【HeadFirst 设计模式】装饰者模式的C++实现

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……）：

#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：

#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;
}