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Re: [wg14/wg21 liaison] How to handle conflicting 'unreachable' definitions

From: Jens Maurer <jens.maurer_at_[hidden]>
Date: Thu, 4 Apr 2024 17:46:33 +0200
On 04/04/2024 16.30, Ville Voutilainen via Liaison wrote:
> On Thu, 4 Apr 2024 at 16:15, Aaron Ballman via Liaison
> <liaison_at_[hidden]> wrote:
>> C23 adds a macro named `unreachable` to <stddef.h>. C++23 adds a
>> function named `std::unreachable` to <utility>. What should the
>> behavior of this code be when compiled in C++23?
>> #include <stddef.h>
>> #ifndef unreachable
>> #error "oh no, my macro!"
>> #endif
>> [support.c.headers.general]p1
>> (https://eel.is/c++draft/support.c.headers#general-1) says:
>> For compatibility with the C standard library, the C++ standard
>> library provides the C headers shown in Table 40. The intended use of
>> these headers is for interoperability only. It is possible that C++
>> source files need to include one of these headers in order to be valid
>> ISO C. Source files that are not intended to also be valid ISO C
>> should not use any of the C headers.
>> I read this as saying that including a C header in a C++
>> implementation should provide all the interfaces from the C header
>> unless the C++ standard says something else. Since C++23 isn't based
>> on top of C23 and thus says nothing about the `unreachable` macro in
>> stddef.h, I think this implies that the code should compile.
>> (Similarly, including stdbit.h should be fine if the header exists, or
>> including string.h and using strdup or memccpy should work, and so on
>> for all the newly introduced C23 library functionality.) I think this
>> matches user expectations and the intent of the standard, but the
>> situation with unreachable is a bit odd and the question came up
>> during a code review for Clang which removes the macro from stddef.h
>> in C++ mode because of the potential for conflict with the function
>> declared in <utility>.
>> I have two concrete questions:
>> 1) Before WG21 has rebased on top of the latest C, are C++
>> implementations expected to strip out all interfaces from C standard
>> library headers that are not in the version of C which C++ has based
>> the library on?
>> 2) If the answer to #1 is "no, C++ implementations are expected to
>> provide the C interfaces present unless otherwise stated", is
>> unreachable likely to be special when C++ rebases on top of C23?
> My spec-squinting suggests that the answer to (1) is yes. The
> normative reference to C
> comes with a date, it refers to C18. I therefore would not expect
> C++23 to bring in C23 entities.


> That is also my technical preference; it's ostensibly so that that
> particular version of C
> has been figured out how to make it work with that particular version
> of C++, and newer
> versions of C simply might not work.

If C_next adds a few more stand-alone (non-type-generic) functions,
it wouldn't seriously hurt to make these available in C++, from a
technical perspective. From a conformance perspective, however,
headers should be kept as-specified.


Received on 2024-04-04 15:46:37