五、Singleton(单件)
情景举例:
保证仅有一个迷宫工厂的实例。
代码示例:
/* 注意静态函数和静态变量,它们实现了单件 构造函数设为protected以供子类访问,若没有子类可以设为private */ class MazeFactory { public: static MazeFactory* Instance(); // existing interface goes here protected: MazeFactory(); private: static MazeFactory* _instance; }; /* */ MazeFactory* MazeFactory::_instance = 0; MazeFactory* MazeFactory::Instance () { if (_instance == 0) { _instance = new MazeFactory; } return _instance; } /* 如果有子类的情况下,则按以下方法实现单件 */ MazeFactory* MazeFactory::Instance () { if (_instance == 0) { const char* mazeStyle = getenv("MAZESTYLE"); /* */ if (strcmp(mazeStyle, "bombed") == 0) { _instance = new BombedMazeFactory; /* */ } else if (strcmp(mazeStyle, "enchanted") == 0) { _instance = new EnchantedMazeFactory; /* */ // ... other possible subclasses /* */ } else { // default _instance = new MazeFactory; } } return _instance; }
个人理解:
单件模式在C++中实现的方法就是使用静态函数和静态变量,注意要将构造函数设置为保护的(有子类的情况)或私有的。