Basic Object-oriented Programming___CH_13

13.1 — Welcome to object-oriented programming

Object-oriented programming (OOP) provides us with the ability to create objects that tie together both properties and behaviors into a self-contained, reusable package. This leads to code that looks more like this:

you.driveTo(work);

Note that the term “object” is overloaded a bit, and this causes some amount of confusion. In traditional programming, an object is a piece of memory to store values. And that’s it. In object-oriented programming, an “object” implies that it is both an object in the traditional programming sense, and that it combines both properties and behaviors. From this point forward, when we use the term “object”, we’ll be referring to “objects” in the object-oriented sense.

13.2 — Classes and class members

Here is an example of a struct used to hold a date:

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

Enumerated types and data-only structs (structs that only contain variables) represent the traditional non-object-oriented programming world, as they can only hold data. We can create and initialize this struct as follows:

DateStruct today { 2020, 10, 14 }; // use uniform initialization
Now, if we want to print the date to the screen (something we probably want to do a lot), it makes sense to write a function to do this. Here’s a full program:

#include 

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

void print(const DateStruct& date)
{
    std::cout << date.year << '/' << date.month << '/' << date.day;
}

int main()
{
    DateStruct today { 2020, 10, 14 }; // use uniform initialization

    today.day = 16; // use member selection operator to select a member of the struct
    print(today);

    return 0;
}

This program prints:

2020/10/16

Classes

In C++, classes and structs are essentially the same. In fact, the following struct and class are effectively identical:

struct DateStruct
{
    int year {};
    int month {};
    int day {};
};

class DateClass
{
public:
    int m_year {};
    int m_month {};
    int m_day {};
};

Note that the only significant difference is the public: keyword in the class. We will discuss the function of this keyword in the next lesson.

Warning

Just like with structs, one of the easiest mistakes to make in C++ is to forget the semicolon at the end of a class declaration. This will cause a compiler error on the next line of code. Modern compilers like Visual Studio 2010 will give you an indication that you may have forgotten a semicolon, but older or less sophisticated compilers may not, which can make the actual error hard to find.

Class (and struct) definitions are like a blueprint – they describe what the resulting object will look like, but they do not actually create the object. To actually create an object of the class, a variable of that class type must be defined:

DateClass today { 2020, 10, 14 }; // declare a variable of class DateClass

A reminder

Initialize the member variables of a class at the point of declaration.

Member Functions

Best practice

Name your classes starting with a capital letter.

With normal non-member functions, a function can’t call a function that’s defined “below” it (without a forward declaration):

void x()
{
// You can't call y() from here unless the compiler has already seen a forward declaration for y()
}

void y()
{
}

With member functions, this limitation doesn’t apply:

class foo
{
public:
     void x() { y(); } // okay to call y() here, even though y() isn't defined until later in this class
     void y() { };
};

Member types

A note about structs in C++

Best practice

Use the struct keyword for data-only structures. Use the class keyword for objects that have both data and functions.

You have already been using classes without knowing it

It turns out that the C++ standard library is full of classes that have been created for your benefit. std::string, std::vector, and std::array are all class types! So when you create an object of any of these types, you’re instantiating a class object. And when you call a function using these objects, you’re calling a member function.

Conclusion

The class keyword lets us create a custom type in C++ that can contain both member variables and member functions. Classes form the basis for Object-oriented programming, and we’ll spend the rest of this chapter and many of the future chapters exploring all they have to offer!

13.3 — Public vs private access specifiers

Public and private members

The code outside of a struct or class is sometimes called the public: the public is only allowed to access the public members of a struct or class, which makes sense.

Access specifiers

Mixing access specifiers

A class can (and almost always does) use multiple access specifiers to set the access levels of each of its members. There is no limit to the number of access specifiers you can use in a class.

In general, member variables are usually made private, and member functions are usually made public. We’ll take a closer look at why in the next lesson.

Best practice

Make member variables private, and member functions public, unless you have a good reason not to.

Some programmers prefer to list private members first, because the public members typically use the private ones, so it makes sense to define the private ones first. However, a good counterargument is that users of the class don’t care about the private members, so the public ones should come first. Either way is fine.

Access controls work on a per-class basis

#include 

class DateClass // members are private by default
{
	int m_month {}; // private by default, can only be accessed by other members
	int m_day {}; // private by default, can only be accessed by other members
	int m_year {}; // private by default, can only be accessed by other members

public:
	void setDate(int month, int day, int year)
	{
		m_month = month;
		m_day = day;
		m_year = year;
	}

	void print()
	{
		std::cout << m_month << '/' << m_day << '/' << m_year;
	}

	// Note the addition of this function
	void copyFrom(const DateClass& d)
	{
		// Note that we can access the private members of d directly
		m_month = d.m_month;
		m_day = d.m_day;
		m_year = d.m_year;
	}
};

int main()
{
	DateClass date;
	date.setDate(10, 14, 2020); // okay, because setDate() is public

	DateClass copy {};
	copy.copyFrom(date); // okay, because copyFrom() is public
	copy.print();
	std::cout << '\n';

	return 0;
}

One nuance of C++ that is often missed or misunderstood is that access control works on a per-class basis, not a per-object basis. This means that when a function has access to the private members of a class, it can access the private members of any object of that class type that it can see.

