Friend functions

In principle, private and protected members of a class cannot be accessed from outside the same class in which they are declared. However, this rule does not apply to "friends".

Friends are functions or classes declared with the friend keyword.

A non-member function can access the private and protected members of a class if it is declared a friend of that class. That is done by including a declaration of this external function within the class, and preceding it with the keyword friend:

// friend functions

#include <iostream>

using namespace std;

class Rectangle {

    int width, height;

  public:

    Rectangle() {}

    Rectangle (int x, int y) : width(x), height(y) {}

    int area() {return width * height;}

    friend Rectangle duplicate (const Rectangle&);

};

Rectangle duplicate (const Rectangle& param)

{

  Rectangle res;

  res.width = param.width*2;

  res.height = param.height*2;

  return res;

}

int main () {

  Rectangle foo;

  Rectangle bar (2,3);

  foo = duplicate (bar);

  cout << foo.area() << '\n';

  return 0;

}

The duplicate function is a friend of class Rectangle. Therefore,

 function duplicate is able to access the members width and height 

(which are private) of different objects of type Rectangle. 

Notice though that neither in the declaration of duplicate nor 

in its later use in main, function duplicate is considered a member 

of class Rectangle. It isn't! It simply has access to its private 

and protected members without being a member.

}