帮我看看这个程序，分析一下它的执行过程！

#include <iostream>
using namespace std;
class Clock
{
public:
    Clock(int NewH=0,int NewM=0,int NewS=0);
    void ShowTime();
    Clock& operator ++();
    Clock operator ++(int);
private:
    int Hour,Minute,Second;
};
Clock::Clock(int NewH,int NewM,int NewS)
{
    if(0<=NewH&&NewH<24&&0<=NewM&&NewM<60&&0<=NewS&&NewS<60)
    {
        Hour=NewH;
        Minute=NewM;
        Second=NewS;
    }
else
cout<<"Time error!"<<endl;
}
void Clock::ShowTime()
{
    cout<<Hour<<":"<<Minute<<":"<<Second<<endl;
}
Clock& Clock::operator ++()
{
    Second++;
    if(Second>=60)
    {
        Second=Second-60;
        Minute++;
        if(Minute>=60)
        {
            Minute=Minute-60;
            Hour++;
            Hour=Hour%24;
        }
    }
    return *this;
}
Clock Clock::operator ++(int)
{
    Clock old=*this;
    ++(*this);
    return old;
}
int main()
{
    Clock myClock(23,59,59);
    cout<<"First time output:";
    myClock.ShowTime();
    cout<<"Show myClock++:";
    (myClock++).ShowTime();
    cout<<"Show ++myClock:";
    (++myClock).ShowTime();
}
帮我分析一下它的执行过程，还有这里这个‘*this’是什么意思？int main()中的为什么用（myclock++）.showtime()来输出结果？谢谢了！