(C++设计模式) ------享元模式-- 结构型模式

素颜马尾好姑娘i 2021-11-22 15:32 246阅读 0赞

设计模式的学习是一个循序渐进的过程，每一个设计师都是在套用这几种设计模式来实现自己的软件构架，很多都是融合会贯通的，一个设计模式中不仅仅是用到一种，所以要灵活运用这些设计模式。理解了设计模式对设计和代码的阅读都有很大的好处！！

今天我们学习结构性模式享元（flyweight）模式;

Flyweight—每天跟MM发短信，手指都累死了，最近买了个新手机，可以把一些常用的句子存在手机里，要用的时候，直接拿出来，在前面加上MM的名字就可以发送了，再不用一个字一个字敲了。共享的句子就是Flyweight，MM的名字就是提取出来的外部特征，根据上下文情况使用。  
        享元模式：FLYWEIGHT在拳击比赛中指最轻量级。享元模式以**共享的方式**高效的支持大量的细粒度对象。说的再具体一些是将所有具有相同状态的对象指向同一个引用,从而解决了系统在创建大量对象时所带来的**内存压力。**  
        享元模式能做到共享的关键是区分内蕴状态和外蕴状态。

**内蕴状态**存储在享元内部并且可以共享，不会随环境的改变而有所不同。  
**外蕴状态**是随环境的改变而改变的，不可以共享。外蕴状态不能影响内蕴状态，它们是相互独立的。将可以共享的状态和不可以共享的状态从常规类中区分开来，将不可以共享的状态从类里剔除出去。  
        **亨元对象的外蕴状态必须由客户端保存，并在亨元对象被创建之后，在需要使用的时候再传入到亨元对象内部。**

**注意：**客户端不可以直接创建被共享的对象，而应当使用一个**工厂对象**负责创建被共享的对象。享元模式大幅度的降低内存中对象的数量。

![20130812153316218][]

** 使用的场合：**

*  需要创建大量对象。
 *  考虑内存成本。
 *  大多数对象属性可以对外共享。
 *  应用程序不必授权给唯一的对象，因为同一对象被实例后需要被重复使用。
    
    **例子：**
    
    比如围棋有300颗棋子，用一般的设计模式，创建一个类,每个棋子都用一个对象的话那就会非常麻烦，并且各自定义各自在棋盘的位置.....等等
    
    而使用亨元模式来实现的话，就用两个对象 ：一个黑，一个白。
    
    这样就可以了，至于棋子的方位不同,那只是对象的不同的外部表现形式或者说是外部状态。
    
    这样三百多个对象就减到了两个对象。
    
    **优缺点：**
    
    **优点：**减少对象数量，节省内存空间  
    **缺点：**维护共享对象，需要额外开销
    
    换句话说就是：享元模式的优点在于它大幅度地降低内存中对象的数量。但是，它做到这一点所付出的代价也是很高的：享元模式使得系统更加复杂。为了使对象可以共享，需要将一些状态外部化，这使得程序的逻辑复杂化。另外它将享元对象的状态外部化，而读取外部状态使得运行时间稍微变长。
    
    **本质：**
    
    分离与共享
 *   
    
    **代码举例：引用自：[http://blog.csdn.net/roynee/article/details/4633474][http_blog.csdn.net_roynee_article_details_4633474]**
 *  以纯文本为例，**每个字符CharFlyweight为共享的轻量级对**象，而由这些字符构成的行则为**非共享的对象RowFlyweight**，而TextFlyweightFactory则管理其共享对象的对象池。当用户向该工厂请求字符时，就从对象池中取得，而当用户向该工厂请求字符串时候则通过创建非共享的RowFlyweight获取，但此种每个字符的获取仍然向TextFlyweightFactory共享对象的对象池提出请求。
    
    ![1][]
    