In the above example, copyFrom() is a member of DateClass, which gives it access to the private members of DateClass. This means copyFrom() can not only directly access the private members of the implicit object it is operating on (copy), it also means it has direct access to the private members of DateClass parameter d! If parameter d were some other type, this would not be the case.

This can be particularly useful when we need to copy members from one object of a class to another object of the same class. We’ll also see this topic show up again when we talk about overloading operator<< to print members of a class in the next chapter.

Structs vs classes revisited

Now that we’ve talked about access specifiers, we can talk about the actual differences between a class and a struct in C++. A class defaults its members to private. A struct defaults its members to public.

That’s it!

(Okay, to be pedantic, there’s one more minor difference – structs inherit from other classes publicly and classes inherit privately. We’ll cover what this means in a future chapter, but this particular point is practically irrelevant since you should never rely on the defaults anyway).

13.4 — Access functions and encapsulation

Encapsulation

Note: The word encapsulation is also sometimes used to refer to the packaging of data and functions that work on that data together. We prefer to just call that object-oriented programming.

Benefit: encapsulated classes are easier to use and reduce the complexity of your programs

Benefit: encapsulated classes help protect your data and prevent misuse

Access functions

Best practice

Getters should return by value or const reference.

13.5 — Constructors

A constructor is a special kind of class member function that is automatically called when an object of that class is created. Constructors are typically used to initialize member variables of the class to appropriate user-provided values, or to do any setup steps necessary for the class to be used (e.g. open a file or database).

After a constructor executes, the object should be in a well-defined, usable state.

Unlike normal member functions, constructors have specific rules for how they must be named:

1. Constructors must have the same name as the class (with the same capitalization)
2. Constructors have no return type (not even void)

Default constructors and default initialization

Value-initialization

Best practice

Favor value-initialization over default-initialization for class objects.

Note that value initialization uses empty braces, not empty parenthesis:

Fraction f1 {}; // value initialization of Fraction f1
Fraction f2();  // forward declaration of function f2

Direct- and list-initialization using constructors with parameters

Best practice

Favor brace initialization to initialize class objects.

Copy initialization using equals with classes

Much like with fundamental variables, it’s also possible to initialize classes using copy initialization:

Fraction six = Fraction{ 6 }; // Copy initialize a Fraction, will call Fraction(6, 1)
Fraction seven = 7; // Copy initialize a Fraction.  The compiler will try to find a way to convert 7 to a Fraction, which will invoke the Fraction(7, 1) constructor.

However, we recommend you avoid this form of initialization with classes, as it may be less efficient. Although direct-initialization, list-initialization, and copy-initialization all work identically with fundamental types, copy-initialization does not work the same with classes (though the end-result is often the same). We’ll explore the differences in more detail in a future chapter.

Reducing your constructors

In the above two-constructor declaration of the Fraction class, the default constructor is actually somewhat redundant. We could simplify this class as follows:

#include 

class Fraction
{
private:
    int m_numerator {};
    int m_denominator {};

public:
    // Default constructor
    Fraction(int numerator=0, int denominator=1)
    {
        assert(denominator != 0);

        m_numerator = numerator;
        m_denominator = denominator;
    }

    int getNumerator() { return m_numerator; }
    int getDenominator() { return m_denominator; }
    double getValue() { return static_cast<double>(m_numerator) / m_denominator; }
};

Although this constructor is still a default constructor, it has now been defined in a way that it can accept one or two user-provided values as well.

Fraction zero; // will call Fraction(0, 1)
Fraction zero{}; // will call Fraction(0, 1)
Fraction six{ 6 }; // will call Fraction(6, 1)
Fraction fiveThirds{ 5, 3 }; // will call Fraction(5, 3)

When implementing your constructors, consider how you might keep the number of constructors down through smart defaulting of values.

A reminder about default parameters

If we want to be able to construct a Something with only a double, we’ll need to add a second (non-default) constructor:

class Something
{
public:
	// Default constructor
	Something(int n = 0, double d = 1.2) // allows us to construct a Something(int, double), Something(int), or Something()
	{
	}

	Something(double d)
	{
	}
};

int main()
{
	Something s1 { 1, 2.4 }; // calls Something(int, double)
	Something s2 { 1 }; // calls Something(int, double)
	Something s3 {}; // calls Something(int, double)

	Something s4 { 2.4 }; // calls Something(double)

	return 0;
}

An implicitly generated default constructor

Best practice

If you have constructors in your class and need a default constructor that does nothing (e.g. because all your members are initialized using non-static member initialization), use = default.

Classes containing class members

Constructor notes

Best practice

Always initialize all member variables in your objects.

