Moderate) ((Foundation)) Warn about functions regarding reference to non-const parameters that do not write to them.
• (Simple) ((Foundation)) Warn when a non-const parameter being passed by reference is moved.

F.18: For “will-move-from” parameters, pass by X&& and std::move the parameter

Reason

It’s efficient and eliminates bugs at the call site: X&& binds to rvalues, which requires an explicit std::move at the call site if passing an lvalue.

Example

void sink(vector&& v)  // sink takes ownership of whatever the argument owned
{
    // usually there might be const accesses of v here
    store_somewhere(std::move(v));
    // usually no more use of v here; it is moved-from
}

Note that the std::move(v) makes it possible for store_somewhere() to leave v in a moved-from state.
That could be dangerous.

Exception

Unique owner types that are move-only and cheap-to-move, such as unique_ptr, can also be passed by value which is simpler to write and achieves the same effect. Passing by value does generate one extra (cheap) move operation, but prefer simplicity and clarity first.

For example:

template
void sink(std::unique_ptr p)
{
    // use p ... possibly std::move(p) onward somewhere else
}   // p gets destroyed

Exception

If the “will-move-from” parameter is a shared_ptr follow R.34 and pass the shared_ptr by value.

Enforcement

• Flag all X&& parameters (where X is not a template type parameter name) where the function body uses them without std::move.
• Flag access to moved-from objects.
• Don’t conditionally move from objects

F.19: For “forward” parameters, pass by TP&& and only std::forward the parameter

Reason

If the object is to be passed onward to other code and not directly used by this function, we want to make this function agnostic to the argument const-ness and rvalue-ness.

In that case, and only that case, make the parameter TP&& where TP is a template type parameter – it both ignores and preserves const-ness and rvalue-ness. Therefore any code that uses a TP&& is implicitly declaring that it itself doesn’t care about the variable’s const-ness and rvalue-ness (because it is ignored), but that intends to pass the value onward to other code that does care about const-ness and rvalue-ness (because it is preserved). When used as a parameter TP&& is safe because any temporary objects passed from the caller will live for the duration of the function call. A parameter of type TP&& should essentially always be passed onward via std::forward in the body of the function.

Example

Usually you forward the entire parameter (or parameter pack, using ...) exactly once on every static control flow path:

template
inline auto invoke(F f, Args&&... args)
{
    return f(forward(args)...);
}

Example

Sometimes you may forward a composite parameter piecewise, each subobject once on every static control flow path:

template
inline auto test(PairLike&& pairlike)
{
    // ...
    f1(some, args, and, forward(pairlike).first);           // forward .first
    f2(and, forward(pairlike).second, in, another, call);   // forward .second
}

Enforcement

• Flag a function that takes a TP&& parameter (where TP is a template type parameter name) and does anything with it other than std::forwarding it exactly once on every static path, or std::forwarding it more than once but qualified with a different data member exactly once on every static path.

F.20: For “out” output values, prefer return values to output parameters

Reason

A return value is self-documenting, whereas a & could be either in-out or out-only and is liable to be misused.

This includes large objects like standard containers that use implicit move operations for performance and to avoid explicit memory management.

If you have multiple values to return, use a tuple or similar multi-member type.

Example

// OK: return pointers to elements with the value x
vector find_all(const vector&, int x);

// Bad: place pointers to elements with value x in-out
void find_all(const vector&, vector& out, int x);

Note

A struct of many (individually cheap-to-move) elements might be in aggregate expensive to move.

Exceptions

• For non-concrete types, such as types in an inheritance hierarchy, return the object by unique_ptr or shared_ptr.
• If a type is expensive to move (e.g., array), consider allocating it on the free store and return a handle (e.g., unique_ptr), or passing it in a reference to non-const target object to fill (to be used as an out-parameter).
• To reuse an object that carries capacity (e.g., std::string, std::vector) across multiple calls to the function in an inner loop: treat it as an in/out parameter and pass by reference.

Example

Assuming that Matrix has move operations (possibly by keeping its elements in a std::vector):

Matrix operator+(const Matrix& a, const Matrix& b)
{
    Matrix res;
    // ... fill res with the sum ...
    return res;
}

Matrix x = m1 + m2;  // move constructor

y = m3 + m3;         // move assignment

Note

The return value optimization doesn’t handle the assignment case, but the move assignment does.

Example

struct Package {      // exceptional case: expensive-to-move object
    char header[16];
    char load[2024 - 16];
};

Package fill();       // Bad: large return value
void fill(Package&);  // OK

int val();            // OK
void val(int&);       // Bad: Is val reading its argument

Enforcement

• Flag reference to non-const parameters that are not read before being written to and are a type that could be cheaply returned; they should be “out” return values.

F.21: To return multiple “out” values, prefer returning a struct or tuple

Reason

A return value is self-documenting as an “output-only” value.
Note that C++ does have multiple return values, by convention of using a tuple (including pair), possibly with the extra convenience of tie or structured bindings (C++17) at the call site.
Prefer using a named struct where there are semantics to the returned value. Otherwise, a nameless tuple is useful in generic code.

Example

// BAD: output-only parameter documented in a comment
int f(const string& input, /*output only*/ string& output_data)
{
    // ...
    output_data = something();
    return status;
}

// GOOD: self-documenting
tuple f(const string& input)
{
    // ...
    return {status, something()};
}

C++98’s standard library already used this style, because a pair is like a two-element tuple.
For example, given a set my_set, consider:

// C++98
result = my_set.insert("Hello");
if (result.second) do_something_with(result.first);    // workaround

With C++11 we can write this, putting the results directly in existing local variables:

Sometype iter;                                // default initialize if we haven't already
Someothertype success;                        // used these variables for some other purpose

tie(iter, success) = my_set.insert("Hello");   // normal return value
if (success) do_something_with(iter);

With C++17 we are able to use “structured bindings” to declare and initialize the multiple variables:

if (auto [ iter, success ] = my_set.insert("Hello"); success) do_something_with(iter);

Exception

Sometimes, we need to pass an object to a function to manipulate its state.
In such cases, passing the object by reference T& is usually the right technique.
Explicitly passing an in-out parameter back out again as a return value is often not necessary.
For example:

istream& operator>>(istream& in, string& s);    // much like std::operator>>()

for (string s; in >> s; ) {
    // do something with line
}

Here, both s and in are used as in-out parameters.
We pass in by (non-const) reference to be able to manipulate its state.
We pass s to avoid repeated allocations.
By reusing s (passed by reference), we allocate new memory only when we need to expand s’s capacity.
This technique is sometimes called the “caller-allocated out” pattern and is particularly useful for types,
such as string and vector, that needs to do free store allocations.

To compare, if we passed out all values as return values, we would something like this:

pair get_string(istream& in)  // not recommended
{
    string s;
    in >> s;
    return {in, move(s)};
}

for (auto p = get_string(cin); p.first; ) {
    // do something with p.second
}

We consider that significantly less elegant with significantly less performance.

For a truly strict reading of this rule (F.21), the exception isn’t really an exception because it relies on in-out parameters,
rather than the plain out parameters mentioned in the rule.
However, we prefer to be explicit, rather than subtle.

Note

In many cases, it can be useful to return a specific, user-defined type.
For example:

struct Distance {
    int value;
    int unit = 1;   // 1 means meters
};

Distance d1 = measure(obj1);        // access d1.value and d1.unit
auto d2 = measure(obj2);            // access d2.value and d2.unit
auto [value, unit] = measure(obj3); // access value and unit; somewhat redundant
                                    // to people who know measure()
auto [x, y] = measure(obj4);        // don't; it's likely to be confusing

The overly-generic pair and tuple should be used only when the value returned represents independent entities rather than an abstraction.

Another example, use a specific type along the lines of variant, rather than using the generic tuple.

Note

When the tuple to be returned is initialized from local variables that are expensive to copy,
explicit move may be helpful to avoid copying:

pair f(const string& input)
{
    LargeObject large1 = g(input);
    LargeObject large2 = h(input);
    // ...
    return { move(large1), move(large2) }; // no copies
}

Alternatively,

pair f(const string& input)
{
    // ...
    return { g(input), h(input) }; // no copies, no moves
}

Note this is different from the return move(...) anti-pattern from ES.56

Enforcement

• Output parameters should be replaced by return values.
  An output parameter is one that the function writes to, invokes a non-const member function, or passes on as a non-const.

F.60: Prefer T* over T& when “no argument” is a valid option

Reason

A pointer (T*) can be a nullptr and a reference (T&) cannot, there is no valid “null reference”.
Sometimes having nullptr as an alternative to indicated “no object” is useful, but if it is not, a reference is notationally simpler and might yield better code.

Example

string zstring_to_string(zstring p) // zstring is a char*; that is a C-style string
{
    if (!p) return string{};    // p might be nullptr; remember to check
    return string{p};
}

void print(const vector& r)
{
    // r refers to a vector; no check needed
}

Note

It is possible, but not valid C++ to construct a reference that is essentially a nullptr (e.g., T* p = nullptr; T& r = *p;).
That error is very uncommon.

Note

If you prefer the pointer notation (-> and/or * vs. .), not_null provides the same guarantee as T&.

Enforcement

• Flag ???

F.22: Use T* or owner to designate a single object

Reason

Readability: it makes the meaning of a plain pointer clear.
Enables significant tool support.

Note

In traditional C and C++ code, plain T* is used for many weakly-related purposes, such as:

• Identify a (single) object (not to be deleted by this function)
• Point to an object allocated on the free store (and delete it later)
• Hold the nullptr
• Identify a C-style string (zero-terminated array of characters)
• Identify an array with a length specified separately
• Identify a location in an array

This makes it hard to understand what the code does and is supposed to do.
It complicates checking and tool support.

Example

void use(int* p, int n, char* s, int* q)
{
    p[n - 1] = 666; // Bad: we don't know if p points to n elements;
                    // assume it does not or use span
    cout << s;      // Bad: we don't know if that s points to a zero-terminated array of char;
                    // assume it does not or use zstring
    delete q;       // Bad: we don't know if *q is allocated on the free store;
                    // assume it does not or use owner
}

better

void use2(span p, zstring s, owner q)
{
    p[p.size() - 1] = 666; // OK, a range error can be caught
    cout << s; // OK
    delete q;  // OK
}

Note

owner represents ownership, zstring represents a C-style string.

Also: Assume that a T* obtained from a smart pointer to T (e.g., unique_ptr) points to a single element.

See also: Support library

See also: Do not pass an array as a single pointer

Enforcement

• (Simple) ((Bounds)) Warn for any arithmetic operation on an expression of pointer type that results in a value of pointer type.

F.23: Use a not_null to indicate that “null” is not a valid value

Reason

Clarity. A function with a not_null parameter makes it clear that the caller of the function is responsible for any nullptr checks that might be necessary.
Similarly, a function with a return value of not_null makes it clear that the caller of the function does not need to check for nullptr.

Example

not_null makes it obvious to a reader (human or machine) that a test for nullptr is not necessary before dereference.
Additionally, when debugging, owner and not_null can be instrumented to check for correctness.

Consider:

int length(Record* p);

When I call length(p) should I check if p is nullptr first? Should the implementation of length() check if p is nullptr?

// it is the caller's job to make sure p != nullptr
int length(not_null p);

// the implementor of length() must assume that p == nullptr is possible
int length(Record* p);

Note

A not_null is assumed not to be the nullptr; a T* might be the nullptr; both can be represented in memory as a T* (so no run-time overhead is implied).

Note

not_null is not just for built-in pointers. It works for unique_ptr, shared_ptr, and other pointer-like types.

Enforcement

• (Simple) Warn if a raw pointer is dereferenced without being tested against nullptr (or equivalent) within a function, suggest it is declared not_null instead.
• (Simple) Error if a raw pointer is sometimes dereferenced after first being tested against nullptr (or equivalent) within the function and sometimes is not.
• (Simple) Warn if a not_null pointer is tested against nullptr within a function.

