#include <iostream>
#include <list>
using namespace std;
 
class Observer
{
public:
     virtual void Update(int) = 0;
};
 
class Subject
{
public:
     virtual void Attach(Observer *) = 0;
     virtual void Detach(Observer *) = 0;
     virtual void Notify() = 0;
};
 
class ConcreteObserver : public Observer
{
public:
     ConcreteObserver(Subject *pSubject) : m_pSubject(pSubject){}
 
     void Update(int value)
     {
          cout<<"ConcreteObserver get the update. New State:"<<value<<endl;
     }
 
private:
     Subject *m_pSubject;
};
 
class ConcreteObserver2 : public Observer
{
public:
     ConcreteObserver2(Subject *pSubject) : m_pSubject(pSubject){}
 
     void Update(int value)
     {
          cout<<"ConcreteObserver2 get the update. New State:"<<value<<endl;
     }
 
private:
     Subject *m_pSubject;
};
 
class ConcreteSubject : public Subject
{
public:
     void Attach(Observer *pObserver);
     void Detach(Observer *pObserver);
     void Notify();
 
     void SetState(int state)
     {
          m_iState = state;
     }
 
private:
     std::list<Observer *> m_ObserverList;
     int m_iState;
};
 
void ConcreteSubject::Attach(Observer *pObserver)
{
     m_ObserverList.push_back(pObserver);
}
 
void ConcreteSubject::Detach(Observer *pObserver)
{
     m_ObserverList.remove(pObserver);
}
 
void ConcreteSubject::Notify()
{
     std::list<Observer *>::iterator it = m_ObserverList.begin();
     while (it != m_ObserverList.end())
     {
          (*it)->Update(m_iState);
          ++it;
     }
}
 
int main()
{
     // Create Subject
     ConcreteSubject *pSubject = new ConcreteSubject();
 
     // Create Observer
     Observer *pObserver = new ConcreteObserver(pSubject);
     Observer *pObserver2 = new ConcreteObserver2(pSubject);
 
     // Change the state
     pSubject->SetState(2);
 
     // Register the observer
     pSubject->Attach(pObserver);
     pSubject->Attach(pObserver2);
 
     pSubject->Notify();
 
     // Unregister the observer
     pSubject->Detach(pObserver);
 
     pSubject->SetState(3);
     pSubject->Notify();
 
     delete pObserver;
     delete pObserver2;
     delete pSubject;
}