Quiz time

Question #2

What happens if you don’t declare a default constructor?

Hide Solution

If you haven’t defined any other constructors, the compiler will create an empty public default constructor for you. This means your objects will be instantiable with no parameters. If you have defined other constructors (default or otherwise), the compiler will not create a default constructor for you. Assuming you haven’t provided a default constructor yourself, your objects will not be instantiable without arguments.

13.6 — Constructor member initializer lists

Member initializer lists

To solve this problem, C++ provides a method for initializing class member variables (rather than assigning values to them after they are created) via a member initializer list (often called a “member initialization list”). Do not confuse these with the similarly named initializer list that we can use to assign values to arrays.

Now let’s write the same code using an initialization list:

#include 

class Something
{
private:
    int m_value1 {};
    double m_value2 {};
    char m_value3 {};

public:
    Something() : m_value1{ 1 }, m_value2{ 2.2 }, m_value3{ 'c' } // Initialize our member variables
    {
    // No need for assignment here
    }

    void print()
    {
         std::cout << "Something(" << m_value1 << ", " << m_value2 << ", " << m_value3 << ")\n";
    }
};

int main()
{
    Something something{};
    something.print();
    return 0;
}

The member initializer list is inserted after the constructor parameters. It begins with a colon (, and then lists each variable to initialize along with the value for that variable separated by a comma.

Note that we no longer need to do the assignments in the constructor body, since the initializer list replaces that functionality. Also note that the initializer list does not end in a semicolon.

Of course, constructors are more useful when we allow the caller to pass in the initialization values:

This prints:

Something(1, 2.2, c)

#include 

class Something
{
private:
    int m_value1 {};
    double m_value2 {};
    char m_value3 {};

public:
    Something(int value1, double value2, char value3='c')
        : m_value1{ value1 }, m_value2{ value2 }, m_value3{ value3 } // directly initialize our member variables
    {
    // No need for assignment here
    }

    void print()
    {
         std::cout << "Something(" << m_value1 << ", " << m_value2 << ", " << m_value3 << ")\n";
    }

};

int main()
{
    Something something{ 1, 2.2 }; // value1 = 1, value2=2.2, value3 gets default value 'c'
    something.print();
    return 0;
}

Best practice

Use member initializer lists to initialize your class member variables instead of assignment.

Initializing const member variables

Rule

Const member variables must be initialized.

Initializing array members with member initializer lists

However, since C++11, you can fully initialize a member array using uniform initialization:

class Something
{
private:
    const int m_array[5];

public:
    Something(): m_array { 1, 2, 3, 4, 5 } // use uniform initialization to initialize our member array
    {
    }

};

Initializing member variables that are classes

#include 

class A
{
public:
    A(int x = 0) { std::cout << "A " << x << '\n'; }
};

class B
{
private:
    A m_a {};
public:
    B(int y)
        : m_a{ y - 1 } // call A(int) constructor to initialize member m_a
    {
        std::cout << "B " << y << '\n';
    }
};

int main()
{
    B b{ 5 };
    return 0;
}

This prints:

A 4
B 5

Formatting your initializer lists

If all of the initializers don’t fit on a single line (or the initializers are non-trivial), then you can space them out, one per line:

class Something
{
private:
    int m_value1 {};
    double m_value2 {};
    char m_value3 {};
    float m_value4 {};

public:
    Something(int value1, double value2, char value3='c', float value4=34.6f) // this line already has a lot of stuff on it
        : m_value1{ value1 } // one per line
        , m_value2{ value2 }
        , m_value3{ value3 }
        , m_value4{ value4 }
    {
    }

};

Initializer list order

Perhaps surprisingly, variables in the initializer list are not initialized in the order that they are specified in the initializer list. Instead, they are initialized in the order in which they are declared in the class.

For best results, the following recommendations should be observed:

1. Don’t initialize member variables in such a way that they are dependent upon other member variables being initialized first (in other words, ensure your member variables will properly initialize even if the initialization ordering is different).
2. Initialize variables in the initializer list in the same order in which they are declared in your class. This isn’t strictly required so long as the prior recommendation has been followed, but your compiler may give you a warning if you don’t do so and you have all warnings turned on.

Summary

Member initializer lists allow us to initialize our members rather than assign values to them. This is the only way to initialize members that require values upon initialization, such as const or reference members, and it can be more performant than assigning values in the body of the constructor. Member initializer lists work both with fundamental types and members that are classes themselves.

13.7 — Non-static member initialization

Note that initializing members using non-static member initialization requires using either an equals sign, or a brace (uniform) initializer – the parenthesis initialization form doesn’t work here:

class A
{
    int m_a = 1;  // ok (copy initialization)
    int m_b{ 2 }; // ok (brace initialization)
    int m_c(3);   // doesn't work (parenthesis initialization)
};

Rule

Favor use of non-static member initialization to give default values for your member variables.

