Friend Function in C++

Introduction

  • Friend functions are special functions of C++ and considered to be a loophole in the Object Oriented Programming concepts.
  • Friend function is defined by its keyword friend.
  • Private and Protected data of class can be accessed using friend function.
  • It is defined inside the body of class either in private or public section.
  • When we define friend function, the entire class and all of its members are friends.
Syntax:
class class_name
{
     friend return_type function_name(arguments);
}

Example : Demonstrates the working structure of Friend function

#include <iostream>
using namespace std;
class Employee
{
     private:
         int empid;
     public:
         Employee(): empid(0){ }
         friend int display(Employee);  //friend function
};
int display(Employee e)            //function definition
{
      e.empid = 100;             //accessing private data from non-member function
      return e.empid;
}
int main()
{
      Employee e;
      cout<<"Employee Id: "<<display(e);
      return 0;
}


Output:
Employee Id: 100

In the above program, friend function display() is declared inside Employee class. So, the private data can be accessed from this function.

Friend Class

  • Friend class is similar to friend function.
  • Friend class is defined by using friend keyword.
  • It increases encapsulation due to interdependency between the classes.
  • It has full access of private data members of another class without being member of that class.
Syntax:
class class_name
{
     friend class class_name2;     //class_name2 is a friend class
}
class class_name2
{
     //Statements;
}

All the member functions of another class can be accessed using friend class.

Example : Demonstrating the basic structure of Friend class

class A
{
     friend class B;   // Class B is a friend class
}
class B
{
     //Statements;
}


In the above example, all the member functions of class B will be friend function of class B. So, any member function of class B can access the private and protected data of class A, but member functions of class A cannot private and protected data of class B.
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