F.24: Use a span or a span_p to designate a half-open sequence

Reason

Informal/non-explicit ranges are a source of errors.

Example

X* find(span r, const X& v);    // find v in r

vector vec;
// ...
auto p = find({vec.begin(), vec.end()}, X{});  // find X{} in vec

Note

Ranges are extremely common in C++ code. Typically, they are implicit and their correct use is very hard to ensure.
In particular, given a pair of arguments (p, n) designating an array [p:p+n),
it is in general impossible to know if there really are n elements to access following *p.
span and span_p are simple helper classes designating a [p:q) range and a range starting with p and ending with the first element for which a predicate is true, respectively.

Example

A span represents a range of elements, but how do we manipulate elements of that range?

void f(span s)
{
    // range traversal (guaranteed correct)
    for (int x : s) cout << x << '\n';

    // C-style traversal (potentially checked)
    for (gsl::index i = 0; i < s.size(); ++i) cout << s[i] << '\n';

    // random access (potentially checked)
    s[7] = 9;

    // extract pointers (potentially checked)
    std::sort(&s[0], &s[s.size() / 2]);
}

Note

A span object does not own its elements and is so small that it can be passed by value.

Passing a span object as an argument is exactly as efficient as passing a pair of pointer arguments or passing a pointer and an integer count.

See also: Support library

Enforcement

(Complex) Warn where accesses to pointer parameters are bounded by other parameters that are integral types and suggest they could use span instead.

F.25: Use a zstring or a not_null to designate a C-style string

Reason

C-style strings are ubiquitous. They are defined by convention: zero-terminated arrays of characters.
We must distinguish C-style strings from a pointer to a single character or an old-fashioned pointer to an array of characters.

If you don’t need null termination, use string_view.

Example

Consider:

int length(const char* p);

When I call length(s) should I check if s is nullptr first? Should the implementation of length() check if p is nullptr?

// the implementor of length() must assume that p == nullptr is possible
int length(zstring p);

// it is the caller's job to make sure p != nullptr
int length(not_null p);

Note

zstring does not represent ownership.

See also: Support library

F.26: Use a unique_ptr to transfer ownership where a pointer is needed

Reason

Using unique_ptr is the cheapest way to pass a pointer safely.

See also: C.50 regarding when to return a shared_ptr from a factory.

Example

unique_ptr get_shape(istream& is)  // assemble shape from input stream
{
    auto kind = read_header(is); // read header and identify the next shape on input
    switch (kind) {
    case kCircle:
        return make_unique(is);
    case kTriangle:
        return make_unique(is);
    // ...
    }
}

Note

You need to pass a pointer rather than an object if what you are transferring is an object from a class hierarchy that is to be used through an interface (base class).

Enforcement

(Simple) Warn if a function returns a locally allocated raw pointer. Suggest using either unique_ptr or shared_ptr instead.

F.27: Use a shared_ptr to share ownership

Reason

Using std::shared_ptr is the standard way to represent shared ownership. That is, the last owner deletes the object.

Example

shared_ptr im { read_image(somewhere) };

std::thread t0 {shade, args0, top_left, im};
std::thread t1 {shade, args1, top_right, im};
std::thread t2 {shade, args2, bottom_left, im};
std::thread t3 {shade, args3, bottom_right, im};

// detach threads
// last thread to finish deletes the image

Note

Prefer a unique_ptr over a shared_ptr if there is never more than one owner at a time.
shared_ptr is for shared ownership.

Note that pervasive use of shared_ptr has a cost (atomic operations on the shared_ptr’s reference count have a measurable aggregate cost).

Alternative

Have a single object own the shared object (e.g. a scoped object) and destroy that (preferably implicitly) when all users have completed.

Enforcement

(Not enforceable) This is a too complex pattern to reliably detect.

F.42: Return a T* to indicate a position (only)

Reason

That’s what pointers are good for.
Returning a T* to transfer ownership is a misuse.

Example

Node* find(Node* t, const string& s)  // find s in a binary tree of Nodes
{
    if (!t || t->name == s) return t;
    if ((auto p = find(t->left, s))) return p;
    if ((auto p = find(t->right, s))) return p;
    return nullptr;
}

If it isn’t the nullptr, the pointer returned by find indicates a Node holding s.
Importantly, that does not imply a transfer of ownership of the pointed-to object to the caller.

Note

Positions can also be transferred by iterators, indices, and references.
A reference is often a superior alternative to a pointer if there is no need to use nullptr or if the object referred to should not change.

Note

Do not return a pointer to something that is not in the caller’s scope; see F.43.

See also: discussion of dangling pointer prevention

Enforcement

• Flag delete, std::free(), etc. applied to a plain T*.
  Only owners should be deleted.
• Flag new, malloc(), etc. assigned to a plain T*.
  Only owners should be responsible for deletion.

F.43: Never (directly or indirectly) return a pointer or a reference to a local object

Reason

To avoid the crashes and data corruption that can result from the use of such a dangling pointer.

Example, bad

After the return from a function its local objects no longer exist:

int* f()
{
    int fx = 9;
    return &fx;  // BAD
}

void g(int* p)   // looks innocent enough
{
    int gx;
    cout << "*p == " << *p << '\n';
    *p = 999;
    cout << "gx == " << gx << '\n';
}

void h()
{
    int* p = f();
    int z = *p;  // read from abandoned stack frame (bad)
    g(p);        // pass pointer to abandoned stack frame to function (bad)
}

Here on one popular implementation I got the output:

*p == 999
gx == 999

I expected that because the call of g() reuses the stack space abandoned by the call of f() so *p refers to the space now occupied by gx.

• Imagine what would happen if fx and gx were of different types.
• Imagine what would happen if fx or gx was a type with an invariant.
• Imagine what would happen if more that dangling pointer was passed around among a larger set of functions.
• Imagine what a cracker could do with that dangling pointer.

Fortunately, most (all?) modern compilers catch and warn against this simple case.

Note

This applies to references as well:

int& f()
{
    int x = 7;
    // ...
    return x;  // Bad: returns reference to object that is about to be destroyed
}

Note

This applies only to non-static local variables.
All static variables are (as their name indicates) statically allocated, so that pointers to them cannot dangle.

Example, bad

Not all examples of leaking a pointer to a local variable are that obvious:

int* glob;       // global variables are bad in so many ways

template
void steal(T x)
{
    glob = x();  // BAD
}

void f()
{
    int i = 99;
    steal([&] { return &i; });
}

int main()
{
    f();
    cout << *glob << '\n';
}

Here I managed to read the location abandoned by the call of f.
The pointer stored in glob could be used much later and cause trouble in unpredictable ways.

Note

The address of a local variable can be “returned”/leaked by a return statement, by a T& out-parameter, as a member of a returned object, as an element of a returned array, and more.

Note

Similar examples can be constructed “leaking” a pointer from an inner scope to an outer one;
such examples are handled equivalently to leaks of pointers out of a function.

A slightly different variant of the problem is placing pointers in a container that outlives the objects pointed to.

See also: Another way of getting dangling pointers is pointer invalidation.
It can be detected/prevented with similar techniques.

Enforcement

• Compilers tend to catch return of reference to locals and could in many cases catch return of pointers to locals.
• Static analysis can catch many common patterns of the use of pointers indicating positions (thus eliminating dangling pointers)

F.44: Return a T& when copy is undesirable and “returning no object” isn’t needed

Reason

The language guarantees that a T& refers to an object, so that testing for nullptr isn’t necessary.

See also: The return of a reference must not imply transfer of ownership:
discussion of dangling pointer prevention and discussion of ownership.

Example

class Car
{
    array w;
    // ...
public:
    wheel& get_wheel(int i) { Expects(i < w.size()); return w[i]; }
    // ...
};

void use()
{
    Car c;
    wheel& w0 = c.get_wheel(0); // w0 has the same lifetime as c
}

Enforcement

Flag functions where no return expression could yield nullptr

F.45: Don’t return a T&&

Reason

It’s asking to return a reference to a destroyed temporary object.
A && is a magnet for temporary objects.

Example

A returned rvalue reference goes out of scope at the end of the full expression to which it is returned:

auto&& x = max(0, 1);   // OK, so far
foo(x);                 // Undefined behavior

This kind of use is a frequent source of bugs, often incorrectly reported as a compiler bug.
An implementer of a function should avoid setting such traps for users.

The lifetime safety profile will (when completely implemented) catch such problems.

Example

Returning an rvalue reference is fine when the reference to the temporary is being passed “downward” to a callee;
then, the temporary is guaranteed to outlive the function call (see F.18 and F.19).
However, it’s not fine when passing such a reference “upward” to a larger caller scope.
For passthrough functions that pass in parameters (by ordinary reference or by perfect forwarding) and want to return values, use simple auto return type deduction (not auto&&).

Assume that F returns by value:

template
auto&& wrapper(F f)
{
    log_call(typeid(f)); // or whatever instrumentation
    return f();          // BAD: returns a reference to a temporary
}

Better:

template
auto wrapper(F f)
{
    log_call(typeid(f)); // or whatever instrumentation
    return f();          // OK
}

Exception

std::move and std::forward do return &&, but they are just casts – used by convention only in expression contexts where a reference to a temporary object is passed along within the same expression before the temporary is destroyed. We don’t know of any other good examples of returning &&.

Enforcement

Flag any use of && as a return type, except in std::move and std::forward.

F.46: int is the return type for main()

Reason

It’s a language rule, but violated through “language extensions” so often that it is worth mentioning.
Declaring main (the one global main of a program) void limits portability.

Example

    void main() { /* ... */ };  // bad, not C++

    int main()
    {
        std::cout << "This is the way to do it\n";
    }

Note

We mention this only because of the persistence of this error in the community.
Note that despite its non-void return type, the main function does not require an explicit return statement.

Enforcement

• The compiler should do it
• If the compiler doesn’t do it, let tools flag it

F.47: Return T& from assignment operators

Reason

The convention for operator overloads (especially on concrete types) is for
operator=(const T&) to perform the assignment and then return (non-const)
*this. This ensures consistency with standard-library types and follows the
principle of “do as the ints do.”

Note

Historically there was some guidance to make the assignment operator return const T&.
This was primarily to avoid code of the form (a = b) = c – such code is not common enough to warrant violating consistency with standard types.

Example

class Foo
{
 public:
    ...
    Foo& operator=(const Foo& rhs)
    {
      // Copy members.
      ...
      return *this;
    }
};

Enforcement

This should be enforced by tooling by checking the return type (and return
value) of any assignment operator.

F.48: Don’t return std::move(local)

Reason

With guaranteed copy elision, it is now almost always a pessimization to expressly use std::move in a return statement.

Example, bad

S f()
{
  S result;
  return std::move(result);
}

Example, good

S f()
{
  S result;
  return result;
}

Enforcement

This should be enforced by tooling by checking the return expression .

F.49: Don’t return const T

Reason

It is not recommended to return a const value.
Such older advice is now obsolete; it does not add value, and it interferes with move semantics.

Example

const vector fct();    // bad: that "const" is more trouble than it is worth

void g(vector& vx)
{
    // ...
    fct() = vx;   // prevented by the "const"
    // ...
    vx = fct(); // expensive copy: move semantics suppressed by the "const"
    // ...
}

The argument for adding const to a return value is that it prevents (very rare) accidental access to a temporary.
The argument against is that it prevents (very frequent) use of move semantics.

See also: F.20, the general item about “out” output values

Enforcement

• Flag returning a const value. To fix: Remove const to return a non-const value instead.

F.50: Use a lambda when a function won’t do (to capture local variables, or to write a local function)

Reason

Functions can’t capture local variables or be defined at local scope; if you need those things, prefer a lambda where possible, and a handwritten function object where not. On the other hand, lambdas and function objects don’t overload; if you need to overload, prefer a function (the workarounds to make lambdas overload are ornate). If either will work, prefer writing a function; use the simplest tool necessary.

