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libstdc++: Use const_cast to workaround tm_zone being non-const

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Iain reported that he's seeing this on Darwin:

include/bits/chrono_io.h:914: warning: ISO C++ forbids converting a string constant to 'char*' [-Wwrite-strings]

This is because the BSD definition ot tm::tm_zone is a char* (and has
been since 1987) rather than const char* as in Glibc and POSIX.1-2024.

We can fix it by using const_cast<char*> when setting the tm_zone
member. This should be safe because libc doesn't actually write anything
to tm_zone; it's only non-const because the BSD definition predates the
addition of the const keyword to C.

For targets where it's a const char* the cast won't matter because it
will be converted back to const char* on assignment anyway.

libstdc++-v3/ChangeLog:

	* include/bits/chrono_io.h (__formatter_chrono::_M_c): Use
	const_cast when setting tm.tm_zone.

Reviewed-by: Iain Sandoe <iain@sandoe.co.uk>
This commit is contained in:
Jonathan Wakely 2025-03-26 17:45:06 +00:00 • committed by Jonathan Wakely
parent fd3bb31405
commit 16766d5a0c
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1 changed files with 4 additions and 3 deletions
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7
libstdc++-v3/include/bits/chrono_io.h
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@ -907,21 +907,22 @@ namespace __format
#ifdef _GLIBCXX_HAVE_STRUCT_TM_TM_ZONE
// POSIX.1-2024 adds tm.tm_zone which will be used for %Z.
// BSD has had tm_zone since 1987 but as char* so cast away const.
if constexpr (__is_time_point_v<_Tp>)
{
// One of sys_time, utc_time, or local_time.
if constexpr (!is_same_v<typename _Tp::clock, local_t>)
__tm.tm_zone = "UTC";
__tm.tm_zone = const_cast<char*>("UTC");
}
else if constexpr (__is_specialization_of<_Tp, __local_time_fmt>)
{
// local-time-format-t is used to provide time zone info for
// one of zoned_time, tai_time, gps_time, or local_time.
if (__t._M_abbrev)
__tm.tm_zone = __t._M_abbrev->c_str();
__tm.tm_zone = const_cast<char*>(__t._M_abbrev->c_str());
}
else
__tm.tm_zone = "UTC";
__tm.tm_zone = const_cast<char*>("UTC");
#endif
auto __d = _S_days(__t); // Either sys_days or local_days.