观察者模式概念：定义对象间的一种一对多的依赖关系，以便当一个对象的状态发生改变时，全部依赖于它的对象都得到通知并自己主动更新。“Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.” – GoF

UML图

特点：

1.观察者模式定义了对象之间一对多的关系。

2.主题（也就是可观察者）用一个共同的接口来更新观察者。

3.观察者和可观察者用松耦合的方式结合，可观察者不知道观察者的细节，仅仅知道观察者实现了观察者接口

4.有多个观察者时，不能够依赖特点的通知次序。

C++源码：

Observer.h

/*观察者模式：在对象之间定义一对多的依赖，这样一来，当一个对象改变状态，依赖它的对象都会收到通知，并自己主动更新。
*/#include <iostream>
#include <vector>
using namespace std;class Observer;//Account抽象类，用作接口
class Account
{
protected:vector<Observer*> observer;double balance;
public:virtual void add_observer(Observer *o)=0;virtual void remove_observer(Observer *o)=0;virtual void notify_observer()=0;virtual void set_balance(double balance)=0;virtual double get_balance()=0;
public:virtual ~Account(){cout<<"in the destructor of Account"<<endl;}
};//Observer抽象类，用作接口
class Observer
{
protected:Account *account;
public:Observer(Account *account):account(account) {}virtual void handle_event()=0;virtual void set_account_balance(double balance)=0;virtual ~Observer(){cout<<"in the destructor of Observer"<<endl;}
};//BankAccount详细类，继承了ACcount抽象类
class BankAccount:public Account
{
public:~BankAccount(){cout<<"in the destructor of BankAccount"<<endl;}public:void add_observer(Observer *o){observer.push_back(o);}void remove_observer(Observer *o){for(vector<Observer*>::iterator it=observer.begin(); it!=observer.end(); it++){if(o==*it){observer.erase(it);return;}}}void notify_observer(){for(vector<Observer*>::const_iterator it=observer.begin(); it!=observer.end(); it++){(*it)->handle_event();}}void set_balance(double balance){this->balance=balance;notify_observer();}double get_balance(){return balance;}};class ATM:public Observer
{
public:ATM(Account *account):Observer(account)//将acount传递给基类Observer的构造函数{}~ATM(){cout<<"in the destructor of ATM"<<endl;}
public:void handle_event(){cout<<"ATM: the balance of bank accont is changed to "<<account->get_balance()<<endl;}void set_account_balance(double balance){account->set_balance(balance);}
};class InternetBankService:public Observer
{
public:InternetBankService(Account *account):Observer(account) {}~InternetBankService(){cout<<"in the destructor InternetBankService "<<endl;}
public:void handle_event(){cout<<"IBS: the balance of bank account is changed to:"<<account->get_balance()<<endl;}void set_account_balance(double balance){account->set_balance(balance);}};

Observer.cpp

#include "Observer.h"
int main(int argc, char **argv)
{Account *account=new BankAccount();Observer *atm_observer=new ATM(account);Observer *ibs_observer=new InternetBankService(account);account->add_observer(atm_observer);account->add_observer(ibs_observer);account->set_balance(1000.12);cout<<"Event is triggered by atm_observer"<<endl;cout<<"============="<<endl;atm_observer->set_account_balance(1000.23);cout<<"Event is triggered by ibs_observer"<<endl;cout<<"============="<<endl;ibs_observer->set_account_balance(1000.36);delete atm_observer;delete ibs_observer;return 0;
}

例2：

Test_Observer.cpp

/*千万要注意对象切割问题的出现。一定要理解。
*/
#include <iostream>
#include <string>
#include <vector>
using namespace std;class Observer
{
protected:string name;
public:virtual void update(){cout<<"Observer"<<endl;}
};class Man:public Observer
{
public:Man(string name){this->name=name;}void update(){cout<<name<<" Man is coming"<<endl;}
};
class Woman:public Observer
{
public:Woman(string name){this->name=name;}void update(){cout<<name<<" Woman is leaving"<<endl;}
};class Subject
{
private:vector<Observer*> obser;
public:void add(Observer &ob){obser.push_back(&ob);}void notify(){for (vector<Observer*>::iterator it=obser.begin(); it!=obser.end(); it++){(*it)->update();}}
};int main()
{Subject secret;Man m1("m1--");Man m2("m2--");Woman w1("w1");Woman w2("w2");secret.add(m1);secret.add(m2);secret.add(w1);secret.add(w2);secret.notify();return 0;}