C++ puzzle answer

x can change and here is how:

#include <stdio.h>

class Puzzle {
  Puzzle(int x_arg) : x(x_arg) {}

  int get_x() const {
    return x;

  void change_x(Puzzle& r, int new_x) const {
    r.x = new_x;

  // Disable default operator=
  Puzzle& operator=(const Puzzle&);

  int x;

int main(void) {
  Puzzle r1(1);
  Puzzle r2(1);
  r1.change_x(r2, 2);
  printf("%d\n", r2.get_x());
  // This is also valid:
  r1.change_x(r1, 2);
  printf("%d\n", r1.get_x());
  return 0;

In C++ access control modifiers (public, protected, private) work on a class level. Objects of the same class can access and modify private fields of each other. The const keyword only guarantees the object on which the const method is called won’t change its state, it is legal to alter state of an object passed via a non-const argument.

Not in all object oriented languages access control modifiers work on a class level. For example, in Ruby an object can’t invoke private methods of other objects of the same class.