Erle Robotics C++ GitBook

friend Functions/Classes

Occasionally you’ll want to allow a function that is not a member of a given class to access the private fields/methods of that class. (This is particularly common in operator overloading.)

We can specify that a given external function gets full access rights by placing the signature of the function inside the class, preceded by the word friend.So, 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.The same happend with classes, a friend class is a class whose members have access to the private or protected members of another class:

// friend class
#include <iostream>
using namespace std;

class Square;

class Rectangle {
    int width, height;
    int area ()
      {return (width * height);}
    void convert (Square a);

class Square {
  friend class Rectangle;
    int side;
    Square (int a) : side(a) {}

void Rectangle::convert (Square a) {
  width = a.side;
  height = a.side;

int main () {
  Rectangle rect;
  Square sqr (4);
  cout << rect.area();

  return 0;

In this example, class Rectangle is a friend of class Square allowing Rectangle's member functions to access private and protected members of Square. More concretely, Rectangle accesses the member variable Square::side, which describes the side of the square.