13.8 — Overlapping and delegating constructors

Constructors with overlapping functionality

class Foo
{
public:
    Foo()
    {
        // code to do A
    }

    Foo(int value)
    {
        // code to do A
        // code to do B
    }
};

The obvious solution doesn’t work

class Foo
{
public:
    Foo()
    {
        // code to do A
    }

    Foo(int value)
    {
        Foo(); // use the above constructor to do A (doesn't work)
        // code to do B
    }
};

What’s happening is that Foo(); instantiates a new Foo object, which is immediately discarded, because it’s not stored in a variable.

Delegating constructors

class Foo
{
private:

public:
    Foo()
    {
        // code to do A
    }

    Foo(int value): Foo{} // use Foo() default constructor to do A
    {
        // code to do B
    }

};

A few additional notes about delegating constructors. First, a constructor that delegates to another constructor is not allowed to do any member initialization itself. So your constructors can delegate or initialize, but not both.

Best practice

If you have multiple constructors that have the same functionality, use delegating constructors to avoid duplicate code

Using a normal member function for setup

13.9 — Destructors

A destructor is another special kind of class member function that is executed when an object of that class is destroyed. Whereas constructors are designed to initialize a class, destructors are designed to help clean up.

When an object goes out of scope normally, or a dynamically allocated object is explicitly deleted using the delete keyword, the class destructor is automatically called (if it exists) to do any necessary clean up before the object is removed from memory. For simple classes (those that just initialize the values of normal member variables), a destructor is not needed because C++ will automatically clean up the memory for you.

However, if your class object is holding any resources (e.g. dynamic memory, or a file or database handle), or if you need to do any kind of maintenance before the object is destroyed, the destructor is the perfect place to do so, as it is typically the last thing to happen before the object is destroyed.

Destructor naming

Like constructors, destructors have specific naming rules:

1. The destructor must have the same name as the class, preceded by a tilde (~).
2. The destructor can not take arguments.
3. The destructor has no return type.
   A class can only have a single destructor.

Generally you should not call a destructor explicitly (as it will be called automatically when the object is destroyed), since there are rarely cases where you’d want to clean up an object more than once. However, destructors may safely call other member functions since the object isn’t destroyed until after the destructor executes.

A destructor example

#include 
#include 
#include 

class IntArray
{
private:
	int* m_array{};
	int m_length{};

public:
	IntArray(int length) // constructor
	{
		assert(length > 0);

		m_array = new int[static_cast<std::size_t>(length)]{};
		m_length = length;
	}

	~IntArray() // destructor
	{
		// Dynamically delete the array we allocated earlier
		delete[] m_array;
	}

	void setValue(int index, int value) { m_array[index] = value; }
	int getValue(int index) { return m_array[index]; }

	int getLength() { return m_length; }
};

int main()
{
	IntArray ar ( 10 ); // allocate 10 integers
	for (int count{ 0 }; count < ar.getLength(); ++count)
		ar.setValue(count, count+1);

	std::cout << "The value of element 5 is: " << ar.getValue(5) << '\n';

	return 0;
} // ar is destroyed here, so the ~IntArray() destructor function is called here

A reminder

In lesson 11.17 – An introduction to std::vector, we note that parentheses based initialization should be used when initializing an array/container/list class with a length (as opposed to a list of elements). For this reason, we initialize IntArray using IntArray ar ( 10 );.

Constructor and destructor timing

#include 

class Simple
{
private:
    int m_nID{};

public:
    Simple(int nID)
        : m_nID{ nID }
    {
        std::cout << "Constructing Simple " << nID << '\n';
    }

    ~Simple()
    {
        std::cout << "Destructing Simple" << m_nID << '\n';
    }

    int getID() { return m_nID; }
};

int main()
{
    // Allocate a Simple on the stack
    Simple simple{ 1 };
    std::cout << simple.getID() << '\n';

    // Allocate a Simple dynamically
    Simple* pSimple{ new Simple{ 2 } };

    std::cout << pSimple->getID() << '\n';

    // We allocated pSimple dynamically, so we have to delete it.
    delete pSimple;

    return 0;
} // simple goes out of scope here

This program produces the following result:

Constructing Simple 1
1
Constructing Simple 2
2
Destructing Simple 2
Destructing Simple 1

Note that “Simple 1” is destroyed after “Simple 2” because we deleted pSimple before the end of the function, whereas simple was not destroyed until the end of main().

Global variables are constructed before main() and destroyed after main().

RAII

RAII (Resource Acquisition Is Initialization) is a programming technique whereby resource use is tied to the lifetime of objects with automatic duration (e.g. non-dynamically allocated objects)

A warning about the std::exit() function

Summary

As you can see, when constructors and destructors are used together, your classes can initialize and clean up after themselves without the programmer having to do any special work! This reduces the probability of making an error, and makes classes easier to use.

