Access private variable in global scope

Question

In this below code, the foo function in the global scope tries to access the private variables of a Box, which ofcourse doesn't work. I have to make the foo function work with one line of code at the place show code for a school assignment.

#include <iostream>

using namespace std;
class Box {
      int x,y;

      public:
             Box(int xi,int yi) {x=xi;y=yi;}
             // One line of code to make foo(Box, Box) work
};

bool foo(Box l,Box r) {return (l.x*l.y)>(r.x*r.y);}

int main(int argc, char* argv[]) {
    Box b1(3,4),b2(1,2);

    if (foo(b1,b2)) cout << "b1>b2\n";

    return cin.get();
}

Answer 1

Look into the friend keyword.

Answer 2

First off, this is not a priori a dirty thing. The placement of the comment line already indicates that the class Box controls who is allowed to touch its privates (pun intended).

Secondly, since this is a school assignment I think that the solution should have been mentioned in class: this can be achieved using a friend declaration.