设计模式-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。

1.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；还有就是对象的大多数状态可以外部状态，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。
2.享元模式可以避免大量非常相似类的开销。在程序设计中，有时就能够受到大幅度地减少需要实例化的类的数量。如果能把那些参数转移到类实例的外面，在方法调用时将他们传递进来，就可以通过共享大幅度地减少单个实例的数目。
享元模式实现代码：

#pragma once
#include <map>
#include <string>
#include <iostream>
using namespace std;

//Flyweight类，它是所有具体享元的超类或接口，通过这个接口，Flyweight可以接受并作用于外部状态。
class CFlyweight
{
public:
	virtual void Operation(int nExtrinsicstate) = 0;
};

//CConcreteFlyweight是继承Flyweight超类或实现Flyweight接口，并为内部状态增加存储空间。
class CConcreteFlyweight : public CFlyweight
{
public:
	void Operation(int nExtrinsicstate)
	{
		cout<<"ju ti Flyweight:"<<nExtrinsicstate<<endl;
	}
};

//UnsharedConcreteFlyweight是指那些不需要共享的Flyweight子类。因为Flyweight接口共享称为可能，
//但它并不强制共享。
class CUnsharedConcreteFlyweight:public CFlyweight
{
public:
	void Operation(int nExtrinsicstate)
	{
		cout<<"bu gong xiang de ju ti Flyweight:"<<nExtrinsicstate<<endl;
	}
};

//FlyweightFactiry,是一个享元工厂，用来创建并管理Flyweight对象。它主要是用来确保合理地共享
//Flyweight，当用户请求一个Flyweigth时，FlyweightFactory对象提供一个已经创建的实例或者创建一个。
class CFlyweightFactory
{
private:
	map<string ,CFlyweight *>m_mpFlyweight;
public:
	CFlyweightFactory()
	{
		m_mpFlyweight.clear();
		m_mpFlyweight["X"] = new CConcreteFlyweight();
		m_mpFlyweight["Y"] = new CConcreteFlyweight();
		m_mpFlyweight["Z"] = new CConcreteFlyweight();
	}
	~CFlyweightFactory()
	{
		delete m_mpFlyweight["X"];
		delete m_mpFlyweight["Y"];
		delete m_mpFlyweight["Z"];
	}
	CFlyweight * GetFlyweight(string strKey)
	{
		return m_mpFlyweight[strKey];
	}
};

客户端调用：

#include "stdafx.h"
#include <iostream>
#include <windows.h>
#include "FlyweightMode.h"

using namespace std;

int main()
{
	int nExtrinsicstate = 22;
	CFlyweightFactory *pF = new CFlyweightFactory();

	CFlyweight *pNowFlyweight = pF->GetFlyweight("X");
	pNowFlyweight->Operation(--nExtrinsicstate);

	pNowFlyweight = pF->GetFlyweight("Y");
	pNowFlyweight->Operation(--nExtrinsicstate);

	pNowFlyweight = pF->GetFlyweight("Z");
	pNowFlyweight->Operation(--nExtrinsicstate);

	CUnsharedConcreteFlyweight *pUF = new CUnsharedConcreteFlyweight();
	pUF->Operation(--nExtrinsicstate);

	delete pUF;
	delete pF;

   return 0;
}

执行结果：