13.10 — The hidden “this” pointer

The hidden *this pointer

Putting it all together:

1. When we call simple.setID(2), the compiler actually calls setID(&simple, 2).
2. Inside setID(), the “this” pointer holds the address of object simple.
3. Any member variables inside setID() are prefixed with “this->”. So when we say m_id = id, the compiler is actually executing this->m_id = id, which in this case updates simple.m_id to id.

The good news is that all of this happens automatically, and it doesn’t really matter whether you remember how it works or not. All you need to remember is that all non-static member functions have a “this” pointer that refers to the object the function was called on.

“this” always points to the object being operated on

Explicitly referencing “this”

Some developers prefer to explicitly add this-> to all class members. We recommend that you avoid doing so, as it tends to make your code less readable for little benefit. Using the m_ prefix is a more readable way to differentiate member variables from non-member (local) variables.

Chaining member functions

Summary

The “this” pointer is a hidden parameter implicitly added to any non-static member function. Most of the time, you will not need to access it directly, but you can if needed. It’s worth noting that “this” is a const pointer – you can change the value of the underlying object it points to, but you can not make it point to something else!

By having functions that would otherwise return void return *this instead, you can make those functions chainable. This is most often used when overloading operators for your classes (something we’ll talk about more in chapter 14).

13.11 — Class code and header files

Defining member functions outside the class definition

So what should I define in the header file vs the cpp file, and what inside the class definition vs outside?

You might be tempted to put all of your member function definitions into the header file, inside the class. While this will compile, there are a couple of downsides to doing so. First, as mentioned above, this clutters up your class definition. Second, if you change anything about the code in the header, then you’ll need to recompile every file that includes that header. This can have a ripple effect, where one minor change causes the entire program to need to recompile (which can be slow). If you change the code in a .cpp file, only that .cpp file needs to be recompiled!

Therefore, we recommend the following:

• For classes used in only one file that aren’t generally reusable, define them directly in the single .cpp file they’re used in.
• For classes used in multiple files, or intended for general reuse, define them in a .h file that has the same name as the class.
• Trivial member functions (trivial constructors or destructors, access functions, etc…) can be defined inside the class.
• Non-trivial member functions should be defined in a .cpp file that has the same name as the class.

In future lessons, most of our classes will be defined in the .cpp file, with all the functions implemented directly in the class definition. This is just for convenience and to keep the examples short. In real projects, it is much more common for classes to be put in their own code and header files, and you should get used to doing so.

Default parameters

Default parameters for member functions should be declared in the class definition (in the header file), where they can be seen by whomever #includes the header.

Libraries

Separating the class definition and class implementation is very common for libraries that you can use to extend your program. Throughout your programs, you’ve #included headers that belong to the standard library, such as iostream, string, vector, array, and other. Notice that you haven’t needed to add iostream.cpp, string.cpp, vector.cpp, or array.cpp into your projects. Your program needs the declarations from the header files in order for the compiler to validate you’re writing programs that are syntactically correct. However, the implementations for the classes that belong to the C++ standard library are contained in a precompiled file that is linked in at the link stage. You never see the code.

Outside of some open source software (where both .h and .cpp files are provided), most 3rd party libraries provide only header files, along with a precompiled library file. There are several reasons for this: 1) It’s faster to link a precompiled library than to recompile it every time you need it, 2) a single copy of a precompiled library can be shared by many applications, whereas compiled code gets compiled into every executable that uses it (inflating file sizes), and 3) intellectual property reasons (you don’t want people stealing your code).

Having your own files separated into declaration (header) and implementation (code file) is not only good form, it also makes creating your own custom libraries easier. Creating your own libraries is beyond the scope of these tutorials, but separating your declaration and implementation is a prerequisite to doing so.

13.12 — Const class objects and member functions

Const classes

Similarly, instantiated class objects can also be made const by using the const keyword. Initialization is done via class constructors:

const Date date1; // initialize using default constructor
const Date date2(2020, 10, 16); // initialize using parameterized constructor
const Date date3 { 2020, 10, 16 }; // initialize using parameterized constructor (C++11)

class Something
{
public:
    int m_value {};

    Something(): m_value{0} { }

    void setValue(int value) { m_value = value; }
    int getValue() { return m_value ; }
};

int main()
{
    const Something something{}; // calls default constructor

    something.m_value = 5; // compiler error: violates const
    something.setValue(5); // compiler error: violates const

    return 0;
}

Const member functions

To make getValue() a const member function, we simply append the const keyword to the function prototype, after the parameter list, but before the function body:

class Something
{
public:
    int m_value {};

    Something(): m_value{0} { }

    void resetValue() { m_value = 0; }
    void setValue(int value) { m_value = value; }

    int getValue() const { return m_value; } // note addition of const keyword after parameter list, but before function body
};

Now getValue() has been made a const member function, which means we can call it on any const objects.

