C++设计模式实现--享元(Flyweight模式)

一、描述

设计模式中的享元模式，避免大量拥有相同内容的小类的开销(如耗费内存),使大家共享一个类(元类).

问题

在面向对象系统的设计何实现中,创建对象是最为常见的操作。这里面就有一个问题:如果一个应用程序使用了太多的对象,就会造成很大的存储开销。特别是对于大量轻量级(细粒度)的对象,比如在文档编辑器的设计过程中,我们如果为没有字母创建一个对象的话,系统可能会因为大量的对象而造成存储开销的浪费。例如一个字母“a”在文档中出现了100000 次,而实际上我们可以让这一万个字母“a”共享一个对象,当然因为在不同的位置可能字母“a”有不同的显示效果(例如字体和大小等设置不同),在这种情况我们可以为将对象的状态分为“外部状态”和“内部状态”,将可以被共享(不会变化)的状态作为内部状态存储在对象中,而外部对象(例如上面提到的字体、大小等)我们可以在适当的时候将外部对象最为参数传递给对象(例如在显示的时候,将字体、大小等信息传递给对象)。

其典型的结构图为:



可以从图 2-1 中看出,Flyweight 模式中有一个类似 Factory 模式的对象构造工厂

FlyweightFactory,当客户程序员(Client)需要一个对象时候就会向 FlyweightFactory 发出请求对象的消息 GetFlyweight()消息,FlyweightFactory 拥有一个管理、存储对象的“仓库”(或者叫对象池,vector 实现),GetFlyweight()消息会遍历对象池中的对象,如果已经存在则直接返回给 Client,否则创建一个新的对象返回给 Client。当然可能也有不想被共享的对象(例如结构图中的
UnshareConcreteFlyweight),但不在本模式的讲解范围,故在实现中不给出。

二、实例

如上所描述的信息，创建类图：



本人的工程目录：



注释：

main:客户程序员(Client)

FlyweightFactory:“仓库”(对象池),

Flyweight：对象池中的对象

ConcreteFlyweight：被共享的对象

代码：

仓库”(对象池)：FlyweightFactory类

FlyweightFactory.h

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#ifndef __Flyweight__FlyweightFactory__

#define __Flyweight__FlyweightFactory__

#include <iostream>

#include "Flyweight.h"

#include <string>

#include <vector>

#include "ConcreteFlyweight.h"

using std::cout;

using std::endl;

using std::string;

using std::vector;

class FlyweightFactory

{

public:

FlyweightFactory();

~FlyweightFactory();

Flyweight* GetFlyweight(const string &key);

private:

vector<Flyweight*> _fly;

};

#endif /* defined(__Flyweight__FlyweightFactory__) */

FlyweightFactory.cpp

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#include "FlyweightFactory.h"

FlyweightFactory::FlyweightFactory()

{

}

FlyweightFactory::~FlyweightFactory()

{

}

Flyweight* FlyweightFactory::GetFlyweight(const string &key)

{

vector<Flyweight*>::iterator it = _fly.begin();

for (; it != _fly.end(); it++)

{

if ((*it)->GetIntrinsicState() == key)

{

cout<<"already created by users...."<<endl;

return *it;

}

}

Flyweight *fn = new ConcreteFlyweight(key);

_fly.push_back(fn);

return fn;

}

对象池中的对象:Flyweight类

Flyweight.h

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#ifndef __Flyweight__Flyweight__

#define __Flyweight__Flyweight__

#include <iostream>

#include <string>

using std::string;

class Flyweight

{

public:

Flyweight(void);

virtual ~Flyweight(void);

virtual void Operation(const string& extrinsicState);

string GetIntrinsicState();

protected:

Flyweight(string intrinsicState);

private:

string _intrinsicState;

};

#endif /* defined(__Flyweight__Flyweight__) */

Flyweight.cpp

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#include "Flyweight.h"

Flyweight::Flyweight(void)

{

}

Flyweight::Flyweight(string intrinsicState)

{

this->_intrinsicState = intrinsicState;

}

Flyweight::~Flyweight()

{

}

void Flyweight::Operation(const string &extrinsicState)

{

}

string Flyweight::GetIntrinsicState()

{

return this->_intrinsicState;

}

被共享的对象:ConcreteFlyweight

ConcreteFlyweight.h

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#ifndef __Flyweight__ConcreteFlyweight__

#define __Flyweight__ConcreteFlyweight__

#include <iostream>

#include "Flyweight.h"

class ConcreteFlyweight:public Flyweight

{

public:

ConcreteFlyweight(void);

ConcreteFlyweight(string intrinsicState);

~ConcreteFlyweight();

void Operation(const string& extrinsicState);

};

#endif /* defined(__Flyweight__ConcreteFlyweight__) */

ConcreteFlyweight.cpp

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#include "ConcreteFlyweight.h"

using std::cout;

using std::endl;

ConcreteFlyweight::ConcreteFlyweight(void)

{

}

ConcreteFlyweight::ConcreteFlyweight(string intrinsicState)

{

cout<<"ConcreteFlyweight Build....."<<intrinsicState<<endl;

}

ConcreteFlyweight::~ConcreteFlyweight()

{

}

void ConcreteFlyweight::Operation(const string &extrinsicState)

{

cout<<"ConcreteFlyweight:内蕴["<<this->GetIntrinsicState()<<"] 外 蕴["<<extrinsicState<<"]"<<endl;

}

客户程序员(Client)

main.cpp

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#include <iostream>

#include "Flyweight.h"

#include "ConcreteFlyweight.h"

#include "FlyweightFactory.h"

using namespace std;

int main(int argc, const char * argv[])

{

FlyweightFactory *fc = new FlyweightFactory();

Flyweight* fw1 = fc->GetFlyweight("hello");

Flyweight* fw2 = fc->GetFlyweight("world!");

Flyweight* fw3 = fc->GetFlyweight("hello2");

// insert code here...

std::cout << "Hello, World!\n";

return 0;

}

结果如下: