观察者模式

股票提醒
当股票的价格上涨或下降5%时，会通知持有该股票的股民，当股民听到价格上涨的消息时会买股票，当价格下降时会大哭一场。

#include <iostream>
#include <list>
using namespace std;

// 观察者接口
class IObserver {
public:
virtual void respond(int rate) = 0;
};

// 主题接口
class ISubject {
protected:
list<IObserver*> observers; // 存储观察者列表
public:
// 注册方法
void attach(IObserver* observer) {
observers.push_back(observer);
}
// 注销方法
void detach(IObserver* observer) {
observers.remove(observer);
}
// 抽象通知方法
virtual void notify(int rate) = 0;
};

// 具体主题类
class Stock : public ISubject {
public:
void notify(int rate) override {
for (auto observer : observers) {
observer->respond(rate);
}
}
};

// 具体观察者类
class Holder : public IObserver {
public:
void respond(int rate) override {
if (rate >= 5) {
cout << "股票上涨，购买股票" << endl;
}
else if (rate <= -5) {
cout << "股票下跌，大哭一场" << endl;
}
else {
cout << "股票还比较稳定哦~" << endl;
}
}
};

int main() {
ISubject* subject = new Stock();
IObserver* observer = new Holder();
subject->attach(observer);

cout << "----------------------股票上涨-----------------------" << endl;
subject->notify(5);
cout << endl;

cout << "----------------------股票下跌-----------------------" << endl;
subject->notify(-5);
cout << endl;

cout << "----------------------股票稳定-----------------------" << endl;
subject->notify(4);
cout << endl;

delete observer;
delete subject;
return 0;
}