// Apple.hpp

#ifndef MYGAME_APPLE_HPP
#define MYGAME_APPLE_HPP

#include "Element.hpp"

class Apple : public Element { ... };

#endif

////////////////////////////////////////////////////////////////////////////////

// Game.hpp

#ifndef MYGAME_GAME_HPP
#define MYGAME_GAME_HPP

class Game { ... Apple mApple; ... }

#endif