Headfirst设计模式的C++实现——装饰者模式(Decorater)

Beverage.h

1 #ifndef BEVERAGE_H_INCLUDED
 2 #define BEVERAGE_H_INCLUDED
 3 
 4 #include <string>
 5 
 6 class Beverage
 7 {
 8 public:
 9     Beverage( std::string description = "Unknown Beverage" ) : m_description ( description ) {}
10     virtual std::string getDescription() { return m_description; }
11     virtual double cost() = 0;
12 private:
13     std::string m_description;
14 };
15 
16 #endif // BEVERAGE_H_INCLUDED

CondimentDecorator.h

1 #ifndef CONDIMENTDECORATOR_H_INCLUDED
 2 #define CONDIMENTDECORATOR_H_INCLUDED
 3 
 4 #include <string>
 5 #include "Beverage.h"
 6 
 7 class CondimentDecorator : public Beverage
 8 {
 9 public:
10     virtual std::string getDescription() = 0;
11 };
12 
13 #endif // CONDIMENTDECORATOR_H_INCLUDED

HouseBlend.h

1 #ifndef HOUSEBLEND_H_INCLUDED
 2 #define HOUSEBLEND_H_INCLUDED
 3 
 4 #include "Beverage.h"
 5 
 6 class HouseBlend : public Beverage
 7 {
 8 public:
 9     HouseBlend () : Beverage( "House Blend Coffee" ) {}
10     double cost() { return .89; }
11 };
12 
13 #endif // HOUSEBLEND_H_INCLUDED

Mocha.h

1 #ifndef MOCHA_H_INCLUDED
 2 #define MOCHA_H_INCLUDED
 3 
 4 #include "CondimentDecorator.h"
 5 
 6 class Mocha : public CondimentDecorator
 7 {
 8 public:
 9     Mocha ( Beverage * p_beverage ) : m_p_beverage ( p_beverage ) {}
10     std::string getDescription() { return m_p_beverage->getDescription() + ", Mocha"; }
11     double cost() { return .20 + m_p_beverage->cost(); }
12 private:
13     Beverage *m_p_beverage;
14 };
15 
16 #endif // MOCHA_H_INCLUDED

main.cpp

1 #include <iostream>
 2 #include "HouseBlend.h"
 3 #include "Mocha.h"
 4 int main()
 5 {
 6     HouseBlend house_blend;
 7     std::cout << house_blend.getDescription() << ": " << house_blend.cost() << std::endl;
 8 
 9     Mocha mocha_house_blend( &house_blend );
10     std::cout << mocha_house_blend.getDescription() << ": " << mocha_house_blend.cost() << std::endl;
11 
12     Mocha mocha_mocha_house_blend( &mocha_house_blend );
13     std::cout << mocha_mocha_house_blend.getDescription() << ": " << mocha_mocha_house_blend.cost() << std::endl;
14 
15     return 0;
16 }

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。
原文链接: https://www.cnblogs.com/ren-yu/p/4947315.html

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