Example

// writing a function that should only take an int or a string
// -- overloading is natural
void f(int);
void f(const string&);

// writing a function object that needs to capture local state and appear
// at statement or expression scope -- a lambda is natural
vector v = lots_of_work();
for (int tasknum = 0; tasknum < max; ++tasknum) {
    pool.run([=, &v] {
        /*
        ...
        ... process 1 / max - th of v, the tasknum - th chunk
        ...
        */
    });
}
pool.join();

Exception

Generic lambdas offer a concise way to write function templates and so can be useful even when a normal function template would do equally well with a little more syntax. This advantage will probably disappear in the future once all functions gain the ability to have Concept parameters.

Enforcement

• Warn on use of a named non-generic lambda (e.g., auto x = [](int i) { /*...*/; };) that captures nothing and appears at global scope. Write an ordinary function instead.

F.51: Where there is a choice, prefer default arguments over overloading

Reason

Default arguments simply provide alternative interfaces to a single implementation.
There is no guarantee that a set of overloaded functions all implement the same semantics.
The use of default arguments can avoid code replication.

Note

There is a choice between using default argument and overloading when the alternatives are from a set of arguments of the same types.
For example:

void print(const string& s, format f = {});

as opposed to

void print(const string& s);  // use default format
void print(const string& s, format f);

There is not a choice when a set of functions are used to do a semantically equivalent operation to a set of types. For example:

void print(const char&);
void print(int);
void print(zstring);

See also

Default arguments for virtual functions

Enforcement

• Warn on an overload set where the overloads have a common prefix of parameters (e.g., f(int), f(int, const string&), f(int, const string&, double)). (Note: Review this enforcement if it’s too noisy in practice.)

F.52: Prefer capturing by reference in lambdas that will be used locally, including passed to algorithms

Reason

For efficiency and correctness, you nearly always want to capture by reference when using the lambda locally. This includes when writing or calling parallel algorithms that are local because they join before returning.

Discussion

The efficiency consideration is that most types are cheaper to pass by reference than by value.

The correctness consideration is that many calls want to perform side effects on the original object at the call site (see example below). Passing by value prevents this.

Note

Unfortunately, there is no simple way to capture by reference to const to get the efficiency for a local call but also prevent side effects.

Example

Here, a large object (a network message) is passed to an iterative algorithm, and it is not efficient or correct to copy the message (which might not be copyable):

std::for_each(begin(sockets), end(sockets), [&message](auto& socket)
{
    socket.send(message);
});

Example

This is a simple three-stage parallel pipeline. Each stage object encapsulates a worker thread and a queue, has a process function to enqueue work, and in its destructor automatically blocks waiting for the queue to empty before ending the thread.

void send_packets(buffers& bufs)
{
    stage encryptor([](buffer& b) { encrypt(b); });
    stage compressor([&](buffer& b) { compress(b); encryptor.process(b); });
    stage decorator([&](buffer& b) { decorate(b); compressor.process(b); });
    for (auto& b : bufs) { decorator.process(b); }
}  // automatically blocks waiting for pipeline to finish

Enforcement

Flag a lambda that captures by reference, but is used other than locally within the function scope or passed to a function by reference. (Note: This rule is an approximation, but does flag passing by pointer as those are more likely to be stored by the callee, writing to a heap location accessed via a parameter, returning the lambda, etc. The Lifetime rules will also provide general rules that flag escaping pointers and references including via lambdas.)

F.53: Avoid capturing by reference in lambdas that will be used non-locally, including returned, stored on the heap, or passed to another thread

Reason

Pointers and references to locals shouldn’t outlive their scope. Lambdas that capture by reference are just another place to store a reference to a local object, and shouldn’t do so if they (or a copy) outlive the scope.

Example, bad

int local = 42;

// Want a reference to local.
// Note, that after program exits this scope,
// local no longer exists, therefore
// process() call will have undefined behavior!
thread_pool.queue_work([&] { process(local); });

Example, good

int local = 42;
// Want a copy of local.
// Since a copy of local is made, it will
// always be available for the call.
thread_pool.queue_work([=] { process(local); });

Note

If a non-local pointer must be captured, consider using unique_ptr; this handles both lifetime and synchronization.

If the this pointer must be captured, consider using [*this] capture, which creates a copy of the entire object.

Enforcement

• (Simple) Warn when capture-list contains a reference to a locally declared variable
• (Complex) Flag when capture-list contains a reference to a locally declared variable and the lambda is passed to a non-const and non-local context

F.54: When writing a lambda that captures this or any class data member, don’t use [=] default capture

Reason

It’s confusing. Writing [=] in a member function appears to capture by value, but actually captures data members by reference because it actually captures the invisible this pointer by value. If you meant to do that, write this explicitly.

Example

class My_class {
    int x = 0;
    // ...

    void f()
    {
        int i = 0;
        // ...

        auto lambda = [=] { use(i, x); };   // BAD: "looks like" copy/value capture
        // [&] has identical semantics and copies the this pointer under the current rules
        // [=,this] and [&,this] are not much better, and confusing

        x = 42;
        lambda(); // calls use(0, 42);
        x = 43;
        lambda(); // calls use(0, 43);

        // ...

        auto lambda2 = [i, this] { use(i, x); }; // ok, most explicit and least confusing

        // ...
    }
};

Note

If you intend to capture a copy of all class data members, consider C++17 [*this].

Enforcement

• Flag any lambda capture-list that specifies a capture-default of [=] and also captures this (whether explicitly or via the default capture and a use of this in the body)

F.55: Don’t use va_arg arguments

Reason

Reading from a va_arg assumes that the correct type was actually passed.
Passing to varargs assumes the correct type will be read.
This is fragile because it cannot generally be enforced to be safe in the language and so relies on programmer discipline to get it right.

Example

int sum(...)
{
    // ...
    while (/*...*/)
        result += va_arg(list, int); // BAD, assumes it will be passed ints
    // ...
}

sum(3, 2); // ok
sum(3.14159, 2.71828); // BAD, undefined

template
auto sum(Args... args) // GOOD, and much more flexible
{
    return (... + args); // note: C++17 "fold expression"
}

sum(3, 2); // ok: 5
sum(3.14159, 2.71828); // ok: ~5.85987

Alternatives

• overloading
• variadic templates
• variant arguments
• initializer_list (homogeneous)

Note

Declaring a ... parameter is sometimes useful for techniques that don’t involve actual argument passing, notably to declare “take-anything” functions so as to disable “everything else” in an overload set or express a catchall case in a template metaprogram.

Enforcement

• Issue a diagnostic for using va_list, va_start, or va_arg.
• Issue a diagnostic for passing an argument to a vararg parameter of a function that does not offer an overload for a more specific type in the position of the vararg. To fix: Use a different function, or [[suppress(types)]].

F.56: Avoid unnecessary condition nesting

Reason

Shallow nesting of conditions makes the code easier to follow. It also makes the intent clearer.
Strive to place the essential code at outermost scope, unless this obscures intent.

Example

Use a guard-clause to take care of exceptional cases and return early.

// Bad: Deep nesting
void foo() {
    ...
    if (x) {
        computeImportantThings(x);
    }
}

// Bad: Still a redundant else.
void foo() {
    ...
    if (!x) {
        return;
    }
    else {
        computeImportantThings(x);
    }
}

// Good: Early return, no redundant else
void foo() {
    ...
    if (!x)
        return;

    computeImportantThings(x);
}

Example

// Bad: Unnecessary nesting of conditions
void foo() {
    ...
    if (x) {
        if (y) {
            computeImportantThings(x);
        }
    }
}

// Good: Merge conditions + return early
void foo() {
    ...
    if (!(x && y))
        return;

    computeImportantThings(x);
}

Enforcement

Flag a redundant else.
Flag a functions whose body is simply a conditional statement enclosing a block.

C: Classes and class hierarchies 类和类层次结构

A class is a user-defined type, for which a programmer can define the representation, operations, and interfaces.
Class hierarchies are used to organize related classes into hierarchical structures.

Class rule summary:

• C.1: Organize related data into structures (structs or classes)
• C.2: Use class if the class has an invariant; use struct if the data members can vary independently
• C.3: Represent the distinction between an interface and an implementation using a class
• C.4: Make a function a member only if it needs direct access to the representation of a class
• C.5: Place helper functions in the same namespace as the class they support
• C.7: Don’t define a class or enum and declare a variable of its type in the same statement
• C.8: Use class rather than struct if any member is non-public
• C.9: Minimize exposure of members

Subsections:

• C.concrete: Concrete types
• C.ctor: Constructors, assignments, and destructors
• C.con: Containers and other resource handles
• C.lambdas: Function objects and lambdas
• C.hier: Class hierarchies (OOP)
• C.over: Overloading and overloaded operators
• C.union: Unions

C.1: Organize related data into structures (structs or classes)

Reason

Ease of comprehension.
If data is related (for fundamental reasons), that fact should be reflected in code.

Example

void draw(int x, int y, int x2, int y2);  // BAD: unnecessary implicit relationships
void draw(Point from, Point to);          // better

Note

A simple class without virtual functions implies no space or time overhead.

Note

From a language perspective class and struct differ only in the default visibility of their members.

Enforcement

Probably impossible. Maybe a heuristic looking for data items used together is possible.

C.2: Use class if the class has an invariant; use struct if the data members can vary independently

Reason

Readability.
Ease of comprehension.
The use of class alerts the programmer to the need for an invariant.
This is a useful convention.

Note

An invariant is a logical condition for the members of an object that a constructor must establish for the public member functions to assume.
After the invariant is established (typically by a constructor) every member function can be called for the object.
An invariant can be stated informally (e.g., in a comment) or more formally using Expects.

If all data members can vary independently of each other, no invariant is possible.

Example

struct Pair {  // the members can vary independently
    string name;
    int volume;
};

but:

class Date {
public:
    // validate that {yy, mm, dd} is a valid date and initialize
    Date(int yy, Month mm, char dd);
    // ...
private:
    int y;
    Month m;
    char d;    // day
};

Note

If a class has any private data, a user cannot completely initialize an object without the use of a constructor.
Hence, the class definer will provide a constructor and must specify its meaning.
This effectively means the definer need to define an invariant.

See also:

• define a class with private data as class
• Prefer to place the interface first in a class
• minimize exposure of members
• Avoid protected data

Enforcement

Look for structs with all data private and classes with public members.

C.3: Represent the distinction between an interface and an implementation using a class

Reason

An explicit distinction between interface and implementation improves readability and simplifies maintenance.

Example

class Date {
public:
    Date();
    // validate that {yy, mm, dd} is a valid date and initialize
    Date(int yy, Month mm, char dd);

    int day() const;
    Month month() const;
    // ...
private:
    // ... some representation ...
};

For example, we can now change the representation of a Date without affecting its users (recompilation is likely, though).

Note

Using a class in this way to represent the distinction between interface and implementation is of course not the only way.
For example, we can use a set of declarations of freestanding functions in a namespace, an abstract base class, or a function template with concepts to represent an interface.
The most important issue is to explicitly distinguish between an interface and its implementation “details.”
Ideally, and typically, an interface is far more stable than its implementation(s).

Enforcement

???

C.4: Make a function a member only if it needs direct access to the representation of a class

Reason

Less coupling than with member functions, fewer functions that can cause trouble by modifying object state, reduces the number of functions that needs to be modified after a change in representation.

Example

class Date {
    // ... relatively small interface ...
};

// helper functions:
Date next_weekday(Date);
bool operator==(Date, Date);

The “helper functions” have no need for direct access to the representation of a Date.

Note

This rule becomes even better if C++ gets “uniform function call”.

Exception

The language requires virtual functions to be members, and not all virtual functions directly access data.
In particular, members of an abstract class rarely do.

Note multi-methods.

Exception

The language requires operators =, (), [], and -> to be members.

Exception

An overload set could have some members that do not directly access private data:

class Foobar {
public:
    void foo(long x) { /* manipulate private data */ }
    void foo(double x) { foo(std::lround(x)); }
    // ...
private:
    // ...
};

Exception

Similarly, a set of functions could be designed to be used in a chain:

x.scale(0.5).rotate(45).set_color(Color::red);

Typically, some but not all of such functions directly access private data.