For member functions defined outside of the class definition, the const keyword must be used on both the function prototype in the class definition and on the function definition:

class Something
{
public:
    int m_value {};

    Something(): m_value{0} { }

    void resetValue() { m_value = 0; }
    void setValue(int value) { m_value = value; }

    int getValue() const; // note addition of const keyword here
};

int Something::getValue() const // and here
{
    return m_value;
}

Futhermore, any const member function that attempts to change a member variable or call a non-const member function will cause a compiler error to occur. For example:

class Something
{
public:
    int m_value {};

    void resetValue() const { m_value = 0; } // compile error, const functions can't change member variables.
};

In this example, resetValue() has been marked as a const member function, but it attempts to change m_value. This will cause a compiler error.

Note that constructors cannot be marked as const. This is because constructors need to be able to initialize their member variables, and a const constructor would not be able to do so. Consequently, the language disallows const constructors.

Const member functions can also be called by non-const objects.

Best practice

Make any member function that does not modify the state of the class object const, so that it can be called by const objects.

Const objects via pass by const reference

---

Const members can not return non-const references to members

When a member function is const, the hidden *this pointer is also const, which means all members are treated as const within that function. Therefore, a const member function can not return a non-const reference to a member, as that would allow the caller to have non-const access to that const member. Const member functions can return const references to members.

We’ll see an example of this in the next section.

Overloading const and non-const function

Finally, although it is not done very often, it is possible to overload a function in such a way to have a const and non-const version of the same function. This works because the const qualifier is considered part of the function’s signature, so two functions which differ only in their const-ness are considered distinct.

#include 

class Something
{
private:
    std::string m_value {};

public:
    Something(const std::string& value=""): m_value{ value } {}

    const std::string& getValue() const { return m_value; } // getValue() for const objects (returns const reference)
    std::string& getValue() { return m_value; } // getValue() for non-const objects (returns non-const reference)
};

Summary

Because passing objects by const reference is common, your classes should be const-friendly. That means making any member function that does not modify the state of the class object const!

13.13 — Static member variables

Member variables of a class can be made static by using the static keyword. Unlike normal member variables, static member variables are shared by all objects of the class. Consider the following program, similar to the above:

#include 

class Something
{
public:
    static int s_value;
};

int Something::s_value{ 1 };

int main()
{
    Something first;
    Something second;

    first.s_value = 2;

    std::cout << first.s_value << '\n';
    std::cout << second.s_value << '\n';
    return 0;
}

This program produces the following output:

2
2

Static members are not associated with class objects

Consequently, it is better to think of static members as belonging to the class itself, not to the objects of the class. Because s_value exists independently of any class objects, it can be accessed directly using the class name and the scope resolution operator (in this case, Something:_value):

#include 

class Something
{
public:
    static int s_value; // declares the static member variable
};

int Something::s_value{ 1 }; // defines the static member variable (we'll discuss this section below)

int main()
{
    // note: we're not instantiating any objects of type Something

    Something::s_value = 2;
    std::cout << Something::s_value << '\n';
    return 0;
}

In the above snippet, s_value is referenced by class name rather than through an object. Note that we have not even instantiated an object of type Something, but we are still able to access and use Something:_value. This is the preferred method for accessing static members.

Best practice

Access static members by class name (using the scope resolution operator) rather than through an object of the class (using the member selection operator).

Defining and initializing static member variables

You must explicitly define the static member outside of the class, in the global scope.

In the example above, we do so via this line:

int Something::s_value{ 1 }; // defines the static member variable

This line serves two purposes: it instantiates the static member variable (just like a global variable), and optionally initializes it. In this case, we’re providing the initialization value 1. If no initializer is provided, C++ initializes the value to 0.

Note that this static member definition is not subject to access controls: you can define and initialize the variable even if it’s declared as private (or protected) in the class.

If the class is defined in a .h file, the static member definition is usually placed in the associated code file for the class (e.g. Something.cpp). If the class is defined in a .cpp file, the static member definition is usually placed directly underneath the class. Do not put the static member definition in a header file (much like a global variable, if that header file gets included more than once, you’ll end up with multiple definitions, which will cause a linker error).

Inline initialization of static member variables

There are a few shortcuts to the above. First, when the static member is a const integral type (which includes char and bool) or a const enum, the static member can be initialized inside the class definition:

class Whatever
{
public:
    static const int s_value{ 4 }; // a static const int can be declared and initialized directly
};

In the above example, because the static member variable is a const int, no explicit definition line is needed.

Second, static constexpr members can be initialized inside the class definition:

#include 

class Whatever
{
public:
    static constexpr double s_value{ 2.2 }; // ok
    static constexpr std::array<int, 3> s_array{ 1, 2, 3 }; // this even works for classes that support constexpr initialization
};

Finally, as of C++17, we can also initialize non-const static members in the class definition by declaring them inline:

class Whatever
{
public:
    static inline int s_value{ 4 }; // a static inline int can be declared and initialized directly (C++17)

Best practice

Prefer initializing static constexpr members at the point of definition.
Prefer making static non-constexpr members inline and initializing them at the point of definition.

