最近在複習C++的時候發現日期類是一個非常有用的類，在現實中是非常實用的（雖然我不知道為什麼這麼實用的類，庫裡沒有）以下是我自己實現的日期類的程式碼，因為大部分都是運算子的過載，所以理解起來應該並不難

#include <iostream>
#include <assert.h>
using namespace std;
class Date
{
        friend  ostream&  operator<<(ostream& out,const  Date&  d);
        friend  istream&  operator>>(istream& in, Date&  d);
        public:
        Date (int year=1900,int month=1,int day=1)
        {
                _year=year;
                _month=month;
                _day=day;


                if (month<1||month>12
                                ||day<1||day>GetMonthDay(year,month))
                {
                        assert (false);
                }
        }





        Date(const Date& d)
        {
                _year=d._year;
                _month=d._month;
                _day=d._day;
        }



        Date&  operator=(const  Date&   d)
        {
                if (this!=&d)
                {
                        _year=d._year;
                        _month=d._month;
                        _day=d._day;
                }
                return *this ;
        }



        bool IsLeapYear(int year)
        {
                if ((year%4==0&&year%100!=0)||year%400==0)
                {
                        return true;
                }
                return false;
        }


        int GetMonthDay(int year,int month)
        {
                assert (month > 0  &&  month < 13);
                static int monthdays[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
                int day=monthdays[month];
                if (month==2&&IsLeapYear(year))
                        day+=1;

                return day;
        }



        bool operator<(const Date&  d)
        {
                if ((_year<d._year)
                                ||(_year==d._year&&_month<d._month)
                                ||(_year==d._year&&_month==d._month&&_day<d._day))
                {
                        return true;
                }
                return false;
        }



        bool operator>(const Date&   d)
        {
                return !(*this <=d);
        }



        bool operator<=(const Date&   d)
        {
                return *this<d||*this==d;
        }



        bool operator>=(const Date&   d)
        {
                return !(*this <d);
        }

        bool operator==(const Date&  d)
        {
                return  _year==d._year&&_month==d._month&&_day==d._day;
        }


        bool operator!=(const Date&  d)
        {
                return !(*this ==d);
        }



        Date operator+ (int day)
        {
                if (day<0)
                {
                        return *this-(-day);
                }
                Date  ret(*this);
                ret._day+=day;


                while (ret._day>GetMonthDay(ret._year,ret._month))
                {
                        ret._day-=GetMonthDay(ret._year,ret._month);
                        ret._month++;
                        if (ret._month==13)
                        {
                                ret._year++;
                                ret._month=1;
                        }
                }
                return ret;
        }



        Date operator- (int day)
        {
                if (day<0)
                {
                        return *this +(-day);
                }
                Date  ret (*this );
                ret._day-=day;
                while (ret._day<=0)
                {
                        ret._month--;
                        if (ret._month==0)
                        {
                                ret._year--;
                                ret._month=12;
                        }
                        ret._day+=GetMonthDay(ret._year,ret._month);
                }
                return ret;
        }



        Date& operator+=(int day)
        {
                *this =*this +day;
                return *this;
        }

        Date&   operator -=(int  day)
        {
                *this =*this -day;
                return *this ;
        }

        Date& operator++()
        {
                *this +=1;
                return *this ;
        }
        Date   operator++(int)
        {
                Date  tmp(*this );
                *this+=1;
                return tmp;
        }

        int operator-(const Date&  d)
        {
                int flag=1;
                Date max=*this,min=d;
                if (*this <d)
                {
                        max=d;
                        min=*this;
                        flag=-1;
                }

                int days=0;
                while (min<max)
                {
                        ++min;
                        ++days;
                }
                return days*flag;
        }


        void Show ()
        {
                cout<<(*this)._year<<"-"<<_month<<"-"<<_day<<endl;
        }



        private:
        int _year;
        int _month;
        int _day;
};
ostream&  operator<<(ostream& out,const  Date&  d)
{
        out<<d._year<<"-"<<d._month<<"-"<<d._day<<endl;
        return out;
}
istream&  operator>>(istream& in, Date&  d)
{
        in>>d._year;
        in>>d._month;
        in>>d._day;


        return in;

}


void TestDate()
{
        Date  d1(2018,3,15);
        d1+=100;
        d1.Show();
}
int main ()
{
        TestDate();
        return 0;
}