转载：C++设计模式之Observer(观察者)模式 http://www.cnblogs.com/chain2012/archive/2011/01/24/1943322.html

作用：定义对象间的一种一对多的依赖关系，当一个对象的状态发生改变时，所有依赖于它的对象都得到通知并被自动更新。比如，QQ中你的好友上线下，他们的头像亮起，熄灭。

 

以下是参考代码

#include <Windows.h>
#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:
    std::list<Observer*> *_observers;
};

void Subject::Attach(Observer* obj)
{
    _observers->push_back(obj);
}

void Subject::Detach(Observer* obj)
{
    typedef std::list<Observer*>::iterator LI;
    for (LI iter=_observers->begin();
        iter!=_observers->end();
        ++iter)
    {
        if (*iter == obj)
        {
            _observers->erase(iter);
            break;
        }
    }
}

void Subject::Notify()
{
    typedef std::list<Observer*>::iterator LI;
    std::list<Observer*> *i(_observers);
    for (LI it=i->begin(); it!=i->end(); ++it)
    {
        (*it)->Update(this);
    }
}

//ClockTimer是一个用于存储和维护一天时间的具体目标。它每秒通知一次它的观察者。
class ClockTimer : public Subject {
public:
    ClockTimer();
    virtual int GetHour();
    virtual int GetMinute();
    virtual int GetSecond();

    void Tick();
};

void ClockTimer::Tick()
{
    Sleep(1000);
    Notify();
}


class Widget;
//DigitalClock类来显示时间。Widget类继承了它的图形功能
class DigitalClock : public Widget, public Observer {
public:
    DigitalClock(ClockTimer*);
    virtual ~DigitalClock();

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

    virtual void Draw(); //overrides Widget operation
private:
    ClockTimer* _subject;
};

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

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

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

void DigitalClock::Draw() 
{
    int hour = _subject->GetHour();
    int minute = _subject->GetMinute();
    int second = _subject->GetSecond();

    //draw the digital clock;
}



int main()
{
    ClockTimer* timer = new ClockTimer;
    DigitalClock* digitalClock = new DigitalClock(timer);
}