CODEWITHSUNDEEP
c++c++14
Shreesha Aithal
Shreesha Aithal

Reputation: 11

How to create a chrono::time_point object for a given calender date without using std::tm and mktime()

I want to create a chrono::time_point object for a given date for example : 2020-03-06 22:48:01 where the format is YYYY-MM-DD HH:MM:SS . I do not want to use mktime() as it is not safe. Also I do not want to use any third party library such as date.

Upvotes: 0

Views: 416

Answers (2)

Howard Hinnant
Howard Hinnant

Reputation: 218770

For C++11 and C++14, none of the chrono clocks have a specified epoch. However one of them: system_clock has an epoch that is the same among the three most popular implementations, gcc, llvm and VS. This epoch is 1970-01-01 00:00:00 UTC, excluding leap seconds. This is also known as Unix Time.

While not guaranteed by the C++11/14 standards, you can program to this de-facto standard for std::chrono::system_clock. But there are no portable epochs for steady_clock or high_resolution_clock.

Since you do not want to involve std::tm, you will have to write your own code to convert YYYY-MM-DD HH:MM:SS to a count of seconds since the Unix Time epoch. If YYYY-MM-DD HH:MM:SS is a local time, you will also have to take into account the UTC offset, and the possibility that the UTC offset itself is a function of time for many time zones (i.e. daylight saving time). If YYYY-MM-DD HH:MM:SS is in string form, you will also need to parse these characters into integral types.

Here are helpful formulas for converting the YYYY-MM-DD portion of your timestamp into a count of days since the Unix Time epoch.

Slight change in my requirement is that, I am okay to use std::tm but not mktime(). Is there anyway to create std::chrono::time_point object?

If you are starting with data in a stream you could use std::get_time from <iomanip> to get the data into a tm. Then there is a non-standard but popular timegm() that converts the tm to a time_t, neglecting timezones (the original timestamp is interpreted as UTC). And then there is a from_time_t static member function of system_clock that will convert the time_t to system_clock::time_point.

Hmm... but timegm() probably qualifies as a 3rd party lib...

Upvotes: 1

Nicol Bolas
Nicol Bolas

Reputation: 473447

All time points are relative to the epoch of the clock that generated them. But in C++14 (and C++17), clock epochs are implementation-specified. Every implementation's clocks have a time point, but there's no way to know what they are. And it makes no sense to ask, since times are all ultimately relative to some fixed date, so the epoch would have to itself be expressed relative to some other date.

So you must either use std::mktime or a 3rd party library. Or you could just write all the code yourself, but that code wouldn't be able to generate a time point from any particular clock unless it were coded in an implementation-specific fashion.

Upvotes: 0

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