Friend Function in C++

Friend Function

 On this page we will discuss about friend function in C++ .A friend function is capable to access the private/protected data of a class in which it is declared as a friend, though it is a nonmember function of that class .A function can be declared as a friend to any number of classes.
Friend Function in C++

Friend Function in C++

Basic Properties of Friend Function in C++

  • A function can be declared as a friend to any number of classes.
  • A friend function tough not a member function has full rights and access to the members of the class

Creating a Friend Function

#include <iostream>
using namespace std;
class demo
  int a=5;
  friend void display(demo);//friend function declaration
void display(demo d)//friend function defination
  cout<<"a: "<<d.a<<endl;//access of private data
int main()
  demo d;
  display(d);//calling a friend function
return 0;
In the above code display() function is made friend with demo class so that display() function can access the private data of the class demo.

Friend class

  • A class  declared with a friend with another class is able to access private/protected  data members of that class
  • When a class is made a friend class, all the member functions of that class becomes friend functions to that class

Creating friend class

  • In this program, all member functions of class test2 will be friend functions of class test1.
  • Thus, any member function of class test2 can access the private and protected data of class test1.
  • But, member functions of class test1 cannot access the data of class test2.


#include <iostream>
using namespace std;
class test1
  int a,b;
  void get()
   cout<<"enter 2 integers"; 
cin>>a>>b; } friend class test2;//defining friend class }; class test2 {    test1 t1;    public:    void put()   {       t1.get();       cout<<"a: "<<t1.a<<endl;        cout<<"b: "<<t1.b<<endl;    } }; int main() {     test2 t2;     t2.put(); }


enter 2 integers 3  4


Code reusability: instead of creating separate member functions to access private data of each class ,a single member function can access private data of many classes


Security: friend functions should be used only when absolutely necessary because friend mechanism alters the security of private data which is against data hiding.

Real-time applications of Friend

  • Device integration is possible only by  friend
    ex: GPRS, Satellite communication
  • Friend class is also used in APIs(Application Programming Interface)
    ex: money transfer from one bank to another

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