Friend function in c++

FRIEND FUNCTION:

Access to the private and protected members of a class by non-member function is not allowed in normal. But it is possible to grant this access to non-member function.

A function which is declared as friend to a class have access to all its members i.e. private and protected as well as public.

To declare a function as friend, following syntax is used:

class class_name

             {      data members and functions;

               public:

                    friend ret_type function_name(arg_list);

                    member functions and variables;

              };

Sample Program:

#include<iostream>

using namespace std;

class sum

{

int a, b;

public:

friend int sumfrd(sum);                          // friend function

sum();                                                     //constructor

}

sum::sum()

{

cout<<"Enter a:";

cin>>a;

cout<<"Enter b:";

cin>>b;

}

int sumfrd(sum x)

{ return x.a+x.b;}

int main()

{   sum s;

     cout<<"Sum is: "<<sumfrd(s);

}

NOTE: friend is a keyword in C++

The circumstances in which friend functions are valuable:

0. Friend functions make creation of some types of I/O functions easier.
1. It is useful when we need to relate two classes in some sort.
2. It is useful in overloading certain types of operators.