Enforcement

• Look for non-virtual member functions that do not touch data members directly.
  The snag is that many member functions that do not need to touch data members directly do.
• Ignore virtual functions.
• Ignore functions that are part of an overload set out of which at least one function accesses private members.
• Ignore functions returning this.

C.5: Place helper functions in the same namespace as the class they support

Reason

A helper function is a function (usually supplied by the writer of a class) that does not need direct access to the representation of the class, yet is seen as part of the useful interface to the class.
Placing them in the same namespace as the class makes their relationship to the class obvious and allows them to be found by argument dependent lookup.

Example

namespace Chrono { // here we keep time-related services

    class Time { /* ... */ };
    class Date { /* ... */ };

    // helper functions:
    bool operator==(Date, Date);
    Date next_weekday(Date);
    // ...
}

Note

This is especially important for overloaded operators.

Enforcement

• Flag global functions taking argument types from a single namespace.

C.7: Don’t define a class or enum and declare a variable of its type in the same statement

Reason

Mixing a type definition and the definition of another entity in the same declaration is confusing and unnecessary.

Example, bad

struct Data { /*...*/ } data{ /*...*/ };

Example, good

struct Data { /*...*/ };
Data data{ /*...*/ };

Enforcement

• Flag if the } of a class or enumeration definition is not followed by a ;. The ; is missing.

C.8: Use class rather than struct if any member is non-public

Reason

Readability.
To make it clear that something is being hidden/abstracted.
This is a useful convention.

Example, bad

struct Date {
    int d, m;

    Date(int i, Month m);
    // ... lots of functions ...
private:
    int y;  // year
};

There is nothing wrong with this code as far as the C++ language rules are concerned,
but nearly everything is wrong from a design perspective.
The private data is hidden far from the public data.
The data is split in different parts of the class declaration.
Different parts of the data have different access.
All of this decreases readability and complicates maintenance.

Note

Prefer to place the interface first in a class, see NL.16.

Enforcement

Flag classes declared with struct if there is a private or protected member.

C.9: Minimize exposure of members

Reason

Encapsulation.
Information hiding.
Minimize the chance of unintended access.
This simplifies maintenance.

Example

template
struct pair {
    T a;
    U b;
    // ...
};

Whatever we do in the //-part, an arbitrary user of a pair can arbitrarily and independently change its a and b.
In a large code base, we cannot easily find which code does what to the members of pair.
This might be exactly what we want, but if we want to enforce a relation among members, we need to make them private
and enforce that relation (invariant) through constructors and member functions.
For example:

class Distance {
public:
    // ...
    double meters() const { return magnitude*unit; }
    void set_unit(double u)
    {
            // ... check that u is a factor of 10 ...
            // ... change magnitude appropriately ...
            unit = u;
    }
    // ...
private:
    double magnitude;
    double unit;    // 1 is meters, 1000 is kilometers, 0.001 is millimeters, etc.
};

Note

If the set of direct users of a set of variables cannot be easily determined, the type or usage of that set cannot be (easily) changed/improved.
For public and protected data, that’s usually the case.

Example

A class can provide two interfaces to its users.
One for derived classes (protected) and one for general users (public).
For example, a derived class might be allowed to skip a run-time check because it has already guaranteed correctness:

class Foo {
public:
    int bar(int x) { check(x); return do_bar(x); }
    // ...
protected:
    int do_bar(int x); // do some operation on the data
    // ...
private:
    // ... data ...
};

class Dir : public Foo {
    //...
    int mem(int x, int y)
    {
        /* ... do something ... */
        return do_bar(x + y); // OK: derived class can bypass check
    }
};

void user(Foo& x)
{
    int r1 = x.bar(1);      // OK, will check
    int r2 = x.do_bar(2);   // error: would bypass check
    // ...
}

Note

protected data is a bad idea.

Note

Prefer the order public members before protected members before private members; see NL.16.

Enforcement

• Flag protected data.
• Flag mixtures of public and private data

C.concrete: Concrete types

Concrete type rule summary:

• C.10: Prefer concrete types over class hierarchies
• C.11: Make concrete types regular
• C.12: Don’t make data members const or references in a copyable or movable type

C.10: Prefer concrete types over class hierarchies

Reason

A concrete type is fundamentally simpler than a type in a class hierarchy:
easier to design, easier to implement, easier to use, easier to reason about, smaller, and faster.
You need a reason (use cases) for using a hierarchy.

Example

class Point1 {
    int x, y;
    // ... operations ...
    // ... no virtual functions ...
};

class Point2 {
    int x, y;
    // ... operations, some virtual ...
    virtual ~Point2();
};

void use()
{
    Point1 p11 {1, 2};   // make an object on the stack
    Point1 p12 {p11};    // a copy

    auto p21 = make_unique(1, 2);   // make an object on the free store
    auto p22 = p21->clone();                // make a copy
    // ...
}

If a class is part of a hierarchy, we (in real code if not necessarily in small examples) must manipulate its objects through pointers or references.
That implies more memory overhead, more allocations and deallocations, and more run-time overhead to perform the resulting indirections.

Note

Concrete types can be stack-allocated and be members of other classes.

Note

The use of indirection is fundamental for run-time polymorphic interfaces.
The allocation/deallocation overhead is not (that’s just the most common case).
We can use a base class as the interface of a scoped object of a derived class.
This is done where dynamic allocation is prohibited (e.g. hard-real-time) and to provide a stable interface to some kinds of plug-ins.

Enforcement

???

C.11: Make concrete types regular

Reason

Regular types are easier to understand and reason about than types that are not regular (irregularities requires extra effort to understand and use).

The C++ built-in types are regular, and so are standard-library classes such as string, vector, and map. Concrete classes without assignment and equality can be defined, but they are (and should be) rare.

Example

struct Bundle {
    string name;
    vector vr;
};

bool operator==(const Bundle& a, const Bundle& b)
{
    return a.name == b.name && a.vr == b.vr;
}

Bundle b1 { "my bundle", {r1, r2, r3}};
Bundle b2 = b1;
if (!(b1 == b2)) error("impossible!");
b2.name = "the other bundle";
if (b1 == b2) error("No!");

In particular, if a concrete type is copyable, prefer to also give it an equality comparison operator, and ensure that a = b implies a == b.

Note

For structs intended to be shared with C code, defining operator== may not be feasible.

Note

Handles for resources that cannot be cloned, e.g., a scoped_lock for a mutex, are concrete types but typically cannot be copied (instead, they can usually be moved),
so they can’t be regular; instead, they tend to be move-only.

Enforcement

???

C.12: Don’t make data members const or references in a copyable or movable type

Reason

const and reference data members are not useful in a copyable or movable type, and make such types difficult to use by making them at least partly uncopyable/unmovable for subtle reasons.

Example; bad

class bad {
    const int i;    // bad
    string& s;      // bad
    // ...
};

The const and & data members make this class “only-sort-of-copyable” – copy-constructible but not copy-assignable.

Note

If you need a member to point to something, use a pointer (raw or smart, and gsl::not_null if it should not be null) instead of a reference.

Enforcement

Flag a data member that is const, &, or && in a type that has any copy or move operation.

C.ctor: Constructors, assignments, and destructors

These functions control the lifecycle of objects: creation, copy, move, and destruction.
Define constructors to guarantee and simplify initialization of classes.

These are default operations:

• a default constructor: X()
• a copy constructor: X(const X&)
• a copy assignment: operator=(const X&)
• a move constructor: X(X&&)
• a move assignment: operator=(X&&)
• a destructor: ~X()

By default, the compiler defines each of these operations if it is used, but the default can be suppressed.

The default operations are a set of related operations that together implement the lifecycle semantics of an object.
By default, C++ treats classes as value-like types, but not all types are value-like.

Set of default operations rules:

• C.20: If you can avoid defining any default operations, do
• C.21: If you define or =delete any copy, move, or destructor function, define or =delete them all
• C.22: Make default operations consistent

Destructor rules:

• C.30: Define a destructor if a class needs an explicit action at object destruction
• C.31: All resources acquired by a class must be released by the class’s destructor
• C.32: If a class has a raw pointer (T*) or reference (T&), consider whether it might be owning
• C.33: If a class has an owning pointer member, define a destructor
• C.35: A base class destructor should be either public and virtual, or protected and non-virtual
• C.36: A destructor must not fail
• C.37: Make destructors noexcept

Constructor rules:

• C.40: Define a constructor if a class has an invariant
• C.41: A constructor should create a fully initialized object
• C.42: If a constructor cannot construct a valid object, throw an exception
• C.43: Ensure that a copyable class has a default constructor
• C.44: Prefer default constructors to be simple and non-throwing
• C.45: Don’t define a default constructor that only initializes data members; use member initializers instead
• C.46: By default, declare single-argument constructors explicit
• C.47: Define and initialize member variables in the order of member declaration
• C.48: Prefer in-class initializers to member initializers in constructors for constant initializers
• C.49: Prefer initialization to assignment in constructors
• C.50: Use a factory function if you need “virtual behavior” during initialization
• C.51: Use delegating constructors to represent common actions for all constructors of a class
• C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization

Copy and move rules:

• C.60: Make copy assignment non-virtual, take the parameter by const&, and return by non-const&
• C.61: A copy operation should copy
• C.62: Make copy assignment safe for self-assignment
• C.63: Make move assignment non-virtual, take the parameter by &&, and return by non-const&
• C.64: A move operation should move and leave its source in a valid state
• C.65: Make move assignment safe for self-assignment
• C.66: Make move operations noexcept
• C.67: A polymorphic class should suppress public copy/move

Other default operations rules:

• C.80: Use =default if you have to be explicit about using the default semantics
• C.81: Use =delete when you want to disable default behavior (without wanting an alternative)
• C.82: Don’t call virtual functions in constructors and destructors
• C.83: For value-like types, consider providing a noexcept swap function
• C.84: A swap must not fail
• C.85: Make swap noexcept
• C.86: Make == symmetric with respect of operand types and noexcept
• C.87: Beware of == on base classes
• C.89: Make a hash noexcept
• C.90: Rely on constructors and assignment operators, not memset and memcpy

C.defop: Default Operations

By default, the language supplies the default operations with their default semantics.
However, a programmer can disable or replace these defaults.

C.20: If you can avoid defining default operations, do

Reason

It’s the simplest and gives the cleanest semantics.

Example

struct Named_map {
public:
    // ... no default operations declared ...
private:
    string name;
    map rep;
};

Named_map nm;        // default construct
Named_map nm2 {nm};  // copy construct

Since std::map and string have all the special functions, no further work is needed.

Note

This is known as “the rule of zero”.

Enforcement

(Not enforceable) While not enforceable, a good static analyzer can detect patterns that indicate a possible improvement to meet this rule.
For example, a class with a (pointer, size) pair of members and a destructor that deletes the pointer could probably be converted to a vector.

C.21: If you define or =delete any copy, move, or destructor function, define or =delete them all

Reason

The semantics of copy, move, and destruction are closely related, so if one needs to be declared, the odds are that others need consideration too.

Declaring any copy/move/destructor function,
even as =default or =delete, will suppress the implicit declaration
of a move constructor and move assignment operator.
Declaring a move constructor or move assignment operator, even as
=default or =delete, will cause an implicitly generated copy constructor
or implicitly generated copy assignment operator to be defined as deleted.
So as soon as any of these are declared, the others should
all be declared to avoid unwanted effects like turning all potential moves
into more expensive copies, or making a class move-only.

Example, bad

struct M2 {   // bad: incomplete set of copy/move/destructor operations
public:
    // ...
    // ... no copy or move operations ...
    ~M2() { delete[] rep; }
private:
    pair* rep;  // zero-terminated set of pairs
};

void use()
{
    M2 x;
    M2 y;
    // ...
    x = y;   // the default assignment
    // ...
}

Given that “special attention” was needed for the destructor (here, to deallocate), the likelihood that the implicitly-defined copy and move assignment operators will be correct is low (here, we would get double deletion).

