observer

#include <iostream>
#include <list>
using namespace std;
class Subject;
class Observer {
public:
  virtual ~Observer(){}
  virtual void Update(Subject* theChangedSubject) = 0;
protected:
  Observer(){}
};
class Subject {
public:
  virtual ~Subject(){}
  virtual void Attach(Observer*);
  virtual void Detach(Observer*);
  virtual void Notify();
protected:
  Subject(){}
private:
  list<Observer*> _observers;
public:
  typedef list<Observer*>::iterator iterator;
};
void Subject::Attach(Observer* o) {
  _observers.push_back(o);
}
void Subject::Detach(Observer* o) {
  _observers.remove(o);
}
void Subject::Notify() {
  iterator itr = _observers.begin();
  while (itr != _observers.end())
    {
      (*itr)->Update(this);
      ++itr;
    }
}
//concrete Subject
class ClockTimer : public Subject {
public:
  ClockTimer(){}
  // virtual int GetHour();
  // virtual int GetMinute();
  // virtual int GetSecond();
  void Tick();
};
void ClockTimer::Tick() {
  Notify(); // call Notify of Subject
}
//concrete Observer1
class DigitalClock : public Observer {
public:
  DigitalClock(ClockTimer*);
  // virtual ~DigitalClock();
  virtual void Update(Subject *); //overrides Observer operation
  virtual void Draw();
private:
  ClockTimer* _subject;
};
DigitalClock::DigitalClock(ClockTimer* s) {
  _subject = s;
  _subject->Attach(this);
}
void DigitalClock::Update(Subject * theChangedSubject) {
  if (theChangedSubject == _subject)
    {
      Draw();
    }
}
void DigitalClock::Draw() {
  cout << "DigitalClock Draw" << endl;
}
//concrete Observer2
class AnalogClock : public Observer {
public:
  AnalogClock(ClockTimer*);
  virtual void Update(Subject *);
  virtual void Draw();
private:
  ClockTimer * _subject;
};
AnalogClock::AnalogClock(ClockTimer *s) {
  _subject = s;
  _subject->Attach(this);
}
void AnalogClock::Update(Subject *theChangedSubject) {
  if (theChangedSubject == _subject)
    Draw();
}
void AnalogClock::Draw() {
  cout << "AnalogClock Draw" << endl;
}
int
main()
{
  ClockTimer *timer = new ClockTimer; //concrete subject;
  AnalogClock* analogClock = new AnalogClock(timer);
  DigitalClock* digitalClock = new DigitalClock(timer);

  timer->Tick();
  delete timer;
  delete analogClock;
  delete digitalClock;
  return 0;
}