【设计模式】Head First 设计模式——装饰者模式 C++实现

设计模式最大的作用就是在变化和稳定中间寻找隔离点，然后分离它们，从而管理变化。将变化像小兔子一样关到笼子里，让它在笼子里随便跳，而不至于跳出来把你整个房间给污染掉。

设计思想

动态地将责任附加到对象上，若要扩展功能，装饰者提供了比继承更有弹性的替代方案。

装饰类继承自超类，继承是为了有正确的类型，而非继承超类的行为。

业务场景

假定你需要给蜜雪冰城设计一套售卖系统。他们家所有的饮料假设只有描述和价格两种属性，你可能又会想到用继承去解决：先抽象一个奶茶父类，然后每种奶茶都继承该父类并实现各自的display和cost方法。

你很快就会发现这么做有很多麻烦：首先，这家奶茶店所有的奶茶种类加一起数量可能有几十种数百种，这也就意味着你的子类会有很多很多；其次，这么设计似乎并不符合实际的业务场景，人们点奶茶的时候并不是一成不变的，有的不要珍珠，有的双倍糖等等，加的调料和份数也不一样，那么最后如何描述这杯奶茶以及计算其价格就成了问题。

代码案例

#include<iostream>
#i#include<iostream>
#include<vector>
#include<algorithm>
#include <string>
#include <memory>//饮料抽象类
class Beverage {
public:virtual ~Beverage() {};virtual std::string getDescription() = 0;virtual double cost() = 0;
protected:std::string description;};
//调料装饰者类
class Condimentecorator :public Beverage {
public:virtual ~Condimentecorator() {};virtual std::string getDescription() = 0;
};//espresso 饮料类
class Espresso :public Beverage {
public:Espresso() :Beverage(){description = "Espresso";}std::string getDescription() { return description; }double cost() {return 1.99;}
};//houseblend 饮料类
class HouseBlend :public Beverage {
public:HouseBlend() :Beverage(){description = "HouseBlend";}std::string getDescription() { return description; }double cost() {return 0.89;}
};//DarkRoast 饮料类
class DarkRoast :public Beverage {
public:DarkRoast() :Beverage(){description = "DarkRoast";}std::string getDescription() { return description; }double cost() {return 0.99;}
};//Decat 饮料类
class Decat :public Beverage {
public:Decat() :Beverage(){description = "Decat";}std::string getDescription() { return description; }double cost(){return 1.05;}
};//Mocha调料装饰者
class Mocha :public Condimentecorator {public:Mocha(std::shared_ptr<Beverage> be) :Condimentecorator(), beverage(be) {}std::string getDescription() {return (beverage->getDescription() + " Mocha");}double cost(){return 0.2 + beverage->cost();}public:std::shared_ptr<Beverage> beverage;
};//Soy调料装饰者
class Soy :public Condimentecorator {
public:Soy(std::shared_ptr<Beverage> be):Condimentecorator(),beverage(be){}std::string getDescription(){return (beverage->getDescription() + " Soy");}double cost(){return 0.15 + beverage->cost();}
public:std::shared_ptr<Beverage> beverage;
};//Whip调料装饰者
class Whip :public Condimentecorator {
public:Whip(std::shared_ptr<Beverage> be) :Condimentecorator(), beverage(be) {}std::string getDescription(){return (beverage->getDescription() + " Whip");}double cost(){return 0.10 + beverage->cost();}
public:std::shared_ptr<Beverage> beverage;
};int main()
{std::shared_ptr<Beverage> beverage = std::make_shared<Espresso>();std::cout << beverage->getDescription()<<" costs: "<<beverage->cost() << std::endl;std::shared_ptr<Beverage> beverage2 = std::make_shared<DarkRoast>();std::cout << beverage2->getDescription() << " costs: " << beverage2->cost() << std::endl;beverage2 = std::make_shared<Mocha>(beverage2);std::cout << beverage2->getDescription() << " costs: " << beverage2->cost() << std::endl;beverage2 = std::make_shared<Mocha>(beverage2);std::cout << beverage2->getDescription() << " costs: " << beverage2->cost() << std::endl;beverage2 = std::make_shared<Whip>(beverage2);std::cout << beverage2->getDescription() << " costs: " << beverage2->cost() << std::endl;auto it = std::make_shared<Whip>(beverage2);std::cout << it->cost() << " " << it->getDescription() << std::endl;std::cout << it->beverage->cost() << " " << it->beverage->getDescription() << std::endl;return 0;
}