C++设计模式之享元模式

概述

想想我们编辑文档用的wps，文档里文字很多都是重复的，我们不可能为每一个出现的汉字都创建独立的空间，这样代价太大，最好的办法就是共享其中相同的部分，使得需要创建的对象降到最小，这个就是享元模式的核心，即运用共享技术有效地支持大量细粒度的对象。

享元对象能做到共享的关键是区分内蕴状态（Internal State）和外蕴状态（External State）。内蕴状态是存储在享元对象内部并且不会随环境改变而改变。因此内蕴状态并可以共享。

外蕴状态是随环境改变而改变的、不可以共享的状态。享元对象的外蕴状态必须由客户端保存，并在享元对象被创建之后，在需要使用的时候再传入到享元对象内部。外蕴状态与内蕴状态是相互独立的。

类图与样例

// Flyweight.cpp : 定义控制台应用程序的入口点。
//

#include "stdafx.h"
#include <iostream>
#include<string>
#include <map>
using namespace std;

class Character
{
public:
    virtual ~Character(){};
    virtual void SetSize(int,int) = 0;
    virtual void Display() = 0;
protected:
    Character() {}
    char m_chSymbol;
    int m_nWeight;
    int m_nHeight;
};

class CharacterA:public Character
{
public:
        CharacterA()
        {
            m_chSymbol = 'A';
            m_nWeight = 100;
            m_nHeight = 200;
        }
        virtual ~CharacterA() {}
        void SetSize(int nWeight,int nHeight)
        {
            m_nWeight = nWeight;
            m_nHeight = nHeight;
        }
        void Display()
        {
            cout << "CharacterA：" << m_chSymbol << "(" << m_nWeight << "," << m_nHeight << ")" << endl;
        }
};

class CharacterB:public Character
{
public:
    CharacterB()
    {
        m_chSymbol = 'B';
        m_nWeight = 100;
        m_nHeight = 200;
    }
    virtual ~CharacterB() {}
    void SetSize(int nWeight,int nHeight)
    {
        m_nWeight = nWeight;
        m_nHeight = nHeight;
    }
    void Display()
    {
        cout << "CharacterB：" << m_chSymbol << "(" << m_nWeight << "," << m_nHeight << ")" << endl;
    }
};

class CharacterFactory
{
public:
    CharacterFactory()
    {
        m_mChar.insert(make_pair<char, Character*>('A', new CharacterA));
        m_mChar.insert(make_pair<char, Character*>('B', new CharacterB));
    }
    virtual ~CharacterFactory() {}
    Character* GetCharacter(char chIn)
    {
        map<char, Character*>::iterator it = m_mChar.find(chIn);
        if(it != m_mChar.end())
        {
            return (Character*)it->second;
        }
        return NULL;
    }
private:
    std::map<char, Character*> m_mChar;

};
int _tmain(int argc, _TCHAR* argv[])
{

    cout<<"享元模式："<<endl;
    CharacterFactory *pFactory = new CharacterFactory();
    //内蕴状态 存储在享元对象内部并且不会随环境改变而改变

    Character* ch1 = pFactory->GetCharacter('A');
    ch1->Display();
    Character* ch2 = pFactory->GetCharacter('B');
    ch2->SetSize(500, 800);
    ch2->Display();
    system("pause");
    return 0;
}