一、案例背景
Starbuzz是以扩张速度最快而闻名的咖啡连锁店。如果你在街角看到它的店，在对面街上肯定还会看到另一家。因为扩张速度实在太快了，他们准备更新订单系统，以合乎他们的饮料供应要求。他们原先的类设计是这样的......
购买咖啡时，可以要求在其中加入各种调料，例如：蒸奶（Steamed Milk）、豆浆（Soy）、摩卡（Mocha，也就是巧克力风味）或覆盖奶泡。星巴克会根据所加入的调料收取不同的费用。所以订单系统必须考虑到这些调料部分。这是他们的第一个尝试......
很明显，Starbuzz为自己制造了一个维护噩梦：如果牛奶的价格上扬怎么办？新增一种焦糖调料风味时怎么办？
二、案例分析
看到这么多类时你肯定也会被震惊到......那么问题来了，如何进行改进呢？一个直截了当的解决方案是利用实例变量和继承，就可以追踪这些调料。比如我们在基类中加上实例变量，这些布尔值代表是否加上该调料（牛奶，豆浆，摩卡，奶泡......）：
c++
#include <iostream>
#include <string>
using namespace std;
class Beverage
{
private:
string description {};
bool milk {};
bool soy {};
bool mocha {};
bool whip {};
public:
const string getDiscription()
{
return description;
};
void setDescription(const string& description)
{
this->description = description + "(Add " + (milk ? "Milk " : "") + (soy ? "& Soy " : "") + (mocha ? "& Mocha " : "") + (whip ? "& Whip " : "") + ")";
}
virtual const float cost()
{
return (milk ? 1 : 0) + (soy ? 2 : 0) + (mocha ? 1 : 0) + (whip ? 1.5 : 0);
}
const bool hasMilk() const
{
return milk;
};
void setMilk(const bool value)
{
milk = value;
};
const bool hasSoy() const
{
return soy;
};
void setSoy(const bool value)
{
soy = value;
};
const bool hasMocha() const
{
return mocha;
};
void setMocha(const bool value)
{
mocha = value;
};
const bool hasWhip() const
{
return whip;
};
void setWhip(const bool value)
{
whip = value;
};
};
class HouseBlend : public Beverage
{
public:
HouseBlend()
{
setMilk(true);
setSoy(true);
setDescription("House Blend");
}
const float cost() override
{
return 5.0 + Beverage::cost();
}
};
class DarkRoast : public Beverage
{
public:
DarkRoast()
{
setMilk(true);
setWhip(true);
setDescription("DarkRoast");
}
const float cost() override
{
return 8.0 + Beverage::cost();
}
};
class Decaf : public Beverage
{
public:
Decaf()
{
setMilk(true);
setWhip(true);
setSoy(true);
setDescription("Decaf");
}
const float cost() override
{
return 10.0 + Beverage::cost();
}
};
int main()
{
cout << "我点了一杯" + HouseBlend().getDiscription() << "，花了" << HouseBlend().cost() << "元"<<endl;
cout << "我点了一杯" + DarkRoast().getDiscription() << "，花了" << DarkRoast().cost() << "元"<<endl;
cout << "我点了一杯" + Decaf().getDiscription() << "，花了" << Decaf().cost() << "元"<<endl;
return 0;
}看起来似乎还行。但是如果将来由于原材料上涨某些调料需要上涨价钱怎么办？如果出现了新的调料呢？如果顾客想要双倍摩卡的咖啡呢？
这些变化都需要我们去直接变更源码。
开放关闭原则：类应该对扩展开放，对修改关闭。
我们的目标是允许类容易扩展，在不修改现有代码的情况下，就可以搭配新的行为。这样的设计具有弹性，可以应对改变，可以接收新的功能来应对改变的需求。
让我们来看看使用装饰者模式是怎么解决问题的：
三、代码分析
这里给出相关案例的C++代码实现：
c++
#include <iostream>
#include <string>
using namespace std;
class Beverage
{
protected:
string description = "unknown Beverage";
public:
virtual const string getDescription() const
{
return description;
}
virtual const double cost() const = 0;
};
class CondimentDecorator : public Beverage
{
public:
virtual const string getDescription() const = 0;
};
class HouseBlend : public Beverage
{
public:
HouseBlend()
{
description = "HouseBlend";
}
const double cost() const override
{
return 5.00;
}
};
class DarkRoast : public Beverage
{
public:
DarkRoast()
{
description = "DarkRoast";
}
const double cost() const override
{
return 8.00;
}
};
class Decaf : public Beverage
{
public:
Decaf()
{
description = "Decaf";
}
const double cost() const override
{
return 10.00;
}
};
class Milk : public CondimentDecorator
{
public:
Beverage* beverage {};
Milk(Beverage* beverage)
{
this->beverage = beverage;
}
const string getDescription() const override
{
return beverage->getDescription() + " & Milk";
}
const double cost() const override
{
return beverage->cost() + 1.0;
}
};
class Soy : public CondimentDecorator
{
public:
Beverage* beverage {};
Soy(Beverage* beverage)
{
this->beverage = beverage;
}
const string getDescription() const override
{
return beverage->getDescription() + " & Soy";
}
const double cost() const override
{
return beverage->cost() + 2.0;
}
};
class Mocha : public CondimentDecorator
{
public:
Beverage* beverage {};
Mocha(Beverage* beverage)
{
this->beverage = beverage;
}
const string getDescription() const override
{
return beverage->getDescription() + " & Mocha";
}
const double cost() const override
{
return beverage->cost() + 2.0;
}
};
class Whip : public CondimentDecorator
{
public:
Beverage* beverage {};
Whip(Beverage* beverage)
{
this->beverage = beverage;
}
const string getDescription() const override
{
return beverage->getDescription() + " & Whip";
}
const double cost() const override
{
return beverage->cost() + 2.0;
}
};
int main()
{
Beverage* houseblend = new Milk(new Soy(new HouseBlend()));
Beverage* darkRoast = new Milk(new Soy(new Whip(new DarkRoast())));
Beverage* decaf = new Milk(new Whip(new Decaf()));
// 双倍摩卡
Beverage* doubleMochaCoffee = new Milk(new Soy(new Mocha(new Mocha(new HouseBlend()))));
cout << "我点了一杯" + houseblend->getDescription() << "，花了" << houseblend->cost() << "元" << endl;
cout << "我点了一杯" + darkRoast->getDescription() << "，花了" << darkRoast->cost() << "元" << endl;
cout << "我点了一杯" + decaf->getDescription() << "，花了" << decaf->cost() << "元" << endl;
cout << "我点了一杯" + doubleMochaCoffee->getDescription() << "，花了" << doubleMochaCoffee->cost() << "元" << endl;
return 0;
}