#ifndef _CONTEXT_H_
#define _CONTEXT_H_
class State;
/**
*
**/
class Context{
  public:
  Context();
  Context(State* state);
  ~Context();
  void Handle();
  void OperationForStateA();
  void OperationForStateB();
  protected:
  private:
  friend class State; //表明在 State 類(lèi)中可以訪問(wèn)
  Context 類(lèi)的 private 字段，重要是訪問(wèn) ChangeState
  void ChangeState(State* state);
  private:
  State* _state;
};
#endif //~_CONTEXT_H_

#include "Context.h"
#include "State.h"
#include <iostream>
using namespace std;
Context::Context(){
}
Context::Context(State* state){
  this->_state = state;
}
Context::~Context(){
  delete _state;
}
void Context::Handle(){
  _state->Handle(this);
}
void Context::ChangeState(State* state){
  ///_state->ChangeState(this,state);
  this->_state = state;
}
void Context::OperationForStateA(){
  cout<<"Do operation in State A ";
}
void Context::OperationForStateB(){
  cout<<"Do operation in State B ";
}

#ifndef _STATE_H_
#define _STATE_H_
class Context; //前置聲明
class State{
  public:
  State();
  virtual ~State();
  virtual void Handle(Context* con) = 0;
  protected:
  void ChangeState(Context* con,State* st);
  private:
  //bool ChangeState(Context* con,State* st);
};
class ConcreteStateA:public State{
  public:
  ConcreteStateA();
  virtual ~ConcreteStateA();
  void Handle(Context* con);
  protected:
  private:
};
class ConcreteStateB:public State{
  public:
  ConcreteStateB();
  virtual ~ConcreteStateB();
  void Handle(Context* con);
  protected:
  private:
};
#endif //~_STATE_H_

#include "State.h"
#include "Context.h"
#include <iostream>
using namespace std;
State::State(){
}
State::~State(){
}
void State::ChangeState(Context* con,State* st){
  con->ChangeState(st);
}
///
ConcreteStateA::ConcreteStateA(){
}
ConcreteStateA::~ConcreteStateA(){
}
void ConcreteStateA::Handle(Context* con){
  con->OperationForStateA();
  cout<<":: State change from A to B"<<endl;
  State::ChangeState(con,new ConcreteStateB());
}
///
ConcreteStateB::ConcreteStateB(){
}
ConcreteStateB::~ConcreteStateB(){
}
void ConcreteStateB::Handle(Context* con){
  con->OperationForStateB();
  cout<<":: State change from B to A"<<endl;
  State::ChangeState(con,new ConcreteStateA());
}

#include "Context.h"
#include "State.h"
#include <iostream>
using namespace std;
int main(int argc,char* argv[]){
  State* st = new ConcreteStateA();
  Context* con = new Context(st);
  con->Handle();
  con->Handle();
  con->Handle();
  if (con != NULL)
    delete con;
  if (st != NULL)
    st = NULL;
  return 0;
}

Do operation in State A :: State change from A to B
Do operation in State B :: State change from B to A
Do operation in State A :: State change from A to B