设计模式(3)-策略模式(Strategy) (三)

Rental::Rental(Movie movie, int daysRented)
{
qDebug()<<"construct Rental";
_movie = movie;
_daysRented = daysRented;
}

Movie Rental::getMovie()
{
return _movie;
}

int Rental::getDaysRented()
{
return _daysRented;
}

double Rental::getCharge(int daysRented)
{
double result = 0;
qDebug()< switch(getMovie().getPriceCode())
{
case REGULAR:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1.5;
}
break;
case NEW_RELEASE:
result += daysRented*3;
break;
case DISCOUNT:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1;
}
break;
}
qDebug()< return result;
}

main.cpp

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#include "movie.h"
#include "rental.h"

int main()
{
int daysRented = 30;
Movie movie("Book1",NEW_RELEASE);
Rental rental(movie,daysRented);
movie.setPriceCode(NEW_RELEASE);
rental.getCharge(daysRented);
return 0;
}
#include "movie.h"
#include "rental.h"

int main()
{
int daysRented = 30;
Movie movie("Book1",NEW_RELEASE);
Rental rental(movie,daysRented);
movie.setPriceCode(NEW_RELEASE);
rental.getCharge(daysRented);
return 0;
}

运行结果如下：

[html]
"construct Movie::Movie(Book1,1)"
construct Movie::Movie()
construct Rental
1
90
"construct Movie::Movie(Book1,1)"
construct Movie::Movie()
construct Rental
1
90
示例调用了getCharge，并返回了priceCode相应的计价。相对于《重构》一书最原始代码，这段代码经过了一定的重构处理。

先看下述代码，

[html]
double Rental::getCharge(int daysRented)
{
double result = 0;
qDebug()< switch(getMovie().getPriceCode())
{
case REGULAR:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1.5;
}
break;
case NEW_RELEASE:
result += daysRented*3;
break;
case DISCOUNT:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1;
}
break;
}
qDebug()< return result;
}
double Rental::getCharge(int daysRented)
{
double result = 0;
qDebug()< switch(getMovie().getPriceCode())
{
case REGULAR:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1.5;
}
break;
case NEW_RELEASE:
result += daysRented*3;
break;
case DISCOUNT:
result +=2;
if(daysRented>2)
{
result += (daysRented-2)*1;
}
break;
}
qDebug()< return result;
}
这段代码，有一些不好的地方：

(1) 这段代码通过priceCode判断电影的租金，采用了switch-case语言，实质上还是面向过程的范畴，代码可重用性很低；

(2) 如果情形较多，switch-case会变得十分庞大。

有什么办法能去掉switch-case吗？下面通过引入策略模式实现这一目的。

先看UML图，

图2

(1) 引入了一个Price基类，提供了getCharge()接口供子类实现；

(2) 将switch-case语句中的3种情形抽取为3个Price子类；

(3) Movie调用了Price提供的接口。

【代码清单】

price.h

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#ifndef PRICE_H
#define PRICE_H

class Price
{
public:
Price();

private:
int _priceCode;

public:
int getPriceCode();