• 定义：允许一个对象在其状态改变时，改变它的行为。看起来对象似乎修改了它的类。
• 解决根据不同的对象状态进行不同的操作的问题
• UML

  

• 代码示例：
  1 //==========================================  2 //Context.h  3 //==========================================  4 #pragma once  5   6 class State;  7 class Context  8 {  9 public: 10     Context(State* pState); 11     ~Context(); 12     void Request(); 13 private: 14     void UpdateState(State* pState); 15     friend class ConcreteStateA; 16     friend class ConcreteStateB; 17 private: 18     State* m_pState; 19 }; 20  21 //========================================== 22 //Context.cpp 23 //========================================== 24 #include "Context.h" 25 #include 
         
           26 #include "State.h" 27  28 Context::Context(State* pState) 29     :m_pState(pState) 30 { 31 } 32  33 Context::~Context() 34 { 35     delete m_pState; 36 } 37  38 void Context::Request() 39 { 40     if (NULL != m_pState) 41     { 42         m_pState->Handle(this); 43     } 44 } 45  46 void Context::UpdateState(State* pState) 47 { 48     delete m_pState; 49     m_pState = pState; 50 } 51  52 //========================================== 53 //State.h 54 //========================================== 55 #pragma once 56  57 class Context; 58 class State 59 { 60 public: 61     virtual ~State(){} 62     virtual void Handle(Context* pContex) = 0; 63 }; 64  65 class ConcreteStateA : public State 66 { 67 public: 68     void Handle(Context* pContex); 69 }; 70  71 class ConcreteStateB : public State 72 { 73 public: 74     void Handle(Context* pContex); 75 }; 76  77 //========================================== 78 //State.cpp 79 //========================================== 80 #include "State.h" 81 #include 
          
            82 #include "Context.h" 83 using namespace std; 84  85 //处理、更新状态 86 void ConcreteStateA::Handle(Context* pContex) 87 { 88     cout<<"ConcreteStateA :: Handle"<
           
            UpdateState(new ConcreteStateB); 90 } 91  92 void ConcreteStateB::Handle(Context* pContex) 93 { 94     cout<<"ConcreteStateB :: Handle"<
            
             UpdateState(new ConcreteStateA); 96 } 97  98 //========================================== 99 //main.cpp100 //==========================================101 #include "Context.h"102 #include "State.h"103 104 int main()105 {106     State *pState = new ConcreteStateA();107     Context *pContext = new Context(pState);108     pContext->Request();109     pContext->Request();110     pContext->Request();111 112     delete pContext;113 114     return 0;115 }