     
 *  【程序】
    
      // test.cpp : Defines the entry point for the console application. // \#include "stdafx.h" \#include "iostream" \#include "string" \#include <vector> using namespace std; /\*\* \* AbstractFlyweight Class\[Abstract Class\] \*/ class AbstractFlyweight \{ public: AbstractFlyweight()\{\} virtual ~AbstractFlyweight()\{\} /// Interface of Writing the Element of Flyweight virtual void Write()=0; \}; class CharFlyweight:public AbstractFlyweight \{ public: /\*constructor and destructor\*/ CharFlyweight(char tChar) \{ this->\_Char=tChar; \} virtual ~CharFlyweight()\{\} /// Interface of Writing the Element of Flyweight virtual void Write() \{ cout<<this->\_Char<<flush; return; \} /// Get the Char virtual char GetChar() \{ return this->\_Char; \} private: char \_Char; \}; class RowFlyweight:public AbstractFlyweight \{ public: RowFlyweight()\{\} virtual ~RowFlyweight()\{\} /// Interface of Writing the Element of Flyweight virtual void Write() \{ for(vector<abstractflyweight>::iterator iter=this->\_vecFlyweight.begin();iter!=this->\_vecFlyweight.end();++iter) (\*iter)->Write(); return; \} virtual void AddFlyweight(AbstractFlyweight \* flyweight) \{ this->\_vecFlyweight.push\_back(flyweight); \} private: vector<abstractflyweight> \_vecFlyweight; \}; /\*\* \* TextFlyweightFactory: provide the CharFlyweight object \*/ class TextFlyweightFactory \{ public: /\*Constructor and destructor\*/ TextFlyweightFactory()\{\} virtual ~TextFlyweightFactory()\{\} /// Get the CharFlyweight Object virtual AbstractFlyweight\* GetFlyweight(char \_char) \{ vector<charflyweight>::iterator iter = this->\_vecCharFlyweight.begin(); for(;iter != this->\_vecCharFlyweight.end();++iter) \{ if((\*iter)->GetChar()==\_char)return \*iter; \} CharFlyweight \*newchar=new CharFlyweight(\_char);/\*new character\*/ this->\_vecCharFlyweight.push\_back(newchar); /\*Record it\*/ return newchar; \} virtual AbstractFlyweight\* GetFlyweight(string str) \{ RowFlyweight \*row=new RowFlyweight(); for(int i=0;i < str.length();++i) row->AddFlyweight(this->GetFlyweight(str\[i\])); /\*Get the char from VecCharFlyweight\*/ return row; \} /// Just be used to show the size of vector virtual int SizeVector() \{ return this->\_vecCharFlyweight.size(); \} private: /\*CharFlyweight Vector\*/ vector<charflyweight> \_vecCharFlyweight; \}; int main (int argc, char \*argv\[\]) \{ /\*Now There is A factory\*/ TextFlyweightFactory \*factoryApi=new TextFlyweightFactory(); /\*Now Get CharFlyweight-shared\*/ factoryApi->GetFlyweight('a')->Write(); factoryApi->GetFlyweight('a')->Write(); factoryApi->GetFlyweight('c')->Write(); cout<<"\\nFactory's Size is: "<<factoryapi->SizeVector(); /\*Now New a Row-unshared\*/ factoryApi->GetFlyweight("hello \\n")->Write(); return(0); \} </factoryapi-></this->
 *  运行结果：
 *  ![Center][]
 *   
    
      virtual AbstractFlyweight\* GetFlyweight(string str)   
        \{   
            RowFlyweight \*row=new RowFlyweight();   
            for(int i=0;i < **str.length() -1**;++i)     //这地方改成减一  
                row->AddFlyweight(this->GetFlyweight(str\[i\]));  /\*Get the char from VecCharFlyweight\*/   
            return row;   
        \} 
 *   然后main函数修改一下更能体现共享的概念： virtual AbstractFlyweight\* GetFlyweight(string str) \{ RowFlyweight \*row=new RowFlyweight(); for(int i=0;i < str.length() -1;++i) //这地方改成减一 row->AddFlyweight(this->GetFlyweight(str\[i\])); /\*Get the char from VecCharFlyweight\*/ return row; \} int main (int argc, char \*argv\[\]) \{ /\*Now There is A factory\*/ TextFlyweightFactory \*factoryApi=new TextFlyweightFactory(); /\*Now Get CharFlyweight-shared\*/ factoryApi->GetFlyweight('a')->Write(); factoryApi->GetFlyweight('a')->Write(); factoryApi->GetFlyweight('a')->Write(); cout<<"\\nFactory's Size is: "<<factoryapi->SizeVector(); /\*Now New a Row-unshared\*/ factoryApi->GetFlyweight("aaaaaaa\\n")->Write(); cout<<"\\nFactory's Size is: "<<factoryapi->SizeVector()<<endl; return(0); \} </factoryapi-></factoryapi->
 *  输出结果是：
 *  ![Center 1][]
 *  不错的相关网站：①[http://www.csdn.net/article/2012-06-25/2806857][http_www.csdn.net_article_2012-06-25_2806857]
 *  ② [ http://blog.csdn.net/roynee/article/details/4633474][http_blog.csdn.net_roynee_article_details_4633474]

[20130812153316218]: /images/20211122/ce3b4d6113f9480bbaddcbaab7c81605.png
[http_blog.csdn.net_roynee_article_details_4633474]: http://blog.csdn.net/roynee/article/details/4633474
[1]: /images/20211122/f555c2c79864410c8ea36d4d4d807230.png
[Center]: /images/20211122/58f94b83f9b24b879d92b2364d86d45b.png
[Center 1]: /images/20211122/40f09fd78d2042378a3dda11702a4424.png
[http_www.csdn.net_article_2012-06-25_2806857]: http://www.csdn.net/article/2012-06-25/2806857