计算时间：一个C++运算符重载示例

Time类是一个用于计算时间的类，其原型如下：
程序清单11.1 mytime0.h

// mytime0.h -- Time class before operator overloading
#ifndef MYTIME0_H_
#define MYTIME0_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 Sum(const Time & t) const;
    void Show() const;
};
#endif // MYTIME0_H_

Time类提供了用于调整和重新设置事时间、显示时间、将两个事件相加的方法。程序清单11.2列出了方法定义。
程序清单11.2 mytime0.cpp

// mytime0.cpp -- implementing Time methods
#include <iostream>
#include "mytime0.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::Sum(const Time & t) const
{
    Time sum;
    sum.minutes = minutes + t.minutes;
    sum.hours = hours + t.hours + sum.minutes / 60;
    sum.minutes %= 60;
    return sum;
}

void Time::Show() const
{
    std::cout << hours << " hours, " << minutes << "minutes";
}

最后，程序清单11.3对Time类中计算时间总和的部分进行了测试。
程序清单11.3 usetime0.cpp

// usetime0.cpp -- using the first draft of the Time class
// conpile usetime0.cpp and mytime0.cpp together
#include <iostream>
#include "mytime0.h"

int main()
{
    using std::cout;
    using std::endl;
    Time planing;
    Time coding(2, 40);
    Time fixing(5, 55);
    Time total;

    cout << "planing time = ";
    planing.Show();
    cout << endl;

    cout << "coding time = ";
    coding.Show();
    cout << endl;

    cout << "fixing time = ";
    fixing.Show();
    cout << endl;

    total = coding.Sum(fixing);
    cout << "coding.Sum(fixing) = ";
    total.Show();
    cout << endl;

    return 0;
}

下面是程序清单11.1、程序清单11.2和程序清单11.3组成的程序的输出：

planing time = 0 hours, 0 minutes
coding time = 2 hours, 40 minutes
fixing time = 5 hours, 55 minutes
coding.Sum(fixing) = 8 hours, 35 minutes

11.2.1 添加加法运算符
将Time类转换为重载的加法运算符很容易，只要将Sum()的名称改为operator +()即可。这样做是对的，只要把运算符（这里为+）防盗operator的后面，并将结果用作方法名即可。在这里，可以在标识符中使用字幕、数字或下划线之外的其他字符。程序清单11.4和程序清单11.5反映了这些细微的修改。
程序清单11.4 mytime1.h

// mytime1.h -- Time class before operator overloading
#ifndef MYTIME1_H_
#define MYTIME1_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;
    void Show() const;
};
#endif // MYTIME1_H_

程序清单11.5 mytime1.cpp

// mytime1.cpp -- implementing Time methods
#include <iostream>
#include "mytime1.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;
}

void Time::Show() const
{
    std::cout << hours << " hours, " << minutes << " minutes";
}

用一个usetime1.cpp做测试，得到和上述一样的结果。

usetime1.cpp:

// usetime1.cpp -- using the first draft of the Time class
// conpile usetime0.cpp and mytime0.cpp together
#include <iostream>
#include "mytime1.h"

int main()
{
    using std::cout;
    using std::endl;
    Time planing;
    Time coding(2, 40);
    Time fixing(5, 55);
    Time total;

    cout << "planing time = ";
    planing.Show();
    cout << endl;

    cout << "coding time = ";
    coding.Show();
    cout << endl;

    cout << "fixing time = ";
    fixing.Show();
    cout << endl;

    total = coding + fixing;
    cout << "coding.Sum(fixing) = ";
    total.Show();
    cout << endl;

    return 0;
}

和Sum()一样，operator+()也是由Time对象调用的，它将第二个Time对象作为参数，并返回一个Time对象。因此，可以像调用Sum()那样来调用operator+()方法：

total = coding.operator+(fixing);    // function notation

但将该方法命令为operator+()后，也可以使用运算符表示法：

total = coding + fixing;        // operator notation

这两种表示法都将调用operator+()方法。注意，在运算符表示法中，运算符左侧的对象（这里为coding）是调用对象，运算符右边的对象（这里为fixing）是作为参数被传递的对象。