Decorator模式

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Decorator模式

在开发过程中，有时会遇到需要向已经定义好的类添加新功能的需求，这时候通常的做法就是定义一个新类，继承这个类，在新类中添加新功能．

采用继承的方式来实现，在使用时，我们经常使用父类的指针或引用通过多态技术来调用子类的接口，但这样就会遇到一个问题：再向子类添加方法时，也需要向父类添加方法，否则通过父类指针或引用就调用不到这个方法．所以Decorator模式没有采用继承来实现，而是采用组合的方式来实现．类结构如下图所示：　
这里写图片描述

类ConcreteComponent和类Decorator有相同的接口，所以都继承自Component．在Decorator中，有一个指向Component的指针(引用)，通过这个指针来调用ConcreteComponent中的函数Operation.

下面是实现：
//Decorator.h

//Decorator.h#ifndef _DECORATOR_H_#define _DECORATOR_H_class Component{public:    virtual ~Component();    virtual void Operation();protected:    Component();};class ConcreteComponent :public Component{public:    ConcreteComponent();    ~ConcreteComponent();    void Operation();};class Decorator :public Component{public:    Decorator(Component* com);    virtual ~Decorator();    void Operation();protected:    Component* _com;};class ConcreteDecorator :public Decorator{public:    ConcreteDecorator(Component* com);    ~ConcreteDecorator();    void Operation();    void AddedBehavior();};#endif

//Decorator.cpp

//Decorator.cpp#include"Decorator.h"#include<iostream>Component::~Component(){}void Component::Operation(){}Component::Component(){}ConcreteComponent::ConcreteComponent(){}ConcreteComponent::~ConcreteComponent(){}void ConcreteComponent::Operation(){    std::cout << "ConcreteComponent operation" << std::endl;}Decorator::Decorator(Component* com){    _com = com;}Decorator::~Decorator(){    delete _com;}void Decorator::Operation(){}ConcreteDecorator::ConcreteDecorator(Component* com) :Decorator(com){}ConcreteDecorator::~ConcreteDecorator(){}void ConcreteDecorator::AddedBehavior(){    std::cout << "ConcreteDecorator::AddedBehavior..." << std::endl;}void ConcreteDecorator::Operation(){    _com->Operation();    AddedBehavior();}

//main.cpp

//main.cpp#include"Decorator.h"int main(){    Component* com = new ConcreteComponent();    Decorator* dec = new ConcreteDecorator(com);    dec->Operation();    delete dec;    return 0;}

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