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Add formatters for chrono::time_point<system_clock> #1837

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vitaut merged 3 commits into from
Aug 28, 2020
Merged

Add formatters for chrono::time_point<system_clock> #1837

vitaut merged 3 commits into from
Aug 28, 2020

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@adamburgess
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@adamburgess adamburgess commented Aug 25, 2020 •
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Fixes #1819

Makes dates a little easier to format:

auto the_time = std::chrono::system_clock::now();
fmt::print("{}", the_time); // the_time is converted using fmt::localtime, which then uses the normal tm formatter
fmt::print("UTC {}", fmt::gmtime(the_time)); // prints UTC time using a new overload on gmtime/localtime

I wanted to test localtime so I did that using setenv/tzset, though I'm not sure if that's the best way.

C++20 does include a formatter for sys_time -- in #985 it was discussed that printing a time_point was not in the standard library, but it seems with time (hah) it has now been added.

I agree that my contributions are licensed under the {fmt} license, and agree to future changes to the licensing.

vitaut
vitaut requested changes Aug 26, 2020
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Thanks for the PR. Mostly looks good but the unit tests shouldn't manipulate environment variables. Instead I suggest comparing the output to the result of formatting time_t with system functions.

@adamburgess
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adamburgess commented Aug 28, 2020

My thoughts were to confirm that passing a time_point would be formatting with the local system time instead of UTC, and that's hard to test without changing the timezone (also: travis etc would be using UTC as well). It's probably up to the stdlib to make sure localtime works ;)

The test now does this: check if time_point formatting is equal to strftime(localtime(to_time_t(now())))

@vitaut vitaut merged commit f39e6fb into fmtlib:master Aug 28, 2020
@LarsGullik
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LarsGullik commented Oct 5, 2020

It is a bit bad that the formatters for chrono clocks are not compatible with C++20. In particular that %S in C++20 also prints out subseconds. Going via tm, truncates this to seconds.

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vitaut commented Oct 5, 2020

A PR to improve compatibility with C++20 is welcome.

@themightyoarfish
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themightyoarfish commented Apr 14, 2025

I might be missing something here, but for me this

auto the_time = std::chrono::system_clock::now();
fmt::print("{}", the_time);

does not compile. The FMT_VERSION is 100201 …

include/fmt/core.h:1600:63: error: implicit instantiation of undefined template 'fmt::detail::type_i
s_unformattable_for<std::chrono::time_point<std::chrono::system_clock>, char>'
 1600 |     type_is_unformattable_for<T, typename Context::char_type> _;

…

include/fmt/core.h:1604:7: error: static assertion failed due to requirement 'formattable': Cannot format an argument. To make type T formattable provide a formatter<T> specialization: https://fmt.dev/latest/api.html#udt
 1604 |       formattable,
      |       ^~~~~~~~~~~

The build is with c++20, what could I be doing wrong?

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vitaut commented Apr 14, 2025

@themightyoarfish Seems to work: https://www.godbolt.org/z/j5rx8MPMv. Make sure to include correct headers.

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themightyoarfish commented Apr 15, 2025

Must be something else with my build, then, the header is included.

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Feature Request: Print std::chrono::time_point

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@adamburgess @LarsGullik @vitaut @themightyoarfish