Note

This is known as “the rule of five.”

Note

If you want a default implementation (while defining another), write =default to show you’re doing so intentionally for that function.
If you don’t want a generated default function, suppress it with =delete.

Example, good

When a destructor needs to be declared just to make it virtual, it can be
defined as defaulted.

class AbstractBase {
public:
    virtual void foo() = 0;  // at least one abstract method to make the class abstract
    virtual ~AbstractBase() = default;
    // ...
};

To prevent slicing as per C.67,
make the copy and move operations protected or =deleted, and add a clone:

class CloneableBase {
public:
    virtual unique_ptr clone() const;
    virtual ~CloneableBase() = default;
    CloneableBase() = default;
    CloneableBase(const CloneableBase&) = delete;
    CloneableBase& operator=(const CloneableBase&) = delete;
    CloneableBase(CloneableBase&&) = delete;
    CloneableBase& operator=(CloneableBase&&) = delete;
    // ... other constructors and functions ...
};

Defining only the move operations or only the copy operations would have the
same effect here, but stating the intent explicitly for each special member
makes it more obvious to the reader.

Note

Compilers enforce much of this rule and ideally warn about any violation.

Note

Relying on an implicitly generated copy operation in a class with a destructor is deprecated.

Note

Writing these functions can be error-prone.
Note their argument types:

class X {
public:
    // ...
    virtual ~X() = default;               // destructor (virtual if X is meant to be a base class)
    X(const X&) = default;                // copy constructor
    X& operator=(const X&) = default;     // copy assignment
    X(X&&) noexcept = default;            // move constructor
    X& operator=(X&&) noexcept = default; // move assignment
};

A minor mistake (such as a misspelling, leaving out a const, using & instead of &&, or leaving out a special function) can lead to errors or warnings.
To avoid the tedium and the possibility of errors, try to follow the rule of zero.

Enforcement

(Simple) A class should have a declaration (even a =delete one) for either all or none of the copy/move/destructor functions.

C.22: Make default operations consistent

Reason

The default operations are conceptually a matched set. Their semantics are interrelated.
Users will be surprised if copy/move construction and copy/move assignment do logically different things. Users will be surprised if constructors and destructors do not provide a consistent view of resource management. Users will be surprised if copy and move don’t reflect the way constructors and destructors work.

Example, bad

class Silly {   // BAD: Inconsistent copy operations
    class Impl {
        // ...
    };
    shared_ptr p;
public:
    Silly(const Silly& a) : p(make_shared()) { *p = *a.p; }   // deep copy
    Silly& operator=(const Silly& a) { p = a.p; return *this; }   // shallow copy
    // ...
};

These operations disagree about copy semantics. This will lead to confusion and bugs.

Enforcement

• (Complex) A copy/move constructor and the corresponding copy/move assignment operator should write to the same member variables at the same level of dereference.
• (Complex) Any member variables written in a copy/move constructor should also be initialized by all other constructors.
• (Complex) If a copy/move constructor performs a deep copy of a member variable, then the destructor should modify the member variable.
• (Complex) If a destructor is modifying a member variable, that member variable should be written in any copy/move constructors or assignment operators.

C.dtor: Destructors

“Does this class need a destructor?” is a surprisingly insightful design question.
For most classes the answer is “no” either because the class holds no resources or because destruction is handled by the rule of zero;
that is, its members can take care of themselves as concerns destruction.
If the answer is “yes”, much of the design of the class follows (see the rule of five).

C.30: Define a destructor if a class needs an explicit action at object destruction

Reason

A destructor is implicitly invoked at the end of an object’s lifetime.
If the default destructor is sufficient, use it.
Only define a non-default destructor if a class needs to execute code that is not already part of its members’ destructors.

Example

template
struct final_action {   // slightly simplified
    A act;
    final_action(A a) : act{a} {}
    ~final_action() { act(); }
};

template
final_action finally(A act)   // deduce action type
{
    return final_action{act};
}

void test()
{
    auto act = finally([] { cout << "Exit test\n"; });  // establish exit action
    // ...
    if (something) return;   // act done here
    // ...
} // act done here

The whole purpose of final_action is to get a piece of code (usually a lambda) executed upon destruction.

Note

There are two general categories of classes that need a user-defined destructor:

• A class with a resource that is not already represented as a class with a destructor, e.g., a vector or a transaction class.
• A class that exists primarily to execute an action upon destruction, such as a tracer or final_action.

Example, bad

class Foo {   // bad; use the default destructor
public:
    // ...
    ~Foo() { s = ""; i = 0; vi.clear(); }  // clean up
private:
    string s;
    int i;
    vector vi;
};

The default destructor does it better, more efficiently, and can’t get it wrong.

Note

If the default destructor is needed, but its generation has been suppressed (e.g., by defining a move constructor), use =default.

Enforcement

Look for likely “implicit resources”, such as pointers and references. Look for classes with destructors even though all their data members have destructors.

C.31: All resources acquired by a class must be released by the class’s destructor

Reason

Prevention of resource leaks, especially in error cases.

Note

For resources represented as classes with a complete set of default operations, this happens automatically.

Example

class X {
    ifstream f;   // might own a file
    // ... no default operations defined or =deleted ...
};

X’s ifstream implicitly closes any file it might have open upon destruction of its X.

Example, bad

class X2 {     // bad
    FILE* f;   // might own a file
    // ... no default operations defined or =deleted ...
};

X2 might leak a file handle.

Note

What about a socket that won’t close? A destructor, close, or cleanup operation should never fail.
If it does nevertheless, we have a problem that has no really good solution.
For starters, the writer of a destructor does not know why the destructor is called and cannot “refuse to act” by throwing an exception.
See discussion.
To make the problem worse, many “close/release” operations are not retryable.
Many have tried to solve this problem, but no general solution is known.
If at all possible, consider failure to close/cleanup a fundamental design error and terminate.

Note

A class can hold pointers and references to objects that it does not own.
Obviously, such objects should not be deleted by the class’s destructor.
For example:

Preprocessor pp { /* ... */ };
Parser p { pp, /* ... */ };
Type_checker tc { p, /* ... */ };

Here p refers to pp but does not own it.

Enforcement

• (Simple) If a class has pointer or reference member variables that are owners
  (e.g., deemed owners by using gsl::owner), then they should be referenced in its destructor.
• (Hard) Determine if pointer or reference member variables are owners when there is no explicit statement of ownership
  (e.g., look into the constructors).

C.32: If a class has a raw pointer (T*) or reference (T&), consider whether it might be owning

Reason

There is a lot of code that is non-specific about ownership.

Example

class legacy_class
{
    foo* m_owning;   // Bad: change to unique_ptr or owner
    bar* m_observer; // OK: keep
}

The only way to determine ownership may be code analysis.

Note

Ownership should be clear in new code (and refactored legacy code) according to R.20 for owning
pointers and R.3 for non-owning pointers. References should never own R.4.

Enforcement

Look at the initialization of raw member pointers and member references and see if an allocation is used.

C.33: If a class has an owning pointer member, define a destructor

Reason

An owned object must be deleted upon destruction of the object that owns it.

Example

A pointer member could represent a resource.
A T* should not do so, but in older code, that’s common.
Consider a T* a possible owner and therefore suspect.

template
class Smart_ptr {
    T* p;   // BAD: vague about ownership of *p
    // ...
public:
    // ... no user-defined default operations ...
};

void use(Smart_ptr p1)
{
    // error: p2.p leaked (if not nullptr and not owned by some other code)
    auto p2 = p1;
}

Note that if you define a destructor, you must define or delete all default operations:

template
class Smart_ptr2 {
    T* p;   // BAD: vague about ownership of *p
    // ...
public:
    // ... no user-defined copy operations ...
    ~Smart_ptr2() { delete p; }  // p is an owner!
};

void use(Smart_ptr2 p1)
{
    auto p2 = p1;   // error: double deletion
}

The default copy operation will just copy the p1.p into p2.p leading to a double destruction of p1.p. Be explicit about ownership:

template
class Smart_ptr3 {
    owner p;   // OK: explicit about ownership of *p
    // ...
public:
    // ...
    // ... copy and move operations ...
    ~Smart_ptr3() { delete p; }
};

void use(Smart_ptr3 p1)
{
    auto p2 = p1;   // OK: no double deletion
}

Note

Often the simplest way to get a destructor is to replace the pointer with a smart pointer (e.g., std::unique_ptr) and let the compiler arrange for proper destruction to be done implicitly.

Note

Why not just require all owning pointers to be “smart pointers”?
That would sometimes require non-trivial code changes and might affect ABIs.

Enforcement

• A class with a pointer data member is suspect.
• A class with an owner should define its default operations.

C.35: A base class destructor should be either public and virtual, or protected and non-virtual

Reason

To prevent undefined behavior.
If the destructor is public, then calling code can attempt to destroy a derived class object through a base class pointer, and the result is undefined if the base class’s destructor is non-virtual.
If the destructor is protected, then calling code cannot destroy through a base class pointer and the destructor does not need to be virtual; it does need to be protected, not private, so that derived destructors can invoke it.
In general, the writer of a base class does not know the appropriate action to be done upon destruction.

Discussion

See this in the Discussion section.

Example, bad

struct Base {  // BAD: implicitly has a public non-virtual destructor
    virtual void f();
};

struct D : Base {
    string s {"a resource needing cleanup"};
    ~D() { /* ... do some cleanup ... */ }
    // ...
};

void use()
{
    unique_ptr p = make_unique();
    // ...
} // p's destruction calls ~Base(), not ~D(), which leaks D::s and possibly more

Note

A virtual function defines an interface to derived classes that can be used without looking at the derived classes.
If the interface allows destroying, it should be safe to do so.

Note

A destructor must be non-private or it will prevent using the type:

class X {
    ~X();   // private destructor
    // ...
};

void use()
{
    X a;                        // error: cannot destroy
    auto p = make_unique();  // error: cannot destroy
}

Exception

We can imagine one case where you could want a protected virtual destructor: When an object of a derived type (and only of such a type) should be allowed to destroy another object (not itself) through a pointer to base. We haven’t seen such a case in practice, though.

Enforcement

• A class with any virtual functions should have a destructor that is either public and virtual or else protected and non-virtual.
• If a class inherits publicly from a base class, the base class should have a destructor that is either public and virtual or else protected and non-virtual.

C.36: A destructor must not fail

Reason

In general we do not know how to write error-free code if a destructor should fail.
The standard library requires that all classes it deals with have destructors that do not exit by throwing.

Example

class X {
public:
    ~X() noexcept;
    // ...
};

X::~X() noexcept
{
    // ...
    if (cannot_release_a_resource) terminate();
    // ...
}

Note

Many have tried to devise a fool-proof scheme for dealing with failure in destructors.
None have succeeded to come up with a general scheme.
This can be a real practical problem: For example, what about a socket that won’t close?
The writer of a destructor does not know why the destructor is called and cannot “refuse to act” by throwing an exception.
See discussion.
To make the problem worse, many “close/release” operations are not retryable.
If at all possible, consider failure to close/cleanup a fundamental design error and terminate.

Note

Declare a destructor noexcept. That will ensure that it either completes normally or terminates the program.

Note

If a resource cannot be released and the program must not fail, try to signal the failure to the rest of the system somehow
(maybe even by modifying some global state and hope something will notice and be able to take care of the problem).
Be fully aware that this technique is special-purpose and error-prone.
Consider the “my connection will not close” example.
Probably there is a problem at the other end of the connection and only a piece of code responsible for both ends of the connection can properly handle the problem.
The destructor could send a message (somehow) to the responsible part of the system, consider that to have closed the connection, and return normally.

Note

If a destructor uses operations that could fail, it can catch exceptions and in some cases still complete successfully
(e.g., by using a different clean-up mechanism from the one that threw an exception).

Enforcement

(Simple) A destructor should be declared noexcept if it could throw.

