C++設計模式淺識享元模式

享元模式(Flyweight)：運用共享技術有效支援大量細粒度的物件。

四個角色類：

Flyweight享元類：所有具體享元類的超類或接口，透過這個接口，Flyweight可以接受並作用於外部狀態。

Flyweight享元工廠類別：一個享元工廠，用來建立並管理Flyweight，當使用者要求一個Flyweight時，FlyweightFactory物件提供一個已建立的實例或建立一個（如果不存在的話）。

ConcreteFlyweight具體享元類別：繼承Flyweight超類別或實作Flyweight接口，並為內部狀態增加儲存空間。

UnSharedConcreteFlyweight不需要共享的具體Flyweight子類別、指那些不需要共享的Flyweight子類別。因為Flyweight介面類別共享成為可能，但並不會強制共享。

模式實作：

[code]//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];
    }
};

客戶端：

[code]//Client
int 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。

物件的大多數狀態可以使用外部狀態，如果刪除物件的外部狀態，那麼可以用相對較少的共享物件取代許多群組對象，此時可以考慮使用享元模式。

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