1 #include <iostream>
  2 #include <list>
  3 using namespace std;
  4 
  5 class Observer
  6 {
  7 public:
  8      virtual void Update(int) = 0;
  9 };
 10 
 11 class Subject
 12 {
 13 public:
 14      virtual void Attach(Observer *) = 0;
 15      virtual void Detach(Observer *) = 0;
 16      virtual void Notify() = 0;
 17 };
 18 
 19 class ConcreteObserver : public Observer
 20 {
 21 public:
 22      ConcreteObserver(Subject *pSubject) : m_pSubject(pSubject){}
 23 
 24      void Update(int value)
 25      {
 26           cout<<"ConcreteObserver get the update. New State:"<<value<<endl;
 27      }
 28 
 29 private:
 30      Subject *m_pSubject;
 31 };
 32 
 33 class ConcreteObserver2 : public Observer
 34 {
 35 public:
 36      ConcreteObserver2(Subject *pSubject) : m_pSubject(pSubject){}
 37 
 38      void Update(int value)
 39      {
 40           cout<<"ConcreteObserver2 get the update. New State:"<<value<<endl;
 41      }
 42 
 43 private:
 44      Subject *m_pSubject;
 45 };
 46 
 47 class ConcreteSubject : public Subject
 48 {
 49 public:
 50      void Attach(Observer *pObserver);
 51      void Detach(Observer *pObserver);
 52      void Notify();
 53 
 54      void SetState(int state)
 55      {
 56           m_iState = state;
 57      }
 58 
 59 private:
 60      std::list<Observer *> m_ObserverList;
 61      int m_iState;
 62 };
 63 
 64 void ConcreteSubject::Attach(Observer *pObserver)
 65 {
 66      m_ObserverList.push_back(pObserver);
 67 }
 68 
 69 void ConcreteSubject::Detach(Observer *pObserver)
 70 {
 71      m_ObserverList.remove(pObserver);
 72 }
 73 
 74 void ConcreteSubject::Notify()
 75 {
 76      std::list<Observer *>::iterator it = m_ObserverList.begin();
 77      while (it != m_ObserverList.end())
 78      {
 79           (*it)->Update(m_iState);
 80           ++it;
 81      }
 82 }
 83 
 84 int main()
 85 {
 86      // Create Subject
 87      ConcreteSubject *pSubject = new ConcreteSubject();
 88 
 89      // Create Observer
 90      Observer *pObserver = new ConcreteObserver(pSubject);
 91      Observer *pObserver2 = new ConcreteObserver2(pSubject);
 92 
 93      // Change the state
 94      pSubject->SetState(2);
 95 
 96      // Register the observer
 97      pSubject->Attach(pObserver);
 98      pSubject->Attach(pObserver2);
 99 
100      pSubject->Notify();
101 
102      // Unregister the observer
103      pSubject->Detach(pObserver);
104 
105      pSubject->SetState(3);
106      pSubject->Notify();
107 
108      delete pObserver;
109      delete pObserver2;
110      delete pSubject;
111 }