宣告：本博文篇幅短，適合review。

一、概念

        運用共享技術，有效地支援大量細粒度的物件。減少記憶體消耗。它有兩個狀態：

       內部狀態：儲存在享元物件內部，並且不會隨環境改變而改變的物件，可以共享。

       外部狀態：隨環境改變而改變的物件，不可共享。

二、結構模式圖

class Flyweight
{
public:
	virtual ~Flyweight();
	virtual void Operation(const std::string & _others){}

	std::string getState(){
		return this->m_state;
	}
protected:
	Flyweight(std::string _state){
		this->m_state = _state;
	}
private:
	std::string m_state;
};

class ConcreteFlyweight : public Flyweight
{
public:
	ConcreteFlyweight(std::string _state) : Flyweight(_state) {}
	~ConcreteFlyweight();

	void Operation(const std::string & _others)
	{
		cout<<"ConcreteFlyweight Operation : "<<m_state<<endl;
	};
};

class FlyweightFactory
{
public:
	FlyweightFactory();
	~FlyweightFactory();

	Flyweight * getFlyweight(const std::string & key){
		std::vector<Flyweight *>::iterator it = m_Vec.begin();

		for (; it != m_Vec.end() ; it++){
			if ( (*it)->getState() == key )
			{
				cout<<"has created"<<endl;
				return *it;
			}
		}

		Flyweight * fw = new ConcreteFlyweight(key);
		m_Vec.push_back(fw);
		return fw;
	}
private:
	std::vector<Flyweight *> m_Vec;
};

void main(){
	FlyweightFactory * fc = new FlyweightFactory();
	Flyweight * fw1 = fc->getFlyweight("xxx");
	Flyweight * fw2 = fc->getFlyweight("yyy");
	Flyweight * fw3 = fc->getFlyweight("xxx");
}
 

三、例子

class Chess
{
public:
	virtual ~Chess();
	virtual void draw(const std::string & _color){}

	std::string getState(){
		return this->m_state;
	}
protected:
	Chess(std::string _state){
		this->m_state = _state;
	}
private:
	std::string m_state;
};

class FiveChess : public Chess
{
public:
	FiveChess(std::string _state) : Chess(_state) {}
	~FiveChess();

	void draw(const std::string & _color)
	{
		cout<<"FiveChess draw : "<<m_state<<" "<<_color<<endl;
	};
};

class ChessFactory
{
public:
	ChessFactory();
	~ChessFactory();

	Chess * getChess(const std::string & key){
		std::vector<Chess *>::iterator it = m_Vec.begin();

		for (; it != m_Vec.end() ; it++){
			if ( (*it)->getState() == key )
			{
				cout<<"has created"<<endl;
				return *it;
			}
		}

		Chess * c = new FiveChess(key);
		m_Vec.push_back(c);
		return c;
	}
private:
	std::vector<Chess *> m_Vec;
};

void main(){
	ChessFactory * cf = new ChessFactory();
	Chess * c1 = cf->getChess("white");
	c1.draw("白色");
	Chess * c2 = cf->getChess("black");
	c2.draw("黑色");
	Chess * c3 = cf->getChess("white");
	c3.draw("白色");
}
 

四、優缺點

       1、優點

            a、極大的減少系統中物件的個數。

            b、享元模式使得享元物件能夠在不同的環境被共享。

       2、缺點

            a、使得系統更加複雜。為了使物件可以共享，需要將一些狀態外部化，這使得程式的邏輯複雜化。

       b、享元模式將享元物件的狀態外部化，而讀取外部狀態使得執行時間稍微變長。