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

Decorator.h

 #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();
 }

Decorator.cpp

 //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 ;
 }

main.cpp

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。