观察者模式（Observer Pattern）

#ifndef OBSERVER_H
#define OBSERVER_H
#include<list>
#include<iostream>
using namespace std;

class Observer
{
public:
    virtual void update()=0;
};

class Subject
{
public:
    virtual void attach(Observer* observer)=0;
    virtual void detach(Observer* observer)=0;
    virtual void notify()=0;
};

class ConcreteSubject:public Subject
{
private:
    list<Observer*> _observers;

    int _state;
    
public:
    ConcreteSubject():_state(0){
    }

    void attach(Observer* observer){
        _observers.push_front(observer);
    }

    void detach(Observer* observer){
        _observers.remove(observer);
    }

    void notify(){
        list<Observer*>::iterator it;
        for(it=_observers.begin();it!=_observers.end();++it){
            (*it)->update();
        }
    }

    int getState()const{
        return _state;
    }

    void setState(int newState){
        _state=newState;

        notify();
    }
};

class ConcreteObserverA:public Observer
{
private:
    Subject* _subject;
public:
    ConcreteObserverA(Subject* subject){
        _subject=subject;
        _subject->attach(this);
    }

    void update(){
        ConcreteSubject* s=dynamic_cast<ConcreteSubject*>(_subject);
        cout<<"ConcreteObserverA\n------"<<endl;
        cout<<"update!\nThe new state is: "<<s->getState()<<endl<<endl;
    }
};

class ConcreteObserverB:public Observer
{
private:
    Subject* _subject;
public:
    ConcreteObserverB(Subject* subject){
        _subject=subject;
        _subject->attach(this);
    }

    void update(){
        ConcreteSubject* s=dynamic_cast<ConcreteSubject*>(_subject);
        cout<<"ConcreteObserverB\n------"<<endl;
        cout<<"update!\nThe new state is: "<<s->getState()<<endl<<endl;
    }
};

class ConcreteObserverC:public Observer
{
private:
    Subject* _subject;
public:
    ConcreteObserverC(Subject* subject){
        _subject=subject;
        _subject->attach(this);
    }

    void update(){
        ConcreteSubject* s=dynamic_cast<ConcreteSubject*>(_subject);
        cout<<"ConcreteObserverC\n------"<<endl;
        cout<<"update!\nThe new state is: "<<s->getState()<<endl<<endl;
    }
};

#endif//OBSERVER_H

int main()
{
    ConcreteSubject subject;
    ConcreteObserverA observerA(&subject);
    ConcreteObserverB observerB(&subject);
    ConcreteObserverC observerC(&subject);

    subject.notify();

    subject.setState(100);



     return 0;
}