在C ++中比较日期的方式是什么 [英] Whats the way to compare dates in c++

问题描述

在c ++中比较日期的最好方法是什么？将日期转换成具有日期，月份和年份变量的结构
，即如何检查自事件以来已经过去了30天等等。我个人发现，当比较同一年中的2个日期时，我会使用一个数组来将两个日期都设为天-日期，该数组保存一年中所有月份的天数，然后将日期转换为天并进行我需要的计算。有没有更简单的方法可以比较日期？

Whats the best way to compare dates in c++ which are made into a structure that has a day,month and year variable i.e how to check that 30 days have passed since a event and so on. I personally found that when comparing 2 dates in the same year I make both dates into "days-dates" using a array that holds the day count of all the months of the year and then convert the dates into days and do the calculations I need. Is there a easier way to compare the dates ?

推荐答案

这是一个很好的用于处理日期的C ++ 11/14库 ¹ 。它允许您具有 {年，月，日} 结构（称为 date :: year_month_day ）和 {day-of-days} 结构（称为 sys_days ）。它还允许在这两种结构之间轻松有效地进行转换。当然也有比较运算符。

Here is a very good C++11/14 library for handling dates¹. It allows you to have {year, month, day} structures (called date::year_month_day), and {count-of-days} structures (called sys_days). It also allows easy and efficient conversions between these two structures. And naturally there are comparison operators.

整个库本质上是< chrono> 到日历类型的扩展。

The entire library is essentially an extension of <chrono> into calendar types.

视频介绍在这里找到：

https://www.youtube.com/watch?v=tzyGjOm8AKo

此处有很多示例代码。

此处详细说明了在 {年，月，日} 结构和 {count- 结构：

Here is a detailed explanation of the underlying algorithms to convert between the {year, month, day} structure and the {count-of-days} structure:

http://howardhinnant.github.io/date_algorithms.html

以下是两个字段的日期创建和比较示例（ year_month_day ）和序列号（ sys_day s ）数据结构：

Here are some date creation and comparison examples for both field (year_month_day) and serial (sys_days) data structures:

#include "date.h"

int
main()
{
    using namespace date::literals;

    // create and compare {year, month, day} structures
    constexpr auto ymd1 = 2017_y/jan/21;
    constexpr auto ymd2 = ymd1 + date::months{15};
    static_assert(ymd2 > ymd1, "ymd2 is 15 months after ymd1");
    static_assert(ymd2 == 2018_y/apr/21, "ymd2 is 2018-04-21");

    // create and compare {count_of_days} structures
    constexpr date::sys_days sd2 = ymd2;
    static_assert(sd2 == ymd2, "sd2 is the same day as ymd2");
    static_assert(sd2.time_since_epoch().count() == 17642, "sd2 is day 17642");
    constexpr date::sys_days sd1 = sd2 - date::days{465};
    static_assert(sd1 < sd2, "sd1 is 465 days before sd2");
    static_assert(sd1.time_since_epoch().count() == 17177, "sd1 is day 17177");
    static_assert(sd1 == 2017_y/jan/11, "sd1 is 2017-01-11");
}

constexpr / static_assert 需要完全符合要求的C ++ 14编译器。对于C ++ 11，删除 constexpr 并将 static_assert 更改为 assert （并消除 static_assert 消息）。

The constexpr / static_assert requires a fully conforming C++14 compiler. For C++11, remove the constexpr and change static_assert to assert (and eliminate the static_assert message).

date :: sys_days 是 chrono :: time_point ：

time_point<system_clock, duration<int, ratio<86400>>

上面的示例代码只需要 date.h ，并且没有其他C ++源文件（无需安装）。在同一github位置上也有一个时区库，但这确实需要一些

The above example code requires only "date.h", and no other C++ source files (no installation). There is also available a timezone library at this same github location, but that does require some installation.

¹ 我是该库的主要作者。

¹ I am the principal author of this library.

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