将单目运算符"++"重载为成员函数形式

　　对于前置单目运算符，重载函数没有形参，对于后置单目运算符，重载函数有一个int型形参。这个int型参数在函数体中并不使用，纯粹是用来区别前置与后置，因此参数表中可以只给出类型名，没有参数名。

#include <iostream>

using namespace std;

class Clock
{
public:
    Clock(int hour = 0,int min = 0,int sec = 0);
    void showTime();
    Clock& operator++();    //前置单目运算符重载
    Clock operator++(int);    //后置单目运算符重载
private:
    int _hour;
    int _min;
    int _sec;
};

Clock::Clock(int hour,int min,int sec)
{
    if(0 <= hour && hour < 24
                    && 0 <= min && min < 60
                    && 0 <= sec && sec < 60)
    {
        _hour = hour;
        _min = min;
        _sec = sec;
    }
    else
    {
        cout << "Time Init error." << endl;
    }
}

void Clock::showTime()
{
    cout << _hour << ":" << _min << ":" << _sec << endl;
}

Clock& Clock::operator++()
{
    std::cout << "Call front operator overloading" << std::endl;
    _sec++;
    if(_sec >= 60)
    {
        _sec = _sec - 60;
        _min++;
        if(_min >= 60)
        {
            _min = _min - 60;
            _hour++;
            _hour = _hour % 24;
        }
    }
    return *this;
}

Clock Clock::operator++(int)
{
    std::cout << "Call post operator overloading" << std::endl;
    Clock old = *this;
    ++(*this);    //调用前置单目运算符重载函数
    return old;
}

int main(int argc,char* argv[])
{
    Clock myClock(23,59,59);
    cout << "current time: ";
    myClock.showTime();
    ++myClock;
    cout << "++myClock: ";
    myClock.showTime();
    myClock++;
    cout << "myClock++: ";
    myClock.showTime();
    return 0;
}

输出结果：

current time: 23:59:59

Call front operator overloading

++myClock: 0:0:0

Call post operator overloading

Call front operator overloading

myClock++: 0:0:1