chrono :: year对象如何构造？ [英] How is a chrono::year Object Constructed?

问题描述

我刚刚注意到c ++ 20 将具有 chrono ::年 。它的构造函数采用 int 的范围为 [-32767，32767] ，但是我不清楚此参数表示什么。

I just noticed that c++20 is going to have chrono::year. It's constructor takes in an int in the range: [-32767, 32767], however I am unclear what this argument represents.

• 这是否与 tm_year 的1900年起源？


• 或者 time_t 的1970年起源？


• 或者它在 Anno Domini 中？起源为0？ li>
  

• Would this be consistent with tm_year's 1900 origin?
• Or perhaps time_t's 1970 origin?
• Or perhaps it's in Anno Domini with a 0 origin?

编辑：

这对于理解< a href = https://en.cppreference.com/w/cpp/chrono/year/is_leap rel = nofollow noreferrer> is_leap 函数 chrono :: year 优惠。没有起源，目前尚不清楚是哪一年。

This is key to the understanding of what is meant by the is_leap function chrono::year offers. Without an origin it's unclear what year is represented here.

推荐答案

在25.8.1 [time.cal.general]中：

In 25.8.1 [time.cal.general]:

25.8中的类型描述了民用（格里高利历）日历及其与 sys_days 和 local_days 。

关于此措辞的意图是具有挑战性的是要为公历建模（就像目前通过C API的C ++一样），而不会冒犯遵循其他日历的人。

The wording on this was (is) challenging as the intent is to model the Gregorian calendar (as does C++ currently via the C API) without offending those who follow other calendars.

我现在也注意到了 proleptic ，应该将其添加到战略位置。

I also am just now noting that the word "proleptic" is missing from the spec, and should probably be added in a strategic spot.

要直接回答这个问题，与 std :: chrono :: year 相关的积分是Anno Domini参考，定义如下由教皇格里高利（1582年）创作，但时间上来回向前和向后。在我撰写本文时，年份是 2018y 。

To directly answer the question, the integral associated with std::chrono::year is the Anno Domini reference, as defined by Pope Gregory in 1582, but running both backwards and forwards in time. As I write this, the year is 2018y.

然后（回答下面的乔纳森·米的评论），该程序是：

And (answering Jonathan Mee's comment below), this program:

#include <chrono>
#include <iostream>

int
main()
{
    using namespace std;
    using namespace std::chrono;
    const auto foo = 2018y;
    cout << int{foo} << '\n';
}

输出：

2018

您可以试用的实时演示，条件是 date.h 示例实现将内容放入命名空间日期而不是命名空间std :: chrono 。

Live demo that you can experiment with with the proviso that the "date.h" example implementation puts things in namespace date instead of namespace std::chrono.

我还应注意，该软件允许用户编写的日历与 std互操作:: chrono 系统。这是朱利安日历的示例。 此处。

I should also note that this software allows for user-written calendars to interoperate with the std::chrono system. Here is an example of the Julian calendar. There are a couple more examples here.

最后，简要说明一下为什么将本年度表示为 year {2018} 的理由（Anno Domini） ，而不是 year {48} （ time_t 的1970年起源）或 year {118} （ tm_year 的1900年起源）：

Finally, a brief note on the rationale as to why the current year is represented as year{2018} (Anno Domini), as opposed to year{48} (time_t's 1970 origin), or year{118} (tm_year's 1900 origin):

在电影中使用时，这种哲学是歇斯底里的。但是在软件设计中使用时会很累。该库尝试执行预期的操作。

This philosophy is hysterical when used in movies. But gets tiresome when used in software design. This library tries to do the expected.

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