下面是观察者模式的一个例子，源代码来自《设计模式精解-Gof 23种设计模式解析附C++实现源码》。

源代码在每个观察者对象析构时有bug，会导致重复删除subject对象。

//Subject.h
#ifndef  SUBJECT_H
#define  SUBJECT_H
#include <list>
#include <string>
using namespace std;
typedef string State;
class Observer;
class Subject
{
public:
	virtual ~Subject();
	virtual void Attach(Observer* obv);// 注册
	virtual void Detach(Observer* obv);// 注销
	virtual void Notify();
	virtual void SetState(const State& st) = 0;
	virtual State GetState() = 0;

protected:
	Subject();// constructor is protected? Subject是抽象类
private:
	list<Observer*> *_obvs;//存放的是指针，又称发布－订阅（publish-subscribe）
};


class ConcreteSubject: public Subject
{
public:
	ConcreteSubject();
	~ConcreteSubject();
	State GetState();
	void SetState(const State& st);

protected:

private:
	State _st;

};

#endif//~SUBJECT_H

Subject.cpp 

#include "Subject.h"
#include "Observer.h"
#include <iostream>
#include <list>
using namespace std;
typedef string state;
Subject::Subject()
{ //在模板的使用之前一定要new，创建
	_obvs = new list<Observer*>;
}
Subject::~Subject()
{//使用完后释放列表
        delete _obvs;
        _obvs = NULL;
}
void Subject::Attach(Observer* obv)
{
	_obvs->push_front(obv);
}
void Subject::Detach(Observer* obv)
{
	if (obv != NULL)
		_obvs->remove(obv);
}
void Subject::Notify()
{
	list<Observer*>::iterator it;
	for (it = _obvs->begin();it != _obvs->end();it++)
	{ //关于模板和iterator的用法
		(*it)->Update(this);
	}
}
ConcreteSubject::ConcreteSubject()
{
	_st = '\0';
}
ConcreteSubject::~ConcreteSubject()
{ }
State ConcreteSubject::GetState()
{
	return _st;
}
void ConcreteSubject::SetState(const State& st)
{ 
        _st = st;
}

Observer.h 

#ifndef OBSERVER_H
#define OBSERVER_H
#include "Subject.h"
#include <string>
using namespace std;
typedef string State;
class Observer
{
public:
	virtual ~Observer();
	virtual void Update(Subject* sub) = 0;
	virtual void PrintInfo() = 0;
protected:
	Observer();
	State _st;
private:	
};

class ConcreteObserverA:public Observer
{
public:
	ConcreteObserverA(Subject* sub);
	virtual ~ConcreteObserverA();
	virtual Subject* GetSubject();
	//传入Subject作为参数，这样可以让一个View属于多个的Subject。
	void Update(Subject* sub);
	void PrintInfo();
protected:
private:
	Subject* _sub;
};

class ConcreteObserverB:public Observer
{
public:
	ConcreteObserverB(Subject* sub);
	virtual ~ConcreteObserverB();
	virtual Subject* GetSubject();
	//传入Subject作为参数，这样可以让一个View属于多个的Subject。
	void Update(Subject* sub);
	void PrintInfo();
protected:
private:
	Subject* _sub;
};
#endif //~OBSERVER_H

Observer.cpp

#include "Observer.h"
#include "Subject.h"
#include <iostream>
#include <string>
using namespace std;
Observer::Observer()
{
	_st = '\0';//字符串的结尾标志
}
Observer::~Observer()
{
}

ConcreteObserverA::ConcreteObserverA(Subject* sub)
{
	_sub = sub;
	_sub->Attach(this);//注册
}

ConcreteObserverA::~ConcreteObserverA()
{
	_sub->Detach(this);//注销
	//if (_sub != 0)
	//	delete _sub;
}
Subject* ConcreteObserverA::GetSubject()
{
	return _sub;
}
void ConcreteObserverA::PrintInfo()
{
	cout<<"ConcreteObserverA observer.... "<<_sub->GetState()<<endl;
}
void ConcreteObserverA::Update(Subject* sub)
{
	_st = sub->GetState();
	PrintInfo();
}
ConcreteObserverB::ConcreteObserverB(Subject* sub)
{
	_sub = sub;
	_sub->Attach(this);
}

ConcreteObserverB::~ConcreteObserverB()
{
	_sub->Detach(this);
	//if (_sub != 0)
	//{
	//	delete _sub;
	//}
}

Subject* ConcreteObserverB::GetSubject()
{
	return _sub;
}

void ConcreteObserverB::PrintInfo()
{
	cout<<"ConcreteObserverB observer.... "<<_sub->GetState()<<endl;
}

void ConcreteObserverB::Update(Subject* sub)
{
	_st = sub->GetState();
	PrintInfo();
}

//main.cpp
#include "Subject.h"
#include "Observer.h"
#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	ConcreteSubject* sub = new ConcreteSubject();
	Observer* o1 = new ConcreteObserverA(sub);
	Observer* o2 = new ConcreteObserverB(sub);
	sub->SetState("old");
	sub->Notify();
	sub->SetState("new"); //也可以由Observer调用
	sub->Notify();

	delete o2;
	o2 = NULL;
	delete o1;
	o1 = NULL;
	delete sub;
	sub =NULL;
	system("pause");
	return 0;
}

设计模式: 观察者模式