C.37: Make destructors noexcept

Reason

A destructor must not fail. If a destructor tries to exit with an exception, it’s a bad design error and the program had better terminate.

Note

A destructor (either user-defined or compiler-generated) is implicitly declared noexcept (independently of what code is in its body) if all of the members of its class have noexcept destructors. By explicitly marking destructors noexcept, an author guards against the destructor becoming implicitly noexcept(false) through the addition or modification of a class member.

Example

Not all destructors are noexcept by default; one throwing member poisons the whole class hierarchy

struct X {
    Details x;  // happens to have a throwing destructor
    // ...
    ~X() { }    // implicitly noexcept(false); aka can throw
};

So, if in doubt, declare a destructor noexcept.

Note

Why not then declare all destructors noexcept?
Because that would in many cases – especially simple cases – be distracting clutter.

Enforcement

(Simple) A destructor should be declared noexcept if it could throw.

C.ctor: Constructors

A constructor defines how an object is initialized (constructed).

C.40: Define a constructor if a class has an invariant

Reason

That’s what constructors are for.

Example

class Date {  // a Date represents a valid date
              // in the January 1, 1900 to December 31, 2100 range
    Date(int dd, int mm, int yy)
        :d{dd}, m{mm}, y{yy}
    {
        if (!is_valid(d, m, y)) throw Bad_date{};  // enforce invariant
    }
    // ...
private:
    int d, m, y;
};

It is often a good idea to express the invariant as an Ensures on the constructor.

Note

A constructor can be used for convenience even if a class does not have an invariant. For example:

struct Rec {
    string s;
    int i {0};
    Rec(const string& ss) : s{ss} {}
    Rec(int ii) :i{ii} {}
};

Rec r1 {7};
Rec r2 {"Foo bar"};

Note

The C++11 initializer list rule eliminates the need for many constructors. For example:

struct Rec2{
    string s;
    int i;
    Rec2(const string& ss, int ii = 0) :s{ss}, i{ii} {}   // redundant
};

Rec2 r1 {"Foo", 7};
Rec2 r2 {"Bar"};

The Rec2 constructor is redundant.
Also, the default for int would be better done as a member initializer.

See also: construct valid object and constructor throws.

Enforcement

• Flag classes with user-defined copy operations but no constructor (a user-defined copy is a good indicator that the class has an invariant)

C.41: A constructor should create a fully initialized object

Reason

A constructor establishes the invariant for a class. A user of a class should be able to assume that a constructed object is usable.

Example, bad

class X1 {
    FILE* f;   // call init() before any other function
    // ...
public:
    X1() {}
    void init();   // initialize f
    void read();   // read from f
    // ...
};

void f()
{
    X1 file;
    file.read();   // crash or bad read!
    // ...
    file.init();   // too late
    // ...
}

Compilers do not read comments.

Exception

If a valid object cannot conveniently be constructed by a constructor, use a factory function.

Enforcement

• (Simple) Every constructor should initialize every member variable (either explicitly, via a delegating ctor call or via default construction).
• (Unknown) If a constructor has an Ensures contract, try to see if it holds as a postcondition.

Note

If a constructor acquires a resource (to create a valid object), that resource should be released by the destructor.
The idiom of having constructors acquire resources and destructors release them is called RAII (“Resource Acquisition Is Initialization”).

C.42: If a constructor cannot construct a valid object, throw an exception

Reason

Leaving behind an invalid object is asking for trouble.

Example

class X2 {
    FILE* f;
    // ...
public:
    X2(const string& name)
        :f{fopen(name.c_str(), "r")}
    {
        if (!f) throw runtime_error{"could not open" + name};
        // ...
    }

    void read();      // read from f
    // ...
};

void f()
{
    X2 file {"Zeno"}; // throws if file isn't open
    file.read();      // fine
    // ...
}

Example, bad

class X3 {     // bad: the constructor leaves a non-valid object behind
    FILE* f;   // call is_valid() before any other function
    bool valid;
    // ...
public:
    X3(const string& name)
        :f{fopen(name.c_str(), "r")}, valid{false}
    {
        if (f) valid = true;
        // ...
    }

    bool is_valid() { return valid; }
    void read();   // read from f
    // ...
};

void f()
{
    X3 file {"Heraclides"};
    file.read();   // crash or bad read!
    // ...
    if (file.is_valid()) {
        file.read();
        // ...
    }
    else {
        // ... handle error ...
    }
    // ...
}

Note

For a variable definition (e.g., on the stack or as a member of another object) there is no explicit function call from which an error code could be returned.
Leaving behind an invalid object and relying on users to consistently check an is_valid() function before use is tedious, error-prone, and inefficient.

Exception

There are domains, such as some hard-real-time systems (think airplane controls) where (without additional tool support) exception handling is not sufficiently predictable from a timing perspective.
There the is_valid() technique must be used. In such cases, check is_valid() consistently and immediately to simulate RAII.

Alternative

If you feel tempted to use some “post-constructor initialization” or “two-stage initialization” idiom, try not to do that.
If you really have to, look at factory functions.

Note

One reason people have used init() functions rather than doing the initialization work in a constructor has been to avoid code replication.
Delegating constructors and default member initialization do that better.
Another reason has been to delay initialization until an object is needed; the solution to that is often not to declare a variable until it can be properly initialized

Enforcement

???

C.43: Ensure that a copyable class has a default constructor

Reason

That is, ensure that if a concrete class is copyable it also satisfies the rest of “semiregular.”

Many language and library facilities rely on default constructors to initialize their elements, e.g. T a[10] and std::vector v(10).
A default constructor often simplifies the task of defining a suitable moved-from state for a type that is also copyable.

Example

class Date { // BAD: no default constructor
public:
    Date(int dd, int mm, int yyyy);
    // ...
};

vector vd1(1000);   // default Date needed here
vector vd2(1000, Date{7, Month::October, 1885});   // alternative

The default constructor is only auto-generated if there is no user-declared constructor, hence it’s impossible to initialize the vector vd1 in the example above.
The absence of a default value can cause surprises for users and complicate its use, so if one can be reasonably defined, it should be.

Date is chosen to encourage thought:
There is no “natural” default date (the big bang is too far back in time to be useful for most people), so this example is non-trivial.
{0, 0, 0} is not a valid date in most calendar systems, so choosing that would be introducing something like floating-point’s NaN.
However, most realistic Date classes have a “first date” (e.g. January 1, 1970 is popular), so making that the default is usually trivial.

class Date {
public:
    Date(int dd, int mm, int yyyy);
    Date() = default; // [See also](#Rc-default)
    // ...
private:
    int dd {1};
    int mm {1};
    int yyyy {1970};
    // ...
};

vector vd1(1000);

Note

A class with members that all have default constructors implicitly gets a default constructor:

struct X {
    string s;
    vector v;
};

X x; // means X{{}, {}}; that is the empty string and the empty vector

Beware that built-in types are not properly default constructed:

struct X {
    string s;
    int i;
};

void f()
{
    X x;    // x.s is initialized to the empty string; x.i is uninitialized

    cout << x.s << ' ' << x.i << '\n';
    ++x.i;
}

Statically allocated objects of built-in types are by default initialized to 0, but local built-in variables are not.
Beware that your compiler might default initialize local built-in variables, whereas an optimized build will not.
Thus, code like the example above might appear to work, but it relies on undefined behavior.
Assuming that you want initialization, an explicit default initialization can help:

struct X {
    string s;
    int i {};   // default initialize (to 0)
};

Notes

Classes that don’t have a reasonable default construction are usually not copyable either, so they don’t fall under this guideline.

For example, a base class should not be copyable, and so does not necessarily need a default constructor:

// Shape is an abstract base class, not a copyable type.
// It might or might not need a default constructor.
struct Shape {
    virtual void draw() = 0;
    virtual void rotate(int) = 0;
    // =delete copy/move functions
    // ...
};

A class that must acquire a caller-provided resource during construction often cannot have a default constructor, but it does not fall under this guideline because such a class is usually not copyable anyway:

// std::lock_guard is not a copyable type.
// It does not have a default constructor.
lock_guard g {mx};  // guard the mutex mx
lock_guard g2;      // error: guarding nothing

A class that has a “special state” that must be handled separately from other states by member functions or users causes extra work
(and most likely more errors). Such a type can naturally use the special state as a default constructed value, whether or not it is copyable:

// std::ofstream is not a copyable type.
// It does happen to have a default constructor
// that goes along with a special "not open" state.
ofstream out {"Foobar"};
// ...
out << log(time, transaction);

Similar special-state types that are copyable, such as copyable smart pointers that have the special state “==nullptr”, should use the special state as their default constructed value.

However, it is preferable to have a default constructor default to a meaningful state such as std::strings "" and std::vectors {}.

Enforcement

• Flag classes that are copyable by = without a default constructor
• Flag classes that are comparable with == but not copyable

C.44: Prefer default constructors to be simple and non-throwing

Reason

Being able to set a value to “the default” without operations that might fail simplifies error handling and reasoning about move operations.

Example, problematic

template
// elem points to space-elem element allocated using new
class Vector0 {
public:
    Vector0() :Vector0{0} {}
    Vector0(int n) :elem{new T[n]}, space{elem + n}, last{elem} {}
    // ...
private:
    own elem;
    T* space;
    T* last;
};

This is nice and general, but setting a Vector0 to empty after an error involves an allocation, which might fail.
Also, having a default Vector represented as {new T[0], 0, 0} seems wasteful.
For example, Vector0 v[100] costs 100 allocations.

Example

template
// elem is nullptr or elem points to space-elem element allocated using new
class Vector1 {
public:
    // sets the representation to {nullptr, nullptr, nullptr}; doesn't throw
    Vector1() noexcept {}
    Vector1(int n) :elem{new T[n]}, space{elem + n}, last{elem} {}
    // ...
private:
    own elem {};
    T* space {};
    T* last {};
};

Using {nullptr, nullptr, nullptr} makes Vector1{} cheap, but a special case and implies run-time checks.
Setting a Vector1 to empty after detecting an error is trivial.

Enforcement

• Flag throwing default constructors

C.45: Don’t define a default constructor that only initializes data members; use in-class member initializers instead

Reason

Using in-class member initializers lets the compiler generate the function for you. The compiler-generated function can be more efficient.

Example, bad

class X1 { // BAD: doesn't use member initializers
    string s;
    int i;
public:
    X1() :s{"default"}, i{1} { }
    // ...
};

Example

class X2 {
    string s {"default"};
    int i {1};
public:
    // use compiler-generated default constructor
    // ...
};

Enforcement

(Simple) A default constructor should do more than just initialize member variables with constants.

C.46: By default, declare single-argument constructors explicit

Reason

To avoid unintended conversions.

Example, bad

class String {
public:
    String(int);   // BAD
    // ...
};

String s = 10;   // surprise: string of size 10

Exception

If you really want an implicit conversion from the constructor argument type to the class type, don’t use explicit:

class Complex {
public:
    Complex(double d);   // OK: we want a conversion from d to {d, 0}
    // ...
};

Complex z = 10.7;   // unsurprising conversion

See also: Discussion of implicit conversions

Note

Copy and move constructors should not be made explicit because they do not perform conversions. Explicit copy/move constructors make passing and returning by value difficult.

Enforcement

(Simple) Single-argument constructors should be declared explicit. Good single argument non-explicit constructors are rare in most code bases. Warn for all that are not on a “positive list”.

C.47: Define and initialize member variables in the order of member declaration

Reason

To minimize confusion and errors. That is the order in which the initialization happens (independent of the order of member initializers).

Example, bad

class Foo {
    int m1;
    int m2;
public:
    Foo(int x) :m2{x}, m1{++x} { }   // BAD: misleading initializer order
    // ...
};

Foo x(1); // surprise: x.m1 == x.m2 == 2

Enforcement

(Simple) A member initializer list should mention the members in the same order they are declared.

See also: Discussion

C.48: Prefer in-class initializers to member initializers in constructors for constant initializers

Reason

Makes it explicit that the same value is expected to be used in all constructors. Avoids repetition. Avoids maintenance problems. It leads to the shortest and most efficient code.

