06-其他重载运算符

11.2.3　其他重载运算符

还有一些其他的操作对Time类来说是有意义的。例如，可能要将两个时间相减或将时间乘以一个因子，这需要重载减法和乘法运算符。这和重载加法运算符采用的技术相同，即创建operator –()和operator *()方法。也就是说，将下面的原型添加到类声明中：

Time operator-(const Time & t) const;
Time operator*(double n) const;

程序清单11.7是新的头文件。

程序清单11.7　mytime2.h

// mytime2.h -- Time class after operator overloading
#ifndef MYTIME2_H_
#define MYTIME2_H_
class Time
{
private:
    int hours;
    int minutes;
public:
    Time();
    Time(int h, int m = 0);
    void AddMin(int m);
    void AddHr(int h);
    void Reset(int h = 0, int m = 0);
    Time operator+(const Time & t) const;
    Time operator-(const Time & t) const;
    Time operator*(double n) const;
    void Show() const;
};
#endif

然后将新增方法的定义添加到实现文件中，如程序清单11.8所示。

程序清单11.8　mytime2.cpp

// mytime2.cpp -- implementing Time methods
#include <iostream>
#include "mytime2.h"
Time::Time()
{
    hours = minutes = 0;
}
Time::Time(int h, int m )
{
    hours = h;
    minutes = m;
}
void Time::AddMin(int m)
{
    minutes += m;
    hours += minutes / 60;
    minutes %= 60;
}
void Time::AddHr(int h)
{
    hours += h;
}
void Time::Reset(int h, int m)
{
    hours = h;
    minutes = m;
}
Time Time::operator+(const Time & t) const
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}
Time Time::operator-(const Time & t) const
{
    Time diff;
    int tot1, tot2;
    tot1 = t.minutes + 60 * t.hours;
    tot2 = minutes + 60 * hours;
    diff.minutes = (tot2 - tot1) % 60;
    diff.hours = (tot2 - tot1) / 60;
    return diff;
}
Time Time::operator*(double mult) const
{
    Time result;
    long totalminutes = hours * mult * 60 + minutes * mult;
    result.hours = totalminutes / 60;
    result.minutes = totalminutes % 60;
    return result;
}
void Time::Show() const
{
    std::cout << hours << " hours, " << minutes << " minutes";
}

完成上述修改后，就可以使用程序清单11.9中的代码来测试新定义了。

程序清单11.9　usetime2.cpp

// usetime2.cpp -- using the third draft of the Time class
// compile usetime2.cpp and mytime2.cpp together
#include <iostream>
#include "mytime2.h"
int main()
{
    using std::cout;
    using std::endl;
    Time weeding(4, 35);
    Time waxing(2, 47);
    Time total;
    Time diff;
    Time adjusted;
    cout << "weeding time = ";
    weeding.Show();
    cout << endl;
    cout << "waxing time = ";
    waxing.Show();
    cout << endl;
    cout << "total work time = ";
    total = weeding + waxing; // use operator+()
    total.Show();
    cout << endl;
    diff = weeding - waxing; // use operator-()
    cout << "weeding time - waxing time = ";
    diff.Show();
    cout << endl;
    adjusted = total * 1.5; // use operator*()
    cout << "adjusted work time = ";
    adjusted.Show();
    cout << endl;
    return 0;
}

下面是程序清单11.7～程序清单11.9组成的程序得到的输出：

weeding time = 4 hours, 35 minutes
waxing time = 2 hours, 47 minutes
total work time = 7 hours, 22 minutes
weeding time - waxing time = 1 hours, 48 minutes
adjusted work time = 11 hours, 3 minutes