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）

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   d1:2022,1,14d = ++d1;//d:2022,1,15   d1:2022,1,15return 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;
}