Example, bad

class X {   // BAD
    int i;
    string s;
    int j;
public:
    X() :i{666}, s{"qqq"} { }   // j is uninitialized
    X(int ii) :i{ii} {}         // s is "" and j is uninitialized
    // ...
};

How would a maintainer know whether j was deliberately uninitialized (probably a bad idea anyway) and whether it was intentional to give s the default value "" in one case and qqq in another (almost certainly a bug)? The problem with j (forgetting to initialize a member) often happens when a new member is added to an existing class.

Example

class X2 {
    int i {666};
    string s {"qqq"};
    int j {0};
public:
    X2() = default;        // all members are initialized to their defaults
    X2(int ii) :i{ii} {}   // s and j initialized to their defaults
    // ...
};

Alternative: We can get part of the benefits from default arguments to constructors, and that is not uncommon in older code. However, that is less explicit, causes more arguments to be passed, and is repetitive when there is more than one constructor:

class X3 {   // BAD: inexplicit, argument passing overhead
    int i;
    string s;
    int j;
public:
    X3(int ii = 666, const string& ss = "qqq", int jj = 0)
        :i{ii}, s{ss}, j{jj} { }   // all members are initialized to their defaults
    // ...
};

Enforcement

• (Simple) Every constructor should initialize every member variable (either explicitly, via a delegating ctor call or via default construction).
• (Simple) Default arguments to constructors suggest an in-class initializer might be more appropriate.

C.49: Prefer initialization to assignment in constructors

Reason

An initialization explicitly states that initialization, rather than assignment, is done and can be more elegant and efficient. Prevents “use before set” errors.

Example, good

class A {   // Good
    string s1;
public:
    A(czstring p) : s1{p} { }    // GOOD: directly construct (and the C-string is explicitly named)
    // ...
};

Example, bad

class B {   // BAD
    string s1;
public:
    B(const char* p) { s1 = p; }   // BAD: default constructor followed by assignment
    // ...
};

class C {   // UGLY, aka very bad
    int* p;
public:
    C() { cout << *p; p = new int{10}; }   // accidental use before initialized
    // ...
};

Example, better still

Instead of those const char*s we could use C++17 std::string_view or gsl::span
as a more general way to present arguments to a function:

class D {   // Good
    string s1;
public:
    D(string_view v) : s1{v} { }    // GOOD: directly construct
    // ...
};

C.50: Use a factory function if you need “virtual behavior” during initialization

Reason

If the state of a base class object must depend on the state of a derived part of the object, we need to use a virtual function (or equivalent) while minimizing the window of opportunity to misuse an imperfectly constructed object.

Note

The return type of the factory should normally be unique_ptr by default; if some uses are shared, the caller can move the unique_ptr into a shared_ptr. However, if the factory author knows that all uses of the returned object will be shared uses, return shared_ptr and use make_shared in the body to save an allocation.

Example, bad

class B {
public:
    B()
    {
        /* ... */
        f(); // BAD: C.82: Don't call virtual functions in constructors and destructors
        /* ... */
    }

    virtual void f() = 0;
};

Example

class B {
protected:
    class Token {};

public:
    explicit B(Token) { /* ... */ }  // create an imperfectly initialized object
    virtual void f() = 0;

    template
    static shared_ptr create()    // interface for creating shared objects
    {
        auto p = make_shared(typename T::Token{});
        p->post_initialize();
        return p;
    }

protected:
    virtual void post_initialize()   // called right after construction
        { /* ... */ f(); /* ... */ } // GOOD: virtual dispatch is safe
};

class D : public B {                 // some derived class
protected:
    class Token {};

public:
    explicit D(Token) : B{ B::Token{} } {}
    void f() override { /* ...  */ };

protected:
    template
    friend shared_ptr B::create();
};

shared_ptr p = D::create();  // creating a D object

make_shared requires that the constructor is public. By requiring a protected Token the constructor cannot be publicly called anymore, so we avoid an incompletely constructed object escaping into the wild.
By providing the factory function create(), we make construction (on the free store) convenient.

Note

Conventional factory functions allocate on the free store, rather than on the stack or in an enclosing object.

See also: Discussion

C.51: Use delegating constructors to represent common actions for all constructors of a class

Reason

To avoid repetition and accidental differences.

Example, bad

class Date {   // BAD: repetitive
    int d;
    Month m;
    int y;
public:
    Date(int dd, Month mm, year yy)
        :d{dd}, m{mm}, y{yy}
        { if (!valid(d, m, y)) throw Bad_date{}; }

    Date(int dd, Month mm)
        :d{dd}, m{mm} y{current_year()}
        { if (!valid(d, m, y)) throw Bad_date{}; }
    // ...
};

The common action gets tedious to write and might accidentally not be common.

Example

class Date2 {
    int d;
    Month m;
    int y;
public:
    Date2(int dd, Month mm, year yy)
        :d{dd}, m{mm}, y{yy}
        { if (!valid(d, m, y)) throw Bad_date{}; }

    Date2(int dd, Month mm)
        :Date2{dd, mm, current_year()} {}
    // ...
};

See also: If the “repeated action” is a simple initialization, consider an in-class member initializer.

Enforcement

(Moderate) Look for similar constructor bodies.

C.52: Use inheriting constructors to import constructors into a derived class that does not need further explicit initialization

Reason

If you need those constructors for a derived class, re-implementing them is tedious and error-prone.

Example

std::vector has a lot of tricky constructors, so if I want my own vector, I don’t want to reimplement them:

class Rec {
    // ... data and lots of nice constructors ...
};

class Oper : public Rec {
    using Rec::Rec;
    // ... no data members ...
    // ... lots of nice utility functions ...
};

Example, bad

struct Rec2 : public Rec {
    int x;
    using Rec::Rec;
};

Rec2 r {"foo", 7};
int val = r.x;   // uninitialized

Enforcement

Make sure that every member of the derived class is initialized.

C.copy: Copy and move

Concrete types should generally be copyable, but interfaces in a class hierarchy should not.
Resource handles might or might not be copyable.
Types can be defined to move for logical as well as performance reasons.

C.60: Make copy assignment non-virtual, take the parameter by const&, and return by non-const&

Reason

It is simple and efficient. If you want to optimize for rvalues, provide an overload that takes a && (see F.18).

Example

class Foo {
public:
    Foo& operator=(const Foo& x)
    {
        // GOOD: no need to check for self-assignment (other than performance)
        auto tmp = x;
        swap(tmp); // see C.83
        return *this;
    }
    // ...
};

Foo a;
Foo b;
Foo f();

a = b;    // assign lvalue: copy
a = f();  // assign rvalue: potentially move

Note

The swap implementation technique offers the strong guarantee.

Example

But what if you can get significantly better performance by not making a temporary copy? Consider a simple Vector intended for a domain where assignment of large, equal-sized Vectors is common. In this case, the copy of elements implied by the swap implementation technique could cause an order of magnitude increase in cost:

template
class Vector {
public:
    Vector& operator=(const Vector&);
    // ...
private:
    T* elem;
    int sz;
};

Vector& Vector::operator=(const Vector& a)
{
    if (a.sz > sz) {
        // ... use the swap technique, it can't be bettered ...
        return *this;
    }
    // ... copy sz elements from *a.elem to elem ...
    if (a.sz < sz) {
        // ... destroy the surplus elements in *this and adjust size ...
    }
    return *this;
}

By writing directly to the target elements, we will get only the basic guarantee rather than the strong guarantee offered by the swap technique. Beware of self-assignment.

Alternatives: If you think you need a virtual assignment operator, and understand why that’s deeply problematic, don’t call it operator=. Make it a named function like virtual void assign(const Foo&).
See copy constructor vs. clone().

Enforcement

• (Simple) An assignment operator should not be virtual. Here be dragons!
• (Simple) An assignment operator should return T& to enable chaining, not alternatives like const T& which interfere with composability and putting objects in containers.
• (Moderate) An assignment operator should (implicitly or explicitly) invoke all base and member assignment operators.
  Look at the destructor to determine if the type has pointer semantics or value semantics.

C.61: A copy operation should copy

Reason

That is the generally assumed semantics. After x = y, we should have x == y.
After a copy x and y can be independent objects (value semantics, the way non-pointer built-in types and the standard-library types work) or refer to a shared object (pointer semantics, the way pointers work).

Example

class X {   // OK: value semantics
public:
    X();
    X(const X&);     // copy X
    void modify();   // change the value of X
    // ...
    ~X() { delete[] p; }
private:
    T* p;
    int sz;
};

bool operator==(const X& a, const X& b)
{
    return a.sz == b.sz && equal(a.p, a.p + a.sz, b.p, b.p + b.sz);
}

X::X(const X& a)
    :p{new T[a.sz]}, sz{a.sz}
{
    copy(a.p, a.p + sz, p);
}

X x;
X y = x;
if (x != y) throw Bad{};
x.modify();
if (x == y) throw Bad{};   // assume value semantics

Example

class X2 {  // OK: pointer semantics
public:
    X2();
    X2(const X2&) = default; // shallow copy
    ~X2() = default;
    void modify();          // change the pointed-to value
    // ...
private:
    T* p;
    int sz;
};

bool operator==(const X2& a, const X2& b)
{
    return a.sz == b.sz && a.p == b.p;
}

X2 x;
X2 y = x;
if (x != y) throw Bad{};
x.modify();
if (x != y) throw Bad{};  // assume pointer semantics

Note

Prefer value semantics unless you are building a “smart pointer”. Value semantics is the simplest to reason about and what the standard-library facilities expect.

Enforcement

(Not enforceable)

C.62: Make copy assignment safe for self-assignment

Reason

If x = x changes the value of x, people will be surprised and bad errors will occur (often including leaks).

Example

The standard-library containers handle self-assignment elegantly and efficiently:

std::vector v = {3, 1, 4, 1, 5, 9};
v = v;
// the value of v is still {3, 1, 4, 1, 5, 9}

Note

The default assignment generated from members that handle self-assignment correctly handles self-assignment.

struct Bar {
    vector> v;
    map m;
    string s;
};

Bar b;
// ...
b = b;   // correct and efficient

Note

You can handle self-assignment by explicitly testing for self-assignment, but often it is faster and more elegant to cope without such a test (e.g., using swap).

class Foo {
    string s;
    int i;
public:
    Foo& operator=(const Foo& a);
    // ...
};

Foo& Foo::operator=(const Foo& a)   // OK, but there is a cost
{
    if (this == &a) return *this;
    s = a.s;
    i = a.i;
    return *this;
}

This is obviously safe and apparently efficient.
However, what if we do one self-assignment per million assignments?
That’s about a million redundant tests (but since the answer is essentially always the same, the computer’s branch predictor will guess right essentially every time).
Consider:

Foo& Foo::operator=(const Foo& a)   // simpler, and probably much better
{
    s = a.s;
    i = a.i;
    return *this;
}

std::string is safe for self-assignment and so are int. All the cost is carried by the (rare) case of self-assignment.

Enforcement

(Simple) Assignment operators should not contain the pattern if (this == &a) return *this; ???

C.63: Make move assignment non-virtual, take the parameter by &&, and return by non-const&

Reason

It is simple and efficient.

See: The rule for copy-assignment.

Enforcement

Equivalent to what is done for copy-assignment.

• (Simple) An assignment operator should not be virtual. Here be dragons!
• (Simple) An assignment operator should return T& to enable chaining, not alternatives like const T& which interfere with composability and putting objects in containers.
• (Moderate) A move assignment operator should (implicitly or explicitly) invoke all base and member move assignment operators.

C.64: A move operation should move and leave its source in a valid state

Reason

That is the generally assumed semantics.
After y = std::move(x) the value of y should be the value x had and x should be in a valid state.

