C++设计模式浅识享元模式

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：

[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];
    }
};

客户端：

[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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