Method compare

Examples/NoModules/Chapter 13/Ex13_11/Box.h:20–25

  }

  int compare(const Box& box) const
  {
    if (volume() < box.volume()) return -1;
    if (volume() == box.volume()) return 0;
    return +1;
  }

  friend std::ostream& operator<<(std::ostream& out, const Box& box)
  {