C++9--前置++和后置++重载，const,日期类的实现（对前几篇知识点的应用）

目录

1.前置++和后置++重载

2.const成员

3.日期类的实现

---

1.前置++和后置++重载

#include<iostream>
using namespace std;

class Date
{
public:
	Date(int year = 2024, int month = 1, int day = 1)
	{
		_year = year;
		_month = month;
		_day = day;
	}
	//前置++：返回+1之后的结果
	//注意：this指向的对象函数结束后不会销毁，故以引用方式返回提高效率
	Date& operator++()
	{
		_day += 1;
		return *this;
	}
	//后置++：
	//前置++和后置++都是一元运算符，为了让前置++与后置++形成能正确的重载
	//C++规定：后置++重载时多增加一个int类型的参数，但调用函数时该参数不用传递，编译器自动传递
	//注意：后置++是先使用后+1，因此需要返回+1之前的旧值，故需在是实现时需要先将this保存一份
	//，然后给this+1
	//而temp是临时对象，因此只能以值的方式返回，不能返回引用
	Date operator++(int)
	{
		Date temp = *this;
		_day += 1;
		return temp;
	}
private:
	int _year;
	int _month;
	int _day;
};

int main()
{
	Date d;
	Date d1(2022, 1, 13);
	d = d1++;//d:2022,1,13   d1:2022,1,14
	d = ++d1;//d:2022,1,15   d1:2022,1,15
	return 0;
}

2.const成员

将const修饰的"成员函数"称之为const成员函数 ，const修饰类成员函数，实际修饰该成员函数隐含的this指针 ，表明在该成员函数中不能对类的任何成员进行修改。

在日期类的实现里面具体应用

3.日期类的实现

//Date.h
#pragma once
#include<iostream>
#include<assert.h>
using namespace std;

class Date
{   //友元
	friend ostream& operator<<(ostream& out, const Date& d);
	friend istream& operator>>(istream& in, Date& d);

public:
	//全缺省构造函数
	Date(int year = 1, int month = 1, int day = 1);
	void Print()const
	{
		cout << _year << "-" << _month << "-" << _day << endl;
	}
	//拷贝构造函数
	Date(const Date& d)
	{
		_year = d._year;
		_month = d._month;
		_day = d._day;
	}

	//运算符重载
	bool operator<(const Date& x)const;
	bool operator==(const Date& x)const;
	bool operator<=(const Date& x)const;
	bool operator>(const Date& x)const;
	bool operator>=(const Date& x)const;
	bool operator!=(const Date& x)const;

	//获取某年某月的天数
	int GetMonthDay(int year, int month);

	//日期+=天数
	Date& operator+=(int day);
	//日期+天数
	Date operator+(int day)const;
	//日期-=天数
	Date& operator-=(int day);
	//日期-天数
	Date operator-(int day)const;
	//前置++
	Date& operator++();
	//后置++
	Date operator++(int);
	//前置--
	Date& operator--();
	//后置--
	Date operator--(int);
	//日期-日期 返回天数
	int operator-(const Date& d)const;
	//流插入不能写成成员函数？
	//因为Date对象默认占用第一个参数，就是做了左操作数
	//写出来就一定是下面这个样子，不符合使用习惯
	//d1 << cout;  //d1.operator<<(cout);
	//void operator<<(ostream& out);


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

ostream& operator<<(ostream& out, const Date& d);
istream& operator>>(istream& in, Date& d);


//Date.cpp
#include"Date.h"
Date::Date(int year, int month, int day)
{
	if (month > 0 && month < 13
		&& day > 0 && day<=GetMonthDay(year,month))
	{
		_year = year;
		_month = month;
		_day = day;
	}
	else
	{
		cout << "非法日期" << endl;
		assert(false);
	}
}
bool Date::operator<(const Date& x)const
{
	if (_year < x._year)
	{
		return true;
	}
	else if (_year == x._year && _month < x._month)
	{
		return true;
	}
	else if (_year == x._year && _month == x._month && _day < x._day)
	{
		return true;
	}

	return false;
}

bool Date::operator==(const Date& x)const
{
	return _year == x._year
		&& _month == x._month
		&& _day == x._day;
}
bool Date::operator<=(const Date& x)const
{
	return *this < x || *this == x;
}

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

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

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

int Date::GetMonthDay(int year, int month)
{
	static int daysArr[13] = { 0,31,28,31,30,31,30,31,31,30,31,30,31 };
	if (month == 2 && ((year % 4 == 0 && year && 100 != 0) || (year % 400 == 0)))
	{
		return 29;
	}
	else
	{
		return daysArr[month];
	}
}

Date& Date::operator+=(int day)
{
	if (day < 0)
	{
		return *this -= -day;
	}
	_day += day;
	while (_day > GetMonthDay(_year, _month))
	{
		_day -= GetMonthDay(_year, _month);
		++_month;
		if (_month == 13)
		{
			_month = 1;
			++_year;
		}
	}
	return *this;
}

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

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

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

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

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

Date Date::operator++(int)
{
	Date tmp = *this;
	*this += 1;

	return tmp;
}

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

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

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

	return n * flag;
}

ostream& operator<<(ostream& out, const Date& d)
{
	out << d._year << "年" << d._month << "月" << d._day << "日" << endl;
	return out;
}

istream& operator>>(istream& in, Date& d)
{
	int year, month, day;
	in >> year >> month >> day;
	
	if (month > 0 && month < 13
		&& day>0 && day <= d.GetMonthDay(year, month))
	{
		d._year = year;
		d._month = month;
		d._day = day;
	}
	else
	{
		cout << "非法日期" << endl;
		assert(false);
	}
	return in;
}