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

 1 #ifndef _DECORATOR_H_ 
 2 #define _DECORATOR_H_
 3 
 4 class Component 
 5 { 
 6 public: 
 7     virtual ~Component();
 8     virtual void Operation();
 9 protected: 
10     Component();
11 private:
12 };
13 
14 class ConcreteComponent:public Component 
15 { 
16 public: 
17     ConcreteComponent();
18     ~ConcreteComponent();
19     void Operation();
20 protected:
21 private:
22 };
23 
24 class Decorator:public Component 
25 { 
26 public: 
27     Decorator(Component* com);
28     virtual ~Decorator();
29     void Operation();
30 protected: 
31     Component* _com;
32 private: 
33 };
34 
35 class ConcreteDecorator:public Decorator 
36 { 
37 public: 
38     ConcreteDecorator(Component* com);
39     ~ConcreteDecorator();
40     void Operation();
41     void AddedBehavior();
42 protected:
43 private:
44 };
45 
46 #endif

Decorator.h

 1 #include "Decorator.h"
 2 #include <iostream>
 3 
 4 Component::Component() 
 5 {
 6 
 7 }
 8 Component::~Component() 
 9 {
10 
11 }
12 void Component::Operation() 
13 {
14 
15 }
16 ConcreteComponent::ConcreteComponent() 
17 {
18 
19 }
20 ConcreteComponent::~ConcreteComponent() 
21 {
22 
23 }
24 void ConcreteComponent::Operation() 
25 { 
26     std::cout<<"ConcreteComponent operation..."<<std::endl; 
27 }
28 Decorator::Decorator(Component* com) 
29 { 
30     this->_com = com; 
31 }
32 Decorator::~Decorator() 
33 { 
34     delete _com; 
35 }
36 void Decorator::Operation() 
37 {
38 
39 }
40 ConcreteDecorator::ConcreteDecorator(Component*com):Decorator(com) 
41 {
42 
43 }
44 ConcreteDecorator::~ConcreteDecorator() 
45 {
46 
47 }
48 void ConcreteDecorator::AddedBehavior() 
49 { 
50     std::cout<<"ConcreteDecorator::AddedBehacior...."<<std::endl; 
51 } 
52 void ConcreteDecorator::Operation() 
53 { 
54     _com->Operation();
55     this->AddedBehavior();
56 }

Decorator.cpp

 1 //main.cpp
 2 #include "Decorator.h"
 3 #include <iostream> 
 4 using namespace std;
 5 
 6 int main(int argc,char* argv[]) 
 7 { 
 8     Component* com = new ConcreteComponent();
 9     Decorator* dec = new ConcreteDecorator(com);
10     dec->Operation();
11     delete dec;
12     return 0; 
13 }

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。