// Rectangle is declared as a friend of Square so that Rectangle member functions
// could have access to the protected and private members of Square, more specifically
// to Square::side, which describes the side width of the square.

// Ther is an empty declaration of class Square.
// This is necessary because within the declaration of Rectangle we refer to
// Square (as a parameter in convert()). The definition of Square is included later,
// so if we did not include an empty declaration for Square this class would not be
// visible from within the definition of Rectangle.

// In this example, Rectangle is considered as a friend class by Square, but Rectangle
// does not consider Square to be a friend, so Rectangle can access the protected
// and private members of Square but not the reverse way. Of course, we could have
// declared also Square as friend of Rectangle if we wanted to.

// Another property of friendships is that they are not transitive: The friend of a
// friend is not considered to be a friend unless explicitly specified.

#include <iostream>
using namespace std;

class Square;

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

class Square {
  private:
    int side;
  public:
    void set_side (int a){ side=a; }
    friend class Rectangle;
};

void Rectangle::convert (Square a) {
  width = a.side;
  height = a.side;
}
  
int main () {
  Square sqr;
  Rectangle rect;
  sqr.set_side(4);
  rect.convert(sqr);
  cout << "The area is " << rect.area();
  getchar();
  return 0;
}