Example

class X {   // OK: value semantics
public:
    X();
    X(X&& a) noexcept;  // move X
    X& operator=(X&& a) noexcept; // move-assign X
    void modify();     // change the value of X
    // ...
    ~X() { delete[] p; }
private:
    T* p;
    int sz;
};

X::X(X&& a) noexcept
    :p{a.p}, sz{a.sz}  // steal representation
{
    a.p = nullptr;     // set to "empty"
    a.sz = 0;
}

void use()
{
    X x{};
    // ...
    X y = std::move(x);
    x = X{};   // OK
} // OK: x can be destroyed

Note

Ideally, that moved-from should be the default value of the type.
Ensure that unless there is an exceptionally good reason not to.
However, not all types have a default value and for some types establishing the default value can be expensive.
The standard requires only that the moved-from object can be destroyed.
Often, we can easily and cheaply do better: The standard library assumes that it is possible to assign to a moved-from object.
Always leave the moved-from object in some (necessarily specified) valid state.

Note

Unless there is an exceptionally strong reason not to, make x = std::move(y); y = z; work with the conventional semantics.

Enforcement

(Not enforceable) Look for assignments to members in the move operation. If there is a default constructor, compare those assignments to the initializations in the default constructor.

C.65: Make move assignment safe for self-assignment

Reason

If x = x changes the value of x, people will be surprised and bad errors can occur. However, people don’t usually directly write a self-assignment that turn into a move, but it can occur. However, std::swap is implemented using move operations so if you accidentally do swap(a, b) where a and b refer to the same object, failing to handle self-move could be a serious and subtle error.

Example

class Foo {
    string s;
    int i;
public:
    Foo& operator=(Foo&& a) noexcept;
    // ...
};

Foo& Foo::operator=(Foo&& a) noexcept  // OK, but there is a cost
{
    if (this == &a) return *this;  // this line is redundant
    s = std::move(a.s);
    i = a.i;
    return *this;
}

The one-in-a-million argument against if (this == &a) return *this; tests from the discussion of self-assignment is even more relevant for self-move.

Note

There is no known general way of avoiding an if (this == &a) return *this; test for a move assignment and still get a correct answer (i.e., after x = x the value of x is unchanged).

Note

The ISO standard guarantees only a “valid but unspecified” state for the standard-library containers. Apparently this has not been a problem in about 10 years of experimental and production use. Please contact the editors if you find a counter example. The rule here is more caution and insists on complete safety.

Example

Here is a way to move a pointer without a test (imagine it as code in the implementation a move assignment):

// move from other.ptr to this->ptr
T* temp = other.ptr;
other.ptr = nullptr;
delete ptr; // in self-move, this->ptr is also null; delete is a no-op
ptr = temp; // in self-move, the original ptr is restored

Enforcement

• (Moderate) In the case of self-assignment, a move assignment operator should not leave the object holding pointer members that have been deleted or set to nullptr.
• (Not enforceable) Look at the use of standard-library container types (incl. string) and consider them safe for ordinary (not life-critical) uses.

C.66: Make move operations noexcept

Reason

A throwing move violates most people’s reasonable assumptions.
A non-throwing move will be used more efficiently by standard-library and language facilities.

Example

template
class Vector {
public:
    Vector(Vector&& a) noexcept :elem{a.elem}, sz{a.sz} { a.elem = nullptr; a.sz = 0; }
    Vector& operator=(Vector&& a) noexcept {
        delete elem;
        elem = a.elem; a.elem = nullptr;
        sz   = a.sz;   a.sz   = 0;
        return *this;
    }
    // ...
private:
    T* elem;
    int sz;
};

These operations do not throw.

Example, bad

template
class Vector2 {
public:
    Vector2(Vector2&& a) noexcept { *this = a; }             // just use the copy
    Vector2& operator=(Vector2&& a) noexcept { *this = a; }  // just use the copy
    // ...
private:
    T* elem;
    int sz;
};

This Vector2 is not just inefficient, but since a vector copy requires allocation, it can throw.

Enforcement

(Simple) A move operation should be marked noexcept.

C.67: A polymorphic class should suppress public copy/move

Reason

A polymorphic class is a class that defines or inherits at least one virtual function. It is likely that it will be used as a base class for other derived classes with polymorphic behavior. If it is accidentally passed by value, with the implicitly generated copy constructor and assignment, we risk slicing: only the base portion of a derived object will be copied, and the polymorphic behavior will be corrupted.

If the class has no data, =delete the copy/move functions. Otherwise, make them protected.

Example, bad

class B { // BAD: polymorphic base class doesn't suppress copying
public:
    virtual char m() { return 'B'; }
    // ... nothing about copy operations, so uses default ...
};

class D : public B {
public:
    char m() override { return 'D'; }
    // ...
};

void f(B& b)
{
    auto b2 = b; // oops, slices the object; b2.m() will return 'B'
}

D d;
f(d);

Example

class B { // GOOD: polymorphic class suppresses copying
public:
    B() = default;
    B(const B&) = delete;
    B& operator=(const B&) = delete;
    virtual char m() { return 'B'; }
    // ...
};

class D : public B {
public:
    char m() override { return 'D'; }
    // ...
};

void f(B& b)
{
    auto b2 = b; // ok, compiler will detect inadvertent copying, and protest
}

D d;
f(d);

Note

If you need to create deep copies of polymorphic objects, use clone() functions: see C.130.

Exception

Classes that represent exception objects need both to be polymorphic and copy-constructible.

Enforcement

• Flag a polymorphic class with a public copy operation.
• Flag an assignment of polymorphic class objects.

C.other: Other default operation rules

In addition to the operations for which the language offers default implementations,
there are a few operations that are so foundational that specific rules for their definition are needed:
comparisons, swap, and hash.

C.80: Use =default if you have to be explicit about using the default semantics

Reason

The compiler is more likely to get the default semantics right and you cannot implement these functions better than the compiler.

Example

class Tracer {
    string message;
public:
    Tracer(const string& m) : message{m} { cerr << "entering " << message << '\n'; }
    ~Tracer() { cerr << "exiting " << message << '\n'; }

    Tracer(const Tracer&) = default;
    Tracer& operator=(const Tracer&) = default;
    Tracer(Tracer&&) noexcept = default;
    Tracer& operator=(Tracer&&) noexcept = default;
};

Because we defined the destructor, we must define the copy and move operations. The = default is the best and simplest way of doing that.

Example, bad

class Tracer2 {
    string message;
public:
    Tracer2(const string& m) : message{m} { cerr << "entering " << message << '\n'; }
    ~Tracer2() { cerr << "exiting " << message << '\n'; }

    Tracer2(const Tracer2& a) : message{a.message} {}
    Tracer2& operator=(const Tracer2& a) { message = a.message; return *this; }
    Tracer2(Tracer2&& a) noexcept :message{a.message} {}
    Tracer2& operator=(Tracer2&& a) noexcept { message = a.message; return *this; }
};

Writing out the bodies of the copy and move operations is verbose, tedious, and error-prone. A compiler does it better.

Enforcement

(Moderate) The body of a special operation should not have the same accessibility and semantics as the compiler-generated version, because that would be redundant

C.81: Use =delete when you want to disable default behavior (without wanting an alternative)

Reason

In a few cases, a default operation is not desirable.

Example

class Immortal {
public:
    ~Immortal() = delete;   // do not allow destruction
    // ...
};

void use()
{
    Immortal ugh;   // error: ugh cannot be destroyed
    Immortal* p = new Immortal{};
    delete p;       // error: cannot destroy *p
}

Example

A unique_ptr can be moved, but not copied. To achieve that its copy operations are deleted. To avoid copying it is necessary to =delete its copy operations from lvalues:

template> class unique_ptr {
public:
    // ...
    constexpr unique_ptr() noexcept;
    explicit unique_ptr(pointer p) noexcept;
    // ...
    unique_ptr(unique_ptr&& u) noexcept;   // move constructor
    // ...
    unique_ptr(const unique_ptr&) = delete; // disable copy from lvalue
    // ...
};

unique_ptr make();   // make "something" and return it by moving

void f()
{
    unique_ptr pi {};
    auto pi2 {pi};      // error: no move constructor from lvalue
    auto pi3 {make()};  // OK, move: the result of make() is an rvalue
}

Note that deleted functions should be public.

Enforcement

The elimination of a default operation is (should be) based on the desired semantics of the class. Consider such classes suspect, but maintain a “positive list” of classes where a human has asserted that the semantics is correct.

C.82: Don’t call virtual functions in constructors and destructors

Reason

The function called will be that of the object constructed so far, rather than a possibly overriding function in a derived class.
This can be most confusing.
Worse, a direct or indirect call to an unimplemented pure virtual function from a constructor or destructor results in undefined behavior.

Example, bad

class Base {
public:
    virtual void f() = 0;   // not implemented
    virtual void g();       // implemented with Base version
    virtual void h();       // implemented with Base version
    virtual ~Base();        // implemented with Base version
};

class Derived : public Base {
public:
    void g() override;   // provide Derived implementation
    void h() final;      // provide Derived implementation

    Derived()
    {
        // BAD: attempt to call an unimplemented virtual function
        f();

        // BAD: will call Derived::g, not dispatch further virtually
        g();

        // GOOD: explicitly state intent to call only the visible version
        Derived::g();

        // ok, no qualification needed, h is final
        h();
    }
};

Note that calling a specific explicitly qualified function is not a virtual call even if the function is virtual.

See also factory functions for how to achieve the effect of a call to a derived class function without risking undefined behavior.

Note

There is nothing inherently wrong with calling virtual functions from constructors and destructors.
The semantics of such calls is type safe.
However, experience shows that such calls are rarely needed, easily confuse maintainers, and become a source of errors when used by novices.

Enforcement

• Flag calls of virtual functions from constructors and destructors.

C.83: For value-like types, consider providing a noexcept swap function

Reason

A swap can be handy for implementing a number of idioms, from smoothly moving objects around to implementing assignment easily to providing a guaranteed commit function that enables strongly error-safe calling code. Consider using swap to implement copy assignment in terms of copy construction. See also destructors, deallocation, and swap must never fail.

Example, good

class Foo {
public:
    void swap(Foo& rhs) noexcept
    {
        m1.swap(rhs.m1);
        std::swap(m2, rhs.m2);
    }
private:
    Bar m1;
    int m2;
};

Providing a non-member swap function in the same namespace as your type for callers’ convenience.

void swap(Foo& a, Foo& b)
{
    a.swap(b);
}

Enforcement

• Non-trivially copyable types should provide a member swap or a free swap overload.
• (Simple) When a class has a swap member function, it should be declared noexcept.

C.84: A swap function must not fail

Reason

swap is widely used in ways that are assumed never to fail and programs cannot easily be written to work correctly in the presence of a failing swap. The standard-library containers and algorithms will not work correctly if a swap of an element type fails.

Example, bad

void swap(My_vector& x, My_vector& y)
{
    auto tmp = x;   // copy elements
    x = y;
    y = tmp;
}

This is not just slow, but if a memory allocation occurs for the elements in tmp, this swap could throw and would make STL algorithms fail if used with them.

Enforcement

(Simple) When a class has a swap member function, it should be declared noexcept.

C.85: Make swap noexcept

Reason

A swap must not fail.
If a swap tries to exit with an exception, it’s a bad design error and the program had better terminate.

Enforcement

(Simple) When a class has a swap member function, it should be declared noexcept.

C.86: Make == symmetric with respect to operand types and noexcept

Reason

Asymmetric treatment of operands is surprising and a source of errors where conversions are possible.
== is a fundamental operation and programmers should be able to use it without fear of failure.

Example

struct X {
    string name;
    int number;
};

bool operator==(const X& a, const X& b) noexcept {
    return a.name == b.name && a.number == b.number;
}

Example, bad

class B {
    string name;
    int number;
    bool operator==(const B& a) const {
        return name == a.name && number == a.number;
    }
    // ...
};

B’s comparison accepts conversions for its second operand, but not its first.

Note

If a class has a failure state, like double’s NaN, there is a temptation to make a comparison against the failure state throw.
The alternative is to make two failure states compare equal and any valid state compare false against the failure state.

Note

This rule