https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110719
Bug ID: 110719
Summary: Should chrono formatters always use std::time_put for
locale's representation?
Product: gcc
Version: 13.1.1
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: redi at gcc dot gnu.org
Target Milestone: ---
auto t = std::chrono::system_clock::now();
auto loc = std::locale::classic();
std::cout << std::format(loc, "{:%EX %OS}\n", t);
This prints something like:
14:30:46.809059031 46
The %EX output is produced by calling std::format again with a format string
based on the locale's D_FMT, which for the C locale is something like:
%H:%M:%S. And using std::format("{:%H:%M:%S}", t) prints fractional seconds for
the %S part.
The %OS output is produced by calling std::time_put::put with the %OS format
string and a struct tm with tm_sec set to the integer number of seconds. This
doesn't print the fractional part.
If chrono::parse("%EX", t) uses std::time_get then this presents a problem for
round-tripping, as the formatted output will have fractional seconds, but the
parsed input will not consume that fractional part.
Should we consistently use std::time_put for all locale-specific output?
Alternatively, we could use time_point_cast and duration_cast to round to
seconds. None of the locale-specific formats print fractional seconds.
It would be useful to profile std::format with and without std::time_get, to
see if reusing std::format performs better. If it doesn't, using std::time_put
might be simpler.
