什么时候std :: chrono epoch？ [英] When is std::chrono epoch?

问题描述

std :: chrono :: time_point :: time_since_epoch（）返回持续时间，引用一些 time_point 过去。什么时候这样的 time_point ？它取决于C ++实现或者它是由C ++标准定义的？或者，它是一个事实上的标准，将时代设置为1970年1月1日UTC？

std::chrono::time_point::time_since_epoch() returns a duration, referred to some time_point in the past. When is such a time_point? It depends on the C++ implementation or it's defined by the C++ standard? Or it is a de facto standard to set the epoch to 1 January 1970 UTC?

推荐答案

clock 指向 time_point ，并执行 clock 。该标准指定了三个不同的时钟：

It is a function of both the specific clock the time_point refers to, and the implementation of that clock. The standard specifies three different clocks:

• system_clock


• steady_clock


• high_resolution_clock

• system_clock
• steady_clock
• high_resolution_clock

标准没有指定任何时钟的纪元。

And the standard does not specify the epoch for any of these clocks.

程序员

有一个事实上的（非官方的）标准 std :: chrono :: system_clock :: time_point 有与 Unix时间一致的纪元。这是定义为自世界协调时间（UTC）（1970年1月1日星期四）00:00:00起，不计算闰秒的时间长度。

There is a de-facto (unofficial) standard that std::chrono::system_clock::time_point has an epoch consistent with Unix Time. This is defined as the time duration that has elapsed since 00:00:00 Coordinated Universal Time (UTC), Thursday, 1 January 1970, not counting leap seconds.

Fwiw ，这里是一个日期/时间库，利用了这个事实上的标准。

Fwiw, here is a date/time library which takes advantage of this de-facto standard.

其他两个std指定的时钟没有事实上的标准。此外， high_resolution_clock 可以是 system_clock 或 steady_clock 。

There is no de-facto standard for the other two std-specified clocks. Additionally high_resolution_clock is permitted to be a type alias for either system_clock or steady_clock.

在OS X上， high_resolution_clock 是 / code>， steady_clock 是计算机启动后的毫微秒数（与UTC无关）。

On OS X, high_resolution_clock is a type alias for steady_clock, and steady_clock is a count of nanoseconds since the computer booted (no relationship whatsoever to UTC).

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