A Friend Function is a function that can have access to even the private, protected members of a class. This can be an ordinary function or a member of another class. It acts as a bridge between two classes.

   #include <iostream.h>
   class friendcl
   {
   	private:
    	  int a,b;
  	public:
      friend int sum ( friendcl x);
      void set( int w, int q);
   };
   void friendcl:: set( int w, int q)
   {
     a= w;
     b=q;
    }
   int sum (friendcl x)
    {
    	return x.a + x.b;
    }
   int main()
   {
   	friendcl r;
	r.set(6,7);
     	cout <<  "The sum of the values is::"  << sum (r);
      return 0;
   }

   The sum of the values is::13 

In the above example the class "sum" is a friend function for the class "friendcl". So this can use even the private variables "a,b" of the class "friendcl". The function "sum" is not a member of any class. The "x" is an object of the class "friendcl" function, which is declared in the friend function to pass arguments.