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) = ;
     virtual void Display() = ;
 protected:
     Character() {}
     char m_chSymbol;
     int m_nWeight;
     int m_nHeight;
 };

 class CharacterA:public Character
 {
 public:
         CharacterA()
         {
             m_chSymbol = 'A';
             m_nWeight = ;
             m_nHeight = ;
         }
         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 = ;
         m_nHeight = ;
     }
     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(, );
     ch2->Display();
     system("pause");
     return ;
 }