《设计模式的艺术》笔记 - 享元模式

介绍

        享元模式运用共享技术有效地支持大量细粒度对象的复用。系统只使用少量的对象，而这些对象都很相似，状态变化很小，可以实现对象的多次复用。由于享元模式要求能够共享的对象必须是细粒度对象，因此它又称为轻量级模式，是一种对象结构型模式。

实现

myclass.h

//
// Created by yuwp on 2024/1/12.
//#ifndef DESIGNPATTERNS_MYCLASS_H
#define DESIGNPATTERNS_MYCLASS_H#include <iostream>
#include <unordered_map>class Flyweight {   // 享元抽象类
public:Flyweight(const std::string &in);virtual void operation(const std::string &out) = 0;const std::string &getMIn() const;private:std::string m_in;
};class ConcreteFlyweight : public Flyweight {    // 具体享元类
public:ConcreteFlyweight(const std::string &in);void operation(const std::string &out) override;
};class FlyweightFactory {    // 配合工厂模式使用享元工厂类
public:~FlyweightFactory();Flyweight *getFlyweight(const std::string &key);
private:std::unordered_map<std::string, Flyweight *> m_flyweights;
};#endif //DESIGNPATTERNS_MYCLASS_H

myclass.cpp

//
// Created by yuwp on 2024/1/12.
//#include "myclass.h"Flyweight::Flyweight(const std::string &in) {m_in = in;
}const std::string &Flyweight::getMIn() const {return m_in;
}ConcreteFlyweight::ConcreteFlyweight(const std::string &in) : Flyweight(in) {}void ConcreteFlyweight::operation(const std::string &out) {std::cout<< "内部状态: " << getMIn() << ", 外部状态: " << out << std::endl;
}FlyweightFactory::~FlyweightFactory() {for (auto it : m_flyweights) {if (it.second) {delete it.second;}}
}Flyweight* FlyweightFactory::getFlyweight(const std::string &key) {auto it = m_flyweights.find(key);if (it != m_flyweights.end()) {return it->second;} else {Flyweight *flyweight = new ConcreteFlyweight(key);m_flyweights.insert(std::make_pair(key, flyweight));return flyweight;}
}

main.cpp

#include <iostream>
#include <mutex>
#include "myclass.h"int main() {FlyweightFactory *factory = new FlyweightFactory();auto fly1 = factory->getFlyweight("a");auto fly2 = factory->getFlyweight("b");auto fly3 = factory->getFlyweight("a");fly1->operation("1");fly2->operation("2");fly3->operation("3");delete factory;return 0;
}

总结

优点

        1. 可以极大减少内存中对象的数量，使得相同或相似对象在内存中只保存一份，从而可以节约系统资源，提高系统性能。

        2. 享元模式的外部状态相对独立，而且不会影响其内部状态，从而使得享元对象可以在不同的环境中被共享。

缺点

        1. 享元模式需要分离出内部状态和外部状态，从而使得系统变得复杂，这使得程序的逻辑复杂化。

        2. 为了使对象可以共享，享元模式需要将享元对象的部分状态外部化，而读取外部状态将使得运行时间变长。

适用场景

        1. 一个系统有大量相同或者相似的对象，造成内存的大量耗费。

        2. 对象的大部分状态都可以外部化，可以将这些外部状态传入对象中。

        3. 在使用享元模式时需要维护一个存储享元对象的享元池，而这需要耗费一定的系统资源。因此，在需要多次重复使用同一享元对象时才值得使用享元模式。

练习

myclass.h

//
// Created by yuwp on 2024/1/12.
//#ifndef DESIGNPATTERNS_MYCLASS_H
#define DESIGNPATTERNS_MYCLASS_H#include <iostream>
#include <unordered_map>class Flyweight {   // 享元抽象类
public:Flyweight(const std::string &in);virtual void display(const std::string &out) = 0;const std::string &getMIn() const;private:std::string m_in;
};class PictureFlyweight : public Flyweight {    // 具体享元类
public:PictureFlyweight(const std::string &in);void display(const std::string &out) override;
};class AnimationFlyweight : public Flyweight {    // 具体享元类
public:AnimationFlyweight(const std::string &in);void display(const std::string &out) override;
};class VideoFlyweight : public Flyweight {    // 具体享元类
public:VideoFlyweight(const std::string &in);void display(const std::string &out) override;
};class FlyweightFactory {    // 配合工厂模式使用享元工厂类
public:~FlyweightFactory();Flyweight *getFlyweight(const std::string &key);static FlyweightFactory *getInstance();
private:FlyweightFactory();std::unordered_map<std::string, Flyweight *> m_flyweights;
};#endif //DESIGNPATTERNS_MYCLASS_H

myclass.cpp

//
// Created by yuwp on 2024/1/12.
//#include "myclass.h"Flyweight::Flyweight(const std::string &in) {m_in = in;
}const std::string &Flyweight::getMIn() const {return m_in;
}PictureFlyweight::PictureFlyweight(const std::string &in) : Flyweight(in) {}void PictureFlyweight::display(const std::string &out) {std::cout << out << ", " << getMIn() << std::endl;
}AnimationFlyweight::AnimationFlyweight(const std::string &in) : Flyweight(in) {}void AnimationFlyweight::display(const std::string &out) {std::cout << out << ", " << getMIn() << std::endl;
}VideoFlyweight::VideoFlyweight(const std::string &in) : Flyweight(in) {}void VideoFlyweight::display(const std::string &out) {std::cout << out << ", " << getMIn() << std::endl;
}FlyweightFactory::FlyweightFactory() {}FlyweightFactory* FlyweightFactory::getInstance() {static FlyweightFactory *factory = new FlyweightFactory();return factory;
}FlyweightFactory::~FlyweightFactory() {for (auto it : m_flyweights) {if (it.second) {delete it.second;}}
}Flyweight* FlyweightFactory::getFlyweight(const std::string &key) {auto it = m_flyweights.find(key);if (it != m_flyweights.end()) {return it->second;} else {Flyweight *flyweight = nullptr;if (key == "图片") {flyweight = new PictureFlyweight(key);} else if (key == "动画") {flyweight = new AnimationFlyweight(key);} else if (key == "视频") {flyweight = new VideoFlyweight(key);}m_flyweights.insert(std::make_pair(key, flyweight));return flyweight;}
}

main.cpp

#include <iostream>
#include <mutex>
#include "myclass.h"int main() {FlyweightFactory *factory = FlyweightFactory::getInstance();factory->getFlyweight("图片")->display("1");factory->getFlyweight("动画")->display("2");factory->getFlyweight("视频")->display("3");factory->getFlyweight("图片")->display("4");return 0;
}