C++設計模式淺識享元模式

享元模式(Flyweight)：運用共享技術有效地支持大量細粒度的對象。

四個角色類：

1. Flyweight享元類：所有具體享元類的超類或接口，通過這個接口，Flyweight可以接受並作用於外部狀態。
2. Flyweight享元工廠類：一個享元工廠，用來創建並管理Flyweight，當用戶請求一個Flyweight時，FlyweightFactory對象提供一個已創建的實例或者創建一個（如果不存在的話）。
3. ConcreteFlyweight具體享元類：繼承Flyweight超類或實現Flyweight接口，併爲內部狀態增加存儲空間。
4. UnSharedConcreteFlyweight不需共享的具體Flyweight子類、指那些不需要共享的Flyweight子類。因爲Flyweight接口類共享成爲可能，但並不強制共享。

模式實現：

//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;
    }
};

//不需共享的Flyweight子類
class UnSharedConcreteFlyweight: public Flyweight{
public:
    virtual void Operation(int extrinsicState){
        std::cout << "UnSharedConcreteFlyweight: " << extrinsicState << std::endl;
    }
};

//享元工廠，用來創建並管理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];
    }
};

客戶端：

//Client
int 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：

1. 如果一個應用程序使用了大量的對象，而大量的這些對象造成了很大的存儲開銷時就應該考慮使用。
2. 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享對象取代很多組對象，此時可以考慮使用享元模式。