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
C++
#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;
}