friend function cpp reference

1// friend functions
2#include <iostream>
3using namespace std;
4
5class Rectangle {
6    int width, height;
7  public:
8    Rectangle() {}
9    Rectangle (int x, int y) : width(x), height(y) {}
10    int area() {return width * height;}
11    friend Rectangle duplicate (const Rectangle&);
12};
13
14Rectangle duplicate (const Rectangle& param)
15{
16  Rectangle res;
17  res.width = param.width*2;
18  res.height = param.height*2;
19  return res;
20}
21
22int main () {
23  Rectangle foo;
24  Rectangle bar (2,3);
25  foo = duplicate (bar);
26  cout << foo.area() << '\n';
27  return 0;
28}