CODE:

class GameState
{
private:
    GameBase *mNowGameState;
public:
    GameState() : mNowGameState(NULL) {}
    ~GameState(){  clear();  }
    
    void set(GameBase *GameState)
    {
        delete mNowGameState;
        mNowGameState = GameState;
    }
    
    GameBase * get(){  return mNowGameState;  }
    const GameBase *get() const {  return mNowGameState;  }  //const版も
    bool isEnd() const {  return !mNowGameState;  }  //これはconstに

    void clear(){  set(NULL);  }  //clear()はpublicに．
};