<First Head设计模式>装饰器模式使用前的代码设计

今天读到了<First Head设计模式>装饰器这一章,对文中一开始的例子比较感兴趣.文中介绍了一个咖啡售卖系统如何设计的问题.一开始是通过设计一个基类来计算所用的调料的价格,而用子类来计算特定品种的整体价格.

这里面比较感兴趣的是文中有一个子类的实现: 在子类中直接通过父类名::方法来调用父类的方法,之前不怎么用继承,感到比较新奇,就自己码代码尝试了下.

程序的UML图大致如下,真正重要的函数是cost()函数,用来得出咖啡的最终价格.

//Beverage.h
#pragma once
#include <iostream>
using namespace std;

class Beverage
{
public:
    Beverage(bool bMilk, bool bSugar, double dCostMilk, double dCostSugar);
    virtual ~Beverage();    //基类析构用virtual,为了保证多态析构时能够找到子类的析构函数(不用virtual,基类指针是找不到子类的析构函数的位置的)

    virtual double cost();
    bool HasMilk();
    bool HasSugar();
protected:
private:
    bool m_bMilk;
    bool m_bSugar;
    double m_dCostMilk;
    double m_dCostSugar;
};

//Beverage.cpp
#include "Beverage.h"

Beverage::~Beverage()
{
    cout << "Beverage is Over~" << endl;
}

Beverage::Beverage(bool bMilk, bool bSugar, double dCostMilk, double dCostSugar):
m_bMilk(bMilk), m_bSugar(bSugar), m_dCostMilk(dCostMilk), m_dCostSugar(dCostSugar)
{
    cout << "Beverage Begins~" << endl;
}

bool Beverage::HasMilk()
{
    return m_bMilk;
}

bool Beverage::HasSugar()
{
    return m_bSugar;
}

//计算配料最终花了多少钱
double Beverage::cost()
{
    double sum0 = 0;
    if(HasMilk())
    {
        sum0 += m_dCostMilk;
    }

    if (HasSugar())
    {
        sum0 += m_dCostSugar;
    }
    return sum0;
}

//DarkRoastCoffee.h
#pragma once
#include "Beverage.h"

class DarkRoastCoffee : public Beverage
{
public:
    DarkRoastCoffee(double myCost,bool bMilk, bool bSugar, double dCostMilk, double dCostSugar);

    ~DarkRoastCoffee();

    double cost();
    char * Description();
    double GetMyCost();

private:
    double m_dMyCost;
};

//DarkRoastCoffee.cpp
#include "DarkRoastCoffee.h"

//在子类的构造函数中,调用父类的有参构造函数.  注意初始化成员列表需要放在函数定义中,因为最后的")"后面要跟着"{"
DarkRoastCoffee::DarkRoastCoffee(double myCost,bool bMilk, bool bSugar, double dCostMilk, double dCostSugar):Beverage(bMilk, bSugar, dCostMilk, dCostSugar), m_dMyCost(myCost)
{
    cout << "DarkRoastCoffee Begins~" << endl;
}

double DarkRoastCoffee::GetMyCost()
{
    return m_dMyCost;
}

DarkRoastCoffee::~DarkRoastCoffee()
{
    cout << "DarkRoastCoffee is Over~" << endl;
}

double DarkRoastCoffee::cost()
{
    return    Beverage::cost() + GetMyCost();    

//注意此时用了父类::方法,这里的Beverage::cost()不必是static,因为在该方法执行时父类对象已经创建了(若是在无继承关系的别的类中,cost必须要是static了)
//当然这里Beverage::cost()也不能是static,因为static函数不能为virtual.
}

输出结果:

//CoffeeCost.cpp

#include "Beverage.h"
#include "DarkRoastCoffee.h"

int main()
{
    DarkRoastCoffee dc(10, false, true, 2, 4);
    cout << dc.cost() << endl;
    return 0;
}