C++设计模式浅识享元模式

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享元模式(Flyweight)：运用共享技术有效地支持大量细粒度的对象。

四个角色类：

Flyweight享元类：所有具体享元类的超类或接口，通过这个接口，Flyweight可以接受并作用于外部状态。
Flyweight享元工厂类：一个享元工厂，用来创建并管理Flyweight，当用户请求一个Flyweight时，FlyweightFactory对象提供一个已创建的实例或者创建一个（如果不存在的话）。
ConcreteFlyweight具体享元类：继承Flyweight超类或实现Flyweight接口，并为内部状态增加存储空间。
UnSharedConcreteFlyweight不需共享的具体Flyweight子类、指那些不需要共享的Flyweight子类。因为Flyweight接口类共享成为可能，但并不强制共享。

模式实现：

//享元类class Flyweight{public:    virtual void Operation(int extrinsicState){}};//具体享元类class ConcreteFlyweight: public Flyweight{public:    virtual void Operation(int extrinsicState)override{        std::cout << "ConcreteFlyweight: " << extrinsicState << std::endl;    }};//不需共享的Flyweight子类class UnSharedConcreteFlyweight: public Flyweight{public:    virtual void Operation(int extrinsicState){        std::cout << "UnSharedConcreteFlyweight: " << extrinsicState << std::endl;    }};//享元工厂，用来创建并管理Flyweight对象class FlyweightFactory{private:    std::map<std::string, Flyweight*> flyweights;public:    FlyweightFactory(){        flyweights["X"] = new ConcreteFlyweight;        flyweights["Y"] = new ConcreteFlyweight;        flyweights["Z"] = new ConcreteFlyweight;    }    Flyweight* GetFlyweight(std::string key){        return (Flyweight*)flyweights[key];    }};

客户端：

//Clientint main(){    //外部状态    int extrinsicState = 22;    //工厂    FlyweightFactory *f = new FlyweightFactory;    Flyweight* fx = f->GetFlyweight("X");    fx->Operation(--extrinsicState);  //Output: ConcreteFlyweight: 21    Flyweight* fy = f->GetFlyweight("Y");    fy->Operation(--extrinsicState);  //Output: ConcreteFlyweight: 20    Flyweight* fz = f->GetFlyweight("Z");    fz->Operation(--extrinsicState);  //Output: ConcreteFlyweight: 19    Flyweight *uf = new UnSharedConcreteFlyweight;  //Output: UnSharedConcreteFlyweight: 18    uf->Operation(--extrinsicState);    return 0;}

享元模式好处：

如果一个应用程序使用了大量的对象，而大量的这些对象造成了很大的存储开销时就应该考虑使用。
对象的大多数状态可以使用外部状态，如果删除对象的外部状态，那么可以用相对较少的共享对象取代很多组对象，此时可以考虑使用享元模式。

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