An example of static member variables

Why use static variables inside classes? One useful example is to assign a unique ID to every instance of the class. Here’s an example of that:

#include 

class Something
{
private:
    static inline int s_idGenerator { 1 }; // C++17
//  static int s_idGenerator;              // Use this instead for C++14 or older
    int m_id { };

public:
    Something()
    : m_id { s_idGenerator++ } // grab the next value from the id generator
    {}

    int getID() const { return m_id; }
};

// For C++14 or older, we have to initialize the non-const static member outside the class definition
// Note that we're defining and initializing s_idGenerator even though it is declared as private above.
// This is okay since the definition isn't subject to access controls.
// int Something::s_idGenerator { 1 }; // start our ID generator with value 1 (uncomment for C++14 or older)

int main()
{
    Something first;
    Something second;
    Something third;

    std::cout << first.getID() << '\n';
    std::cout << second.getID() << '\n';
    std::cout << third.getID() << '\n';
    return 0;
}

This program prints:

1
2
3

Because s_idGenerator is shared by all Something objects, when a new Something object is created, the constructor grabs the current value out of s_idGenerator and then increments the value for the next object. This guarantees that each instantiated Something object receives a unique id (incremented in the order of creation). This can really help when debugging multiple items in an array, as it provides a way to tell multiple objects of the same class type apart!

Static member variables can also be useful when the class needs to utilize an internal lookup table (e.g. an array used to store a set of pre-calculated values). By making the lookup table static, only one copy exists for all objects, rather than making a copy for each object instantiated. This can save substantial amounts of memory.

13.14 — Static member functions

Like static member variables, static member functions are not attached to any particular object. Here is the above example with a static member function accessor:

#include 

class Something
{
private:
    static int s_value;
public:
    static int getValue() { return s_value; } // static member function
};

int Something::s_value{ 1 }; // initializer

int main()
{
    std::cout << Something::getValue() << '\n';
}

Because static member functions are not attached to a particular object, they can be called directly by using the class name and the scope resolution operator. Like static member variables, they can also be called through objects of the class type, though this is not recommended.

Static member functions have no *this pointer

Another example

Static member functions can also be defined outside of the class declaration. This works the same way as for normal member functions.

Here’s an example:

#include 

class IDGenerator
{
private:
    static int s_nextID; // Here's the declaration for a static member

public:
     static int getNextID(); // Here's the declaration for a static function
};

// Here's the definition of the static member outside the class.  Note we don't use the static keyword here.
// We'll start generating IDs at 1
int IDGenerator::s_nextID{ 1 };

// Here's the definition of the static function outside of the class.  Note we don't use the static keyword here.
int IDGenerator::getNextID() { return s_nextID++; }

int main()
{
    for (int count{ 0 }; count < 5; ++count)
        std::cout << "The next ID is: " << IDGenerator::getNextID() << '\n';

    return 0;
}

This program prints:

The next ID is: 1
The next ID is: 2
The next ID is: 3
The next ID is: 4
The next ID is: 5

Note that because all the data and functions in this class are static, we don’t need to instantiate an object of the class to make use of its functionality! This class utilizes a static member variable to hold the value of the next ID to be assigned, and provides a static member function to return that ID and increment it.

---

A word of warning about classes with all static members

C++ does not support static constructors

Summary

Static member functions can be used to work with static member variables in the class. An object of the class is not required to call them.

Classes can be created with all static member variables and static functions. However, such classes are essentially the equivalent of declaring functions and global variables in a globally accessible namespace, and should generally be avoided unless you have a particularly good reason to use them.

13.15 — Friend functions and classes

Friend functions

A friend function is a function that can access the private members of a class as though it was a member of that class.

Multiple friends

Friend classes

A few additional notes on friend classes. First, even though Display is a friend of Storage, Display has no direct access to the *this pointer of Storage objects. Second, just because Display is a friend of Storage, that does not mean Storage is also a friend of Display. If you want two classes to be friends of each other, both must declare the other as a friend. Finally, if class A is a friend of B, and B is a friend of C, that does not mean A is a friend of C.

Be careful when using friend functions and classes, because it allows the friend function or class to violate encapsulation. If the details of the class change, the details of the friend will also be forced to change. Consequently, limit your use of friend functions and classes to a minimum.

Friend member functions

#include 

class Storage; // forward declaration for class Storage

class Display
{
private:
	bool m_displayIntFirst {};

public:
	Display(bool displayIntFirst)
		: m_displayIntFirst { displayIntFirst }
	{
	}

	void displayItem(const Storage& storage); // forward declaration above needed for this declaration line
};

class Storage // full definition of Storage class
{
private:
	int m_nValue {};
	double m_dValue {};
public:
	Storage(int nValue, double dValue)
		: m_nValue { nValue }, m_dValue { dValue }
	{
	}

	// Make the Display::displayItem member function a friend of the Storage class (requires seeing the full definition of class Display, as above)
	friend void Display::displayItem(const Storage& storage);
};

// Now we can define Display::displayItem, which needs to have seen the full definition of class Storage
void Display::displayItem(const Storage& storage)
{
	if (m_displayIntFirst)
		std::cout << storage.m_nValue << ' ' << storage.m_dValue << '\n';
	else // display double first
		std::cout << storage.m_dValue << ' ' << storage.m_nValue << '\n';
}

int main()
{
    Storage storage(5, 6.7);
    Display display(false);

    display.displayItem(storage);

    return 0;
}

Now everything will compile properly: the forward declaration of class Storage is enough to satisfy the declaration of Display::displayItem(), the full definition of Display satisfies declaring Display::displayItem() as a friend of Storage, and the full definition of class Storage is enough to satisfy the definition of member function Display::displayItem(). If that’s a bit confusing, see the comments in the program above.

