Headfirst设计模式的C++实现——装饰者模式(Decorater)

Beverage.h

 #ifndef BEVERAGE_H_INCLUDED
 #define BEVERAGE_H_INCLUDED

 #include <string>

 class Beverage
 {
 public:
     Beverage( std::string description = "Unknown Beverage" ) : m_description ( description ) {}
     virtual std::string getDescription() { return m_description; }
     virtual double cost() = ;
 private:
     std::string m_description;
 };

 #endif // BEVERAGE_H_INCLUDED

CondimentDecorator.h

 #ifndef CONDIMENTDECORATOR_H_INCLUDED
 #define CONDIMENTDECORATOR_H_INCLUDED

 #include <string>
 #include "Beverage.h"

 class CondimentDecorator : public Beverage
 {
 public:
     virtual std::string getDescription() = ;
 };

 #endif // CONDIMENTDECORATOR_H_INCLUDED

HouseBlend.h

 #ifndef HOUSEBLEND_H_INCLUDED
 #define HOUSEBLEND_H_INCLUDED

 #include "Beverage.h"

 class HouseBlend : public Beverage
 {
 public:
     HouseBlend () : Beverage( "House Blend Coffee" ) {}
     double cost() { return .; }
 };

 #endif // HOUSEBLEND_H_INCLUDED

Mocha.h

 #ifndef MOCHA_H_INCLUDED
 #define MOCHA_H_INCLUDED

 #include "CondimentDecorator.h"

 class Mocha : public CondimentDecorator
 {
 public:
     Mocha ( Beverage * p_beverage ) : m_p_beverage ( p_beverage ) {}
     std::string getDescription() { return m_p_beverage->getDescription() + ", Mocha"; }
     double cost() { return . + m_p_beverage->cost(); }
 private:
     Beverage *m_p_beverage;
 };

 #endif // MOCHA_H_INCLUDED

main.cpp

 #include <iostream>
 #include "HouseBlend.h"
 #include "Mocha.h"
 int main()
 {
     HouseBlend house_blend;
     std::cout << house_blend.getDescription() << ": " << house_blend.cost() << std::endl;

     Mocha mocha_house_blend( &house_blend );
     std::cout << mocha_house_blend.getDescription() << ": " << mocha_house_blend.cost() << std::endl;

     Mocha mocha_mocha_house_blend( &mocha_house_blend );
     std::cout << mocha_mocha_house_blend.getDescription() << ": " << mocha_mocha_house_blend.cost() << std::endl;

     return ;
 }

一点个人理解：

起初对于为什么Mocha类要从CondimentDecorator类继承也就是CondimentDecorator类存在的意义感到疑惑，并做了一次尝试：跳过CondimentDecorator类，让Mocha类直接从Beverage类继承，发现效果是一样的。那为什么还要有CondimentDecorator类的存在呢？

原书上有这么一段解释：

“所有的调料装饰着都必须重新实现getDescription()方法。稍后我们会解释为什么......”

"我们希望叙述不只是描述饮料，而是完整的连调料都描述出来......"

我推测作者的意思是说通过CondimentDecorator类，并且把getDescription方法设置为纯虚函数就可以强制让所有调料类实现此方法。如果不通过getDescription类，那么调料类就可以不实现此方法从而不符合需求。