C++設計模式淺識享元模式

享元模式(Flyweight)：運用共用技術有效地支援大量細粒度的對象。

四個角色類：

Flyweight享元類：所有具體享元類的超類或介面，通過這個介面，Flyweight可以接受並作用於外部狀態。

Flyweight享元工廠類：一個享元工廠，用來建立並管理Flyweight，當使用者請求一個Flyweight時，FlyweightFactory對象提供一個已建立的執行個體或者建立一個（如果不存在的話）。

ConcreteFlyweight具體享元類：繼承Flyweight超類或實現Flyweight介面，並為內部狀態增加儲存空間。

UnSharedConcreteFlyweight不需共用的具體Flyweight子類、指那些不需要共用的Flyweight子類。因為Flyweight介面類共用成為可能，但並不強制共用。

模式實現：

[code]//享元類class Flyweight{public:    virtual void Operation(int extrinsicState){}};//具體享元類class ConcreteFlyweight: public Flyweight{public:    virtual void Operation(int extrinsicState)override{        std::cout << "ConcreteFlyweight: " << extrinsicState << std::endl;    }};//不需共用的Flyweight子類class UnSharedConcreteFlyweight: public Flyweight{public:    virtual void Operation(int extrinsicState){        std::cout << "UnSharedConcreteFlyweight: " << extrinsicState << std::endl;    }};//享元工廠，用來建立並管理Flyweight對象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];    }};

用戶端：

[code]//Clientint main(){    //外部狀態    int extrinsicState = 22;    //工廠    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;}

享元模式好處：

如果一個應用程式使用了大量的對象，而大量的這些對象造成了很大的儲存開銷時就應該考慮使用。

對象的大多數狀態可以使用外部狀態，如果刪除對象的外部狀態，那麼可以用相對較少的共用對象取代很多組對象，此時可以考慮使用享元模式。

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