If this seems like a pain – it is. Fortunately, this dance is only necessary because we’re trying to do everything in a single file. A better solution is to put each class definition in a separate header file, with the member function definitions in corresponding .cpp files. That way, all of the class definitions would have been visible immediately in the .cpp files, and no rearranging of classes or functions is necessary!

Summary

A friend function or class is a function or class that can access the private members of another class as though it was a member of that class. This allows the friend function or friend class to work intimately with the other class, without making the other class expose its private members (e.g. via access functions).

Friending is commonly used when defining overloaded operators (which we’ll cover in the next chapter), or less commonly, when two or more classes need to work together in an intimate way.

Note that making a specific member function a friend requires the full definition for the class of the member function to have been seen first.

13.16 — Anonymous objects

Summary

In C++, anonymous objects are primarily used either to pass or return values without having to create lots of temporary variables to do so. Memory allocated dynamically is also done so anonymously (which is why its address must be assigned to a pointer, otherwise we’d have no way to refer to it).

It is also worth noting that because anonymous objects have expression scope, they can only be used once (unless bound to a constant l-value reference, which will extend the lifetime of the temporary object to match the lifetime of the reference). If you need to reference a value in multiple expressions, you should use a named variable instead.

13.17 — Nested types in classes

Nesting types

Much like functions and data can be members of a class, in C++, types can also be defined (nested) inside of a class. To do this, you simply define the type inside the class, under the appropriate access specifier.

Here’s the same program as above, with FruitType defined inside the class:

#include 

class Fruit
{
public:
	// Note: we've moved FruitType inside the class, under the public access specifier
	// We've also changed it from an enum class to an enum
	enum FruitType
	{
		apple,
		banana,
		cherry
	};

private:
	FruitType m_type {};
	int m_percentageEaten { 0 };

public:
	Fruit(FruitType type) :
		m_type { type }
	{
	}

	FruitType getType() const { return m_type; }
	int getPercentageEaten() const { return m_percentageEaten; }
};

int main()
{
	// Note: we access the FruitType via Fruit now
	Fruit apple { Fruit::apple };

	if (apple.getType() == Fruit::apple)
		std::cout << "I am an apple";
	else
		std::cout << "I am not an apple";

	return 0;
}

First, note that FruitType is now defined inside the class. Second, note that we’ve defined it under the public access specifier, so the type definition can be accessed from outside the class.

Classes essentially act as a namespace for any nested types, much as enum classes do. In the prior example, because we used an enum class, we had to qualify our enumerators with the FruitType:: scope qualifier. In this example, because FruitType is a normal enum that is part of the class, we access our enumerators using the Fruit:: scope qualifier.

Note that because enum classes also act like namespaces, if we’d nested FruitType inside Fruit as an enum class instead of an enum, we’d access the enumeration via a Fruit::FruitType:: scope qualifier. This double-scoping is unnecessary, so we’ve used a normal enum.

Other types can be nested too

13.18 — Timing your code

When writing your code, sometimes you’ll run across cases where you’re not sure whether one method or another will be more performant. So how do you tell?

One easy way is to time your code to see how long it takes to run. C++11 comes with some functionality in the chrono library to do just that. However, using the chrono library is a bit arcane. The good news is that we can easily encapsulate all the timing functionality we need into a class that we can then use in our own programs.

Here’s the class:

#include  // for std::chrono functions

class Timer
{
private:
	// Type aliases to make accessing nested type easier
	using Clock = std::chrono::steady_clock;
	using Second = std::chrono::duration<double, std::ratio<1> >;

	std::chrono::time_point<Clock> m_beg { Clock::now() };

public:
	void reset()
	{
		m_beg = Clock::now();
	}

	double elapsed() const
	{
		return std::chrono::duration_cast<Second>(Clock::now() - m_beg).count();
	}
};

That’s it! To use it, we instantiate a Timer object at the top of our main function (or wherever we want to start timing), and then call the elapsed() member function whenever we want to know how long the program took to run to that point.

#include 

int main()
{
    Timer t;

    // Code to time goes here

    std::cout << "Time elapsed: " << t.elapsed() << " seconds\n";

    return 0;
}

A few caveats about timing

13.x — Chapter 13 comprehensive quiz