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：
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; }