日期類（運算子的過載）

在C++裡可以實現運算子的過載，所以可以實現直接對日期類進行加減、比較等操作。

以下是整個Date.h標頭檔案的原始碼：

#pragma once
#include<cassert>
using namespace std;
class Date
{
public:
	Date(int year = 1900, int month = 1, int day = 1)
		:_year(year), _month(month), _day(day)
	{
		assert(IsInvalid());    //判斷日期是否合法
	}
	Date(const Date& d)
	{
		_year = d._year;
		_month = d._month;
		_day = d._day;
	}
	~Date()
	{}
	void Display()     //列印日期類
	{
		cout << _year << "-" << _month << "-" << _day << endl;
	}

	bool IsInvalid()   //判斷日期是否合法
	{
		if (_year >= 1900 && _month > 0 && _month < 13 && _day>0 && _day <= GetMonthDays(_year, _month))
		{
			return true;
		}
		return false;
	}
	int GetMonthDays(int year, int month)   //獲取當前月份對應的天數
	{
		static int MonthDays[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
		if (month != 2)
		{
			return MonthDays[month];
		}
		else
		{
			if ((year % 4 == 0 && year % 100 != 0) || (year % 400 == 0))   //閏年
			{
				return 29;
			}
			return 28;
		}
	}
	Date operator+(int day);  //過載+
	Date& operator+=(int day);//過載++
	Date& operator++();    //前置++
	Date operator++(int);  //後置++

	Date operator-(int day);  //過載-
	Date& operator-=(int day);//過載-=
	Date& operator--();    //前置--
	Date operator--(int);  //後置--
	int operator-(const Date& d);   //兩日期類物件相減返回一個整數

	bool operator>(const Date& d);  //過載>
	bool operator<(const Date& d);
	bool operator>=(const Date& d);
	bool operator<=(const Date& d);
	bool operator==(const Date& d);
	bool operator!=(const Date& d);

private:
	int _year;
	int _month;
	int _day;
};

inline Date Date::operator+(int day)
{
	Date tmp(*this);
	if (day < 0)
	{
		return *this - (-day);
	}
	tmp._day = _day + day;
	while (!tmp.IsInvalid())
	{
		int MonthDay = GetMonthDays(tmp._year, tmp._month);
		tmp._day -= MonthDay;
		tmp._month++;
		if (tmp._month == 13)
		{
			tmp._year++;
			tmp._month = 1;
		}
	}
	return tmp;
}
inline Date&  Date::operator+=(int day)
{
	*this = *this + day;   //對加法過載的程式碼複用
	return *this;
}
inline Date&  Date::operator++()    //前置++
{
	return *this+=1;
}
inline Date  Date::operator++(int)  //後置++
{
	Date tmp(*this);
	*this += 1;
	return tmp;
}
inline Date Date::operator-(int day)
{
	Date tmp(*this);
	if (day < 0)
	{
		return *this + (-day);
	}
	tmp._day = _day - day;
	while (!tmp.IsInvalid())
	{
		if (tmp._month != 1)
		{
			tmp._month--;
		}
		else   //month==1的情況
		{
			tmp._year--;
			tmp._month = 12;
		}
		int MonthDay = GetMonthDays(tmp._year, tmp._month);
		tmp._day += MonthDay;
	}
	return tmp;

}
inline Date& Date::operator-=(int day)
{
	*this = *this - day;
	return *this;
}
inline Date& Date::operator--()    //前置--
{
	return *this -= 1;
}
inline Date Date::operator--(int)  //後置--
{
	Date tmp(*this);
	*this -= 1;
	return tmp;
}
inline bool Date::operator>(const Date& d)
{
	if (_year > d._year || (_year == d._year && (_month > d._month || (_month == d._month && _day > d._day))))
	{
		return true;
	}
	return false;
}
inline bool Date::operator<(const Date& d)
{
	return !(*this > d || *this == d);
}
inline bool Date::operator>=(const Date& d)
{
	return (*this > d || *this == d);
}
inline bool Date::operator<=(const Date& d)
{
	return (*this < d || *this == d);
}
inline bool Date::operator==(const Date& d)
{
	if (_year == d._year && _month == d._month && _day == d._day)
	{
		return true;
	}
	return false;
}
inline bool Date::operator!=(const Date& d)
{
	return !(*this == d);
}
inline int Date::operator-(const Date& d)     //兩日期類物件相減返回一個整數
{
	int count = 0;
	Date tmp(*this);
	Date tmp2(d);
	
	if (tmp < tmp2)   //第一個日期比第二個小交換順序
	{
		Date x;
		x = tmp;
		tmp = tmp2;
		tmp2 = x;
	}

	//處理_day使兩個日期類的_day相同
	count = GetMonthDays(tmp2._year, tmp2._month) - tmp2._day + tmp._day;
	tmp2._day = tmp._day;
	if (tmp._day >= tmp2._day)  //count多加了一個月
	{
		tmp2._month++;
		if (tmp2._month == 13)
		{
			tmp2._year++;
			tmp2._month = 1;
		}
	}
	//處理_month使兩日期類_month相同
	while (tmp._month != tmp2._month)
	{
		count = count + GetMonthDays(tmp2._year, tmp2._month);
		tmp2._month++;
		if (tmp2._month == 13) 
		{
			tmp2._year++;
			tmp2._month = 1;
		}
	}
	//處理兩日期類的_year
	while (tmp._year > tmp2._year)
	{
		//判斷是否為閏年，並且在二月份一下的才能算366天
		if (((tmp2._year % 4 == 0 && tmp2._year % 100 != 0) || (tmp2._year % 400 == 0)) && (tmp2._month <= 2))
		{
			count += 366;
		}
		else
		{
			count += 365;
		}
		tmp2._year++;
	}
	return count;
}