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C++ Friend Function

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There are private and protected members in a class that is inaccessible from outside of the class, constituting one of the concepts of object-oriented programming i.e. data hiding. However, these rules can be broken by a friend function.

A function defined with a friend keyword is called a friend function. It can access both the private and protected members from outside the class.

Declaration of friend function:

class class_name    
{    
    .......
    friend data_type function_name(arguments);

    .......
};
  • A friend can be a member’s function, function template, or function, or a class or class template,  in which case the entire class and all of its members are friends.
  • Although the friend function appears inside the class, they are not membersfunction.
  • They can be declared anywhere in the class.

Example: C++ program for a friend function

#include <iostream>    
using namespace std;    
class Number    
{    
    private:    
        int num;    
    public:    
        Number(): num(0) { }    
        friend int add(Number); //friend function    
};    
int add(Number n)    
{    
   n.num += 20;    
    return n.num;    
}    
int main()    
{    
    Number num;    
    cout<<"Number: "<< add(num)<<endl;    
    return 0;    
}

Output:

Number: 20

C++ friend class

friend keyword is also used to declare a friend class in C++. When a class is declared a friend class, all the member functions of the friend class become friend functions. 

class Two;

class One {
   // class Two is a friend class of class One
   friend class Two;
   ... .. ...
}

class Two{
   ... .. ...
}

In the above example, class Two is a friend class of class One indicating that we can access all the members of class One from class Two. However, vice-versa is not granted.

Example: C++ program for a friend class.

#include <iostream>  
using namespace std;  
  
class One  
{  
    int num = 10;  
    friend class Two; // friend class.  
};  

class Two  
{  
  public:  
    void print(One &o)  
    {  
        cout<<"Num = "<<o.num;  
    }  
};  
int main()  
{  
    One o;  
    Two t;  
    t.print(o);  
    return 0;  
}

Output:

Num = 10

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