Decorator Pattern

1.Decorator 模式通过组合的方式提供了一种给类增加职责（操作）的方法。

2.Decorator模式结构图

3.实现

#ifndef _DECORATOR_H_ 
#define _DECORATOR_H_

class Component 
{ 
public: 
    virtual ~Component();
    virtual void Operation();
protected: 
    Component();
private:
};

class ConcreteComponent:public Component 
{ 
public: 
    ConcreteComponent();
    ~ConcreteComponent();
    void Operation();
protected:
private:
};

class Decorator:public Component 
{ 
public: 
    Decorator(Component* com);
    virtual ~Decorator();
    void Operation();
protected: 
    Component* _com;
private: 
};

class ConcreteDecorator:public Decorator 
{ 
public: 
    ConcreteDecorator(Component* com);
    ~ConcreteDecorator();
    void Operation();
    void AddedBehavior();
protected:
private:
};

#endif

#include "Decorator.h"
#include <iostream>

Component::Component() 
{

}
Component::~Component() 
{

}
void Component::Operation() 
{

}
ConcreteComponent::ConcreteComponent() 
{

}
ConcreteComponent::~ConcreteComponent() 
{

}
void ConcreteComponent::Operation() 
{ 
    std::cout<<"ConcreteComponent operation..."<<std::endl; 
}
Decorator::Decorator(Component* com) 
{ 
    this->_com = com; 
}
Decorator::~Decorator() 
{ 
    delete _com; 
}
void Decorator::Operation() 
{

}
ConcreteDecorator::ConcreteDecorator(Component*com):Decorator(com) 
{

}
ConcreteDecorator::~ConcreteDecorator() 
{

}
void ConcreteDecorator::AddedBehavior() 
{ 
    std::cout<<"ConcreteDecorator::AddedBehacior...."<<std::endl; 
} 
void ConcreteDecorator::Operation() 
{ 
    _com->Operation();
    this->AddedBehavior();
}

//main.cpp
#include "Decorator.h"
#include <iostream> 
using namespace std;

int main(int argc,char* argv[]) 
{ 
    Component* com = new ConcreteComponent();
    Decorator* dec = new ConcreteDecorator(com);
    dec->Operation();
    delete dec;
    return 0; 
}

4.Decorator模式的讨论

　　为了多态，通过父类指针指向其具体子类，但是这就带来另外一个问题，当具体子类要添加新的职责，就必须向其父类添加一个这个职责的抽象接口，否则是通过父类指针是调用不到这个方法了。这样处于高层的父类就承载了太多的特征（方法），并且继承自这个父类的所有子类都不可避免继承了父类的这些接口，但是可能这并不是这个具体子类所需要的。而在Decorator模式提供了一种较好的解决方法，当需要添加一个操作的时候就可以通过Decorator模式来解决，你可以一步步添加新的职责。