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一、设计模式是什么？

        设计模式是指在软件开发中，经过验证的，用于解决在特定环境下，重复出现的，特定问题的解决方案。

二、设计模式有哪些？

1. 观察者模式

        定义对象间的一种一对多（变化）的依赖关系，以便当一个对象(Subject)的状态发生改变时，所有依赖于它的对象都得到通知并自动更新。

1.1. 背景

        气象站发布气象资料给数据中心，数据中心经过处理，将气象信息更新到多个不同的显示终端（A 、B、C）。

1.2. 伪代码

#include <list>
#include <algorithm>
#include <iostream>using namespace std;
//
class IDisplay {
public:virtual void Show(float temperature) = 0;virtual ~IDisplay() {}
};class DisplayA : public IDisplay {
public:virtual void Show(float temperature) {cout << "DisplayA Show" << endl;}
};class DisplayB : public IDisplay{
public:virtual void Show(float temperature) {cout << "DisplayB Show" << endl;}
};class DisplayC : public IDisplay{
public:virtual void Show(float temperature) {cout << "DisplayC Show" << endl;}
};class DisplayD : public IDisplay{
public:virtual void Show(float temperature) {cout << "DisplayC Show" << endl;}
};class WeatherData {
};class DataCenter {
public:void Attach(IDisplay * ob) {obs.emplace_back(ob);}void Detach(IDisplay * ob) {if(obs.empty()){return;}for(auto it = obs.begin(); it != obs.end(); ++it){if(*it == ob){obs.erase(it);return;}}}void Notify() {float temper = CalcTemperature();for (auto iter : obs) {iter->Show(temper);}}// 接口隔离
private:WeatherData * GetWeatherData();float CalcTemperature() {WeatherData * data = GetWeatherData();// ...float temper/* = */;return temper;}std::list<IDisplay*> obs;
};int main() {// 单例模式DataCenter *center = new DataCenter;// ... 某个模块IDisplay *da = new DisplayA();center->Attach(da);// ...IDisplay *db = new DisplayB();center->Attach(db);IDisplay *dc = new DisplayC();center->Attach(dc);center->Notify();//-----center->Detach(db);center->Notify();//....center->Attach(dd);center->Notify();return 0;
}

2. 策略模式

        定义一系列算法，把它们一个个封装起来，并且使它们可互相替换。该模式使得算法可独立于使用它的客户程序而变化。

2.1 背景

        某商场节假日有固定促销活动，为了加大促销力度，现提升国庆节促销活动规格；

2.2 伪代码

class Context {};class ProStategy {
public:virtual double CalcPro(const Context &ctx) = 0;virtual ~ProStategy(); 
};class VAC_Spring : public ProStategy {
public:virtual double CalcPro(const Context &ctx){}
};class VAC_QiXi : public ProStategy {
public:virtual double CalcPro(const Context &ctx){}
};class VAC_QiXi1  : public VAC_QiXi {
public:virtual double CalcPro(const Context &ctx){}
};class VAC_Wuyi : public ProStategy {
public:virtual double CalcPro(const Context &ctx){}
};class VAC_GuoQing : public ProStategy {
public:virtual double CalcPro(const Context &ctx){}
};class VAC_Shengdan : public ProStategy {
public:virtual double CalcPro(const Context &ctx){}
};class Promotion {
public:Promotion(ProStategy *sss) : s(sss){}~Promotion(){}double CalcPromotion(const Context &ctx){return s->CalcPro(ctx);}
private:ProStategy *s;
};int main () {Context ctx;ProStategy *s = new VAC_QiXi1();Promotion *p = new Promotion(s);p->CalcPromotion(ctx);return 0;
}

3. 单例模式

        保证一个类仅有一个实例，并提供一个该实例的全局访问点。

3.1 代码实现

版本一 

#include <bits/stdc++.h>class Singleton
{
public:static Singleton *getInstance(){if (nullptr == _instance){_instance = new Singleton();}return _instance;}~Singleton(){if (_instance){delete _instance;_instance = nullptr;}}private:Singleton() {}Singleton(const Singelton &) = delete;Singleton &operator=(const Singleton &) = delete;Singleton(Singleton &&) == delete;Singleton &operator=(Singleton &&) = delete;private:static Singleton *_instance;
};Singleton *Singleton::_instance = nullptr;

版本二  

#include <bits/stdc++.h>class Singleton
{
public:static Singleton *getInstance(){if (nullptr == _instance){_instance = new Singleton();atexit(destroy); // 存在线程安全问题}return _instance;}private:static void destroy(){if (_instance){delete _instance;_instance = nullptr;}}Singleton() {}~Singleton() {}Singleton(const Singelton &) = delete;Singleton &operator=(const Singleton &) = delete;Singleton(Singleton &&) == delete;Singleton &operator=(Singleton &&) = delete;private:static Singleton *_instance;
};Singleton *Singleton::_instance = nullptr;

版本三 饿汉模式 解决线程不安全

#include <bits/stdc++.h>class Singleton
{
public:static Singleton *getInstance(){if (nullptr == _instance){_instance = new Singleton();atexit(destroy); // 存在线程安全问题}return _instance;}private:static void destroy(){if (_instance){delete _instance;_instance = nullptr;}}Singleton() {}~Singleton() {}Singleton(const Singelton &) = delete;Singleton &operator=(const Singleton &) = delete;Singleton(Singleton &&) == delete;Singleton &operator=(Singleton &&) = delete;private:static Singleton *_instance;
};Singleton *Singleton::_instance = Singleton::getinstance(); //饿汉模式

版本四 懒汉模式（饱汉模式）加锁解决线程不安全

#include <bits/stdc++.h>class Singleton
{
public:static Singleton *getInstance(){std::lock_guard<std::mutex> lock(_mutex);  //加锁if (nullptr == _instance){_instance = new Singleton();atexit(destroy); }return _instance;}private:static void destroy(){if (_instance){delete _instance;_instance = nullptr;}}Singleton() {}~Singleton() {}Singleton(const Singelton &) = delete;Singleton &operator=(const Singleton &) = delete;Singleton(Singleton &&) == delete;Singleton &operator=(Singleton &&) = delete;private:static Singleton *_instance;static std::mutex _mutex;
};Singleton *Singleton::_instance = nullptr;  //懒汉模式std::mutex Singleton::_mutex;