FRIEND FUNCTION IN C++ With Example

FRIEND FUNCTION IN C++ :

                     The concepts of encapsulation and data hiding dictate that nonmember functions should not be able to access an object’s private or protected data. The policy is, if you’re not a member, you can’t get in. However, there are situations where such rigid discrimination leads to considerable inconvenience.

The following properties explain the concept of Friend Function clearly4;

1-Friend function is not a member function of a class to which it is a friend.

2-Declare in class with "Friend " keyword.

3-It must be define outside the class but declare inside the class.

4-It can not access member of the class directly.

5-It has no caller Object.

6-It should not be define with membership label.  

For Example:

#include<iostream>
using namespace std;

class B;                                                   /*formal declaration of class B because of its                                                                                                             use in class */

class A

{

private:

int a;

public:

     A():a(3)  { }

    friend void fun(A,B);

/*friend function can be declare in more then one class and it become friend of those /////classes in which it is declare /////it can be declare either in private or public because it is not a member function*/

};

class B

{

private:

int b;

public:

B():b(9) { }

       friend void fun(A,B);                    

 /*friend function also declare here to make it  friend of this class/////it can be declare either in private or public because it is not a member function*/

void fun(A o1,B o2)                                        

/*friend function always define outside the class and having no membership label*/

 {

       cout<<"sum is "<<o1.a+o2.b<<endl;  

}

 int main()                                                       //main function 

{

       A a1;                                                         

//Declare Object with Type of Class name A    

       B b1;     

       fun(a1,b1);                                              

 /*friend function can access the private members  of  more than one class *////* simultaneously like here it access the a private member of class A and b of class B */ 
}