类和对象(5)：Date类详解

// Date.h
#include 
using namespace std;
#include 


class Date
{
public:
	Date(int year = 1, int month = 1, int day = 1)
	{
		assert(month >= 1 && month <= 12);
		_year = year;
		_month = month;
		_day = day;
	}

	void Print()
    {
        cout << _year << " " << _month << " " << _day << endl;
    }
	int GetMonthDay(int year, int month);

	bool operator==(const Date& d);
	bool operator>(const Date& d);

	Date& operator+=(int x);
	Date operator+(int x);
	Date& operator-=(int x);
	Date operator-(int x);
	
	Date& operator++();
	Date operator++(int);

    int operator-(const Date& d);

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

1. operator==、operator>

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

bool Date::operator>(const Date& d)
{
	if (_year > d._year)
	{
		return true;
	}
	else if (_year == d._year && _month > d._month)
	{
		return true;
	}
	else if (_year == d._year && _month == d._month && _day > d._day)
	{
		return true;
	}

	return false;
}

!=、>=、< 等运算符，皆可都通过对==和>进行逻辑运算复用实现。

2. operator+= (/+)、operator-= (/-)

int Date::GetMonthDay(int year, int month)
{
	assert(month >= 1 && month <= 12);
	int Day[13] = { 0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };
    // 判断闰二月
	if (month == 2 && (year % 400 == 0 || (year % 4 == 0 && year % 100 != 0)))
	{
		return 29;
	}
	return Day[month];
}

Date& Date::operator+=(int x)
{
	if (x < 0)
	{
		return *this -= (-x);
	}

	_day += x;
	while (_day > GetMonthDay(_year, _month))
	{
		_day -= GetMonthDay(_year, _month);
		_month++;
		if (_month == 13)
		{
			_year++;
			_month = 1;
		}
	}
	return *this;
}

Date Date::operator+(int x)
{
	Date tmp(*this);
	tmp += x;
	return tmp;
}

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

	return *this;
}

Date Date::operator-(int x)
{
	Date tmp(*this);
	tmp -= x;
	return tmp;
}

此处有一种重点，以**+=—>+** 和 **+—>+=**进行对比举例。(暂不考虑 x < 0 的情况)

Date Date::operator+(int x)
{
    Date tmp(*this);
    
    tmp._day += x;
	while (tmp._day > GetMonthDay(tmp._year, tmp._month))
	{
		tmp._day -= GetMonthDay(tmp._year, tmp._month);
		tmp._month++;
		if (tmp._month == 13)
		{
			tmp._year++;
			tmp._month = 1;
		}
	}
	return tmp;
}

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

+=—>+：

// += 0次对象拷贝
// + 2次对象拷贝

+—>+=：

// + 2次对象拷贝
// += 3次对象拷贝

使用前者的复用方法，效率更高。

3. operator++()、operator++(int)

Date& Date::operator++()// 后置++
{
	*this += 1;
	return *this;
}

Date Date::operator++(int)// 前置++
{
	Date tmp(*this);
	*this += 1;
	return tmp;
}

4. int operator-(const Date& d)

int Date::operator-(const Date& d)
{
	int flag = 1;// 假设左大右小
	Date max(*this);
	Date min(d);

	// 左小右大进行调整
	if (max < min)
	{
		max = d;
		min = *this;
		flag = -1;
	}

	int n = 0;
	while (min != max)
	{
		min++;
		n++;
	}
	return n * flag;
}