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<stddef.h>
etc.)Section: 17.14.1 [support.c.headers.general] Status: New Submitter: Jiang An Opened: 2023-07-06 Last modified: 2024-02-22
Priority: 3
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Discussion:
Several feature-test macros are available in C++ versions of C headers (e.g. __cpp_lib_byte
is available in
<cstddef>
). However, the current standard wording doesn't seem to make the distinction between C++ library
feature-test macros and macros from C, so it's not very clear that whether <stddef.h>
and its friends are
required to provide __cpp_lib_*
macros in C++.
name.h
should provide macros shown in the synopsis of its corresponding cname
(as required in C), but should not be required to provide C++ library feature-test macros because such requirement would
generally require implementations of the C standard library to change. I think we should make clarification in
17.14.1 [support.c.headers.general].
[2024-02-22; Reflector poll]
Set priority to 3 after reflector poll in July 2023.
[Jonathan commented]
The issue says that "such requirement would generally require implementations
of the C standard library to change" but this is not true.
A conforming C++ library already needs a C++-aware <stdlib.h>
and <math.h>
, and has done so since C++98! We should be cautious about expanding the set of C headers that need to be C++-aware, but if we require <stdlib.h>
to define these functions:
constexpr long abs(long);
constexpr long long abs(long long);
then it can also define __cpp_lib_constexpr_math
.
We have these <cxxx>
headers providing feature test macros:
<cmath>: __cpp_lib_constexpr_cmath __cpp_lib_hypot __cpp_lib_interpolate __cpp_lib_math_special_functions <cstddef>: __cpp_lib_byte <cstdlib>: __cpp_lib_constexpr_cmath
For <stdlib.h>
and <math.h>
the implementation already needs a C++-specific version of the header,
because ::abs
is required to be overloaded (and constexpr) in
<stdlib.h>
and <math.h>
(and all the math functions have to be constexpr even if you include
<math.h>
).
So I see no issue here: the <xxx.h>
headers should obviously
define the same macros as the <cxxx>
headers.
We do not require ::byte
to be in <stddef.h>
,
so maybe we should not require the macro there either.
Except that std::byte
is permitted to be in
<stddef.h>
,
just not in the global namespace (see LWG 3883(i)).
So maybe SD-6 should simply clarify that the macro indicates the presence
of std::byte
, not ::byte
,
and whether std::byte
and its macro are defined
by <stddef.h>
is unspecified.
Also related to LWG 3484(i).
[Jens commented]
It seems 17.14 [support.c.headers] is silent on which macros
from the <cxxx>
headers are made available via the <xxx.h>
headers,
given that 17.14.7 [support.c.headers.other] talks about names placed in
the standard library namespace (macros don't fit that description).
[Ben noted some additional macros]
#define __cpp_lib_freestanding_cstdlib new-val // freestanding, also in <cstdlib>, <cmath>
#define __cpp_lib_freestanding_cstring new-val // freestanding, also in <cstring>
#define __cpp_lib_freestanding_cwchar new-val // freestanding, also in <cwchar>
#define __cpp_lib_freestanding_errc new-val // freestanding, also in <cerrno>, <system_error>
Proposed resolution: