C++ Friend function

If a function is defined as a friend function in C++, then the protected and private data of a class can be accessed using the function.

By using the keyword friend compiler knows the given function is a friend function.

For accessing the data, the declaration of a friend function should be done inside the body of a class starting with the keyword friend.

Declaration of friend function in C++

class class_name  
{  
    friend data_type function_name(argument/s);            // syntax of friend function.
};  

In the above declaration, the friend function is preceded by the keyword friend. The function can be defined anywhere in the program like a normal C++ function. The function definition does not use either the keyword friend or scope resolution operator.

Characteristics of a Friend function:

The function is not in the scope of the class to which it has been declared as a friend.
It cannot be called using the object as it is not in the scope of that class.
It can be invoked like a normal function without using the object.
It cannot access the member names directly and has to use an object name and dot membership operator with the member name.
It can be declared either in the private or the public part.

C++ friend function Example

Let's see the simple example of C++ friend function used to print the length of a box.

#include <iostream>  
using namespace std;  
class Box  
{  
    private:  
        int length;  
    public:  
        Box(): length(0) { }  
        friend int printLength(Box); //friend function  
};  
int printLength(Box b)  
{  
   b.length += 10;  
    return b.length;  
}  
int main()  
{  
    Box b;  
    cout<<"Length of box: "<< printLength(b)<<endl;  
    return 0;  
}  

Output:

Length of box: 10

Let's see a simple example when the function is friendly to two classes.

#include <iostream>
using namespace std;
class B;          // forward declarartion.
class A
{
    int x;
    public:
    void setdata(int i)
    {
        x=i;
    }
    friend void min(A,B);         // friend function.
};
class B
{
    int y;
    public:
    void setdata(int i)
    {
        y=i;
    }
    friend void min(A,B);                    // friend function
};
void min(A a,B b)
{
    if(a.x<=b.y)
    std::cout << a.x << std::endl;
    else
    std::cout << b.y << std::endl;
}
   int main()
{
   A a;
   B b;
   a.setdata(10);
   b.setdata(20);
   min(a,b);
    return 0;
 }

Output:

In the above example, min() function is friendly to two classes, i.e., the min() function can access the private members of both the classes A and B.

C++ Friend class

A friend class can access both private and protected members of the class in which it has been declared as friend.

Let's see a simple example of a friend class.

#include <iostream>

using namespace std;

class A
{
    int x =5;
    friend class B;           // friend class.
};
class B
{
  public:
    void display(A &a)
    {
        cout<<"value of x is : "<<a.x;
    }
};
int main()
{
    A a;
    B b;
    b.display(a);
    return 0;
}

Output:

value of x is : 5

In the above example, class B is declared as a friend inside the class A. Therefore, B is a friend of class A. Class B can access the private members of class A.

Next TopicC++ Inheritance

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