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;
};

void Subject::Attach (Observer* o) {
     _observers->Append(o);
}

void Subject::Detach (Observer* o) {
     _observers->Remove(o);
}

void Subject::Notify () {
     ListIterator<Observer*> i(_observers);

     for (i.First(); !i.IsDone(); i.Next()) {
     i.CurrentItem()->Update(this);
     }
}

class ClockTimer : public Subject {
public:
     ClockTimer();

     virtual int GetHour();
     virtual int GetMinute();
     virtual int GetSecond();

     void Tick();
};

void ClockTimer::Tick () {
     // update intermal time-keeping state
     // ...
     Notify();
}

Class DigitalClock: public GraphicalElement, public Observer {
public:
     DigitalClock (ClockTimer*);
     virtual ~DigitalClock();

     virtual void Update(Subject*);
         // overrides Observer operation

     virtual void Draw();
     // overrides GraphicalElement operation
     // defines how to draw the digital clock
private:
     ClockTimer* _subject;
};

DigitalClock::DigitalClock(ClockTimer* s) {
     _subject = s;
     _subject->Attach(this);
}

DigitalClock::~DigitalClock() {
     _subject->Detach(this);
}

void DigitalClock::Update(Subject* theChangedSubject) {
     if (theChangedSubject == _subject) {
         Draw();
     }
}

void DigitalClock::Draw () {
     // get the new values from the subject

     int hour = _subject->GetHour();
     int minute = _subject->GetMinute();
     // etc.

     // draw the digital clock
}

class AnalogClock : public GraphicalElement, public Observer {
public:
     AnalogClock (ClockTimer*);
     virtual void Update (Subject*);
     virtual void Draw();
     // ...
};

ClockTimer* timer = new ClockTimer;
AnalogClock* analogClock = new AnalogClock (timer);
DigitalClock* digitalClock = new DigitalClock (timer);