观察者模式定义了一种一对多的依赖关系，让多个观察者对象同时监听某一个主题对象，这个主题对象在状态发生变化时会通知所有观察者对象，使他们能够自动更新自己。

**这种交互也称为发布-订阅，**发布者，它发出通知时并不需要知道谁是它的观察者，可以有任意数目的观察者订阅并接收通知。

二、实现

1.主题

#ifndef _TEACHERSUBJECT_HEAD_H
#define _TEACHERSUBJECT_HEAD_H
#include <iostream>

using namespace std;

class StudentObserver;

class TeacherSubject
{
public:
	virtual void add(StudentObserver *student) = 0;
	virtual void remove(StudentObserver *student) = 0;
	virtual void notify(void) = 0;
	void setMessage(const string &message) {
		this->message = message;
	}

	string getMessage(void) const {
		return message;
	}

private:
	string message;
};

#endif

//------------------------------------------------------
#ifndef _CPPTEACHERSUBJECT_HEAD_H
#define _CPPTEACHERSUBJECT_HEAD_H

#include "TeacherSubject.hpp"
#include <list>
#include "StudentObserver.hpp"

class CppTeacherSubject:public TeacherSubject
{
public:
	void add(StudentObserver *student) {
		obseverList.push_back(student);
	}

	void remove(StudentObserver *student) {
		obseverList.remove(student);
	}

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

private:
	list<StudentObserver *> obseverList;
};

#endif

2.观察者

#ifndef _STUDENTOBSERVER_HEAD_H
#define _STUDENTOBSERVER_HEAD_H

class TeacherSubject;

class StudentObserver
{
public:
	StudentObserver(TeacherSubject *subject) {
		this->subject = subject;
	}
	virtual void update(void) = 0;
protected:
	TeacherSubject *subject;
};

#endif

//------------------------------------------------------
#ifndef _GAMESTUDENTOBSERVER_HEAD_H
#define _GAMESTUDENTOBSERVER_HEAD_H
#include "StudentObserver.hpp"

class GameStudentObserver:public StudentObserver
{
public:
	GameStudentObserver(TeacherSubject *subject):StudentObserver(subject) {

	}

	void update(void) {
		cout << "Can not play game,Go to class" << endl;
		cout << subject->getMessage() << endl;

	}
};

#endif

//------------------------------------------------------
#ifndef _SLEEPSTUDENTOBSERVER_HEAD_H
#define _SLEEPSTUDENTOBSERVER_HEAD_H
#include "StudentObserver.hpp"

class SleepStudentObserver:public StudentObserver
{
public:
	SleepStudentObserver(TeacherSubject *subject):StudentObserver(subject) {

	}

	void update(void) {
		cout << "Can not sleep,Go to class" << endl;
		cout << subject->getMessage() << endl;

	}
};

#endif

4.使用

#include "CppTeacherSubject.hpp"
#include "SleepStudentObserver.hpp"
#include "GameStudentObserver.hpp"

int main(int argc, const char *argv[])
{
	CppTeacherSubject *teacherSubject = new CppTeacherSubject();
	SleepStudentObserver *sleepObserver = new SleepStudentObserver(teacherSubject);
	GameStudentObserver *gameObserver  = new GameStudentObserver(teacherSubject);

	teacherSubject->add(sleepObserver);
	teacherSubject->add(gameObserver);

	cout << "-----------------------------" << endl;

	teacherSubject->setMessage("Start Cpp Class");
	teacherSubject->notify();

	return 0;
}