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Subject: Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)
From: Jonathan Wakely (cxx_at_[hidden])
Date: 2021-01-28 04:22:32


On Thu, 28 Jan 2021 at 10:18, Jonathan Wakely <cxx_at_[hidden]> wrote:

>
>
> On Thu, 28 Jan 2021 at 10:15, Ville Voutilainen <
> ville.voutilainen_at_[hidden]> wrote:
>
>> On Thu, 28 Jan 2021 at 12:13, Jonathan Wakely via SG10
>> <sg10_at_[hidden]> wrote:
>> >
>> >
>> >
>> > On Wed, 27 Jan 2021 at 20:54, Barry Revzin via Liaison <
>> liaison_at_[hidden]> wrote:
>> >>
>> >> Eric Niebler actually asked this on StackOverflow a few years ago:
>> https://stackoverflow.com/q/48045470/2069064
>> >>
>> >> The accepted answer there is:
>> >>
>> >> #define PP_THIRD_ARG(a,b,c,...) c
>> >> #define VA_OPT_SUPPORTED_I(...) PP_THIRD_ARG(__VA_OPT__(,),true,false,)
>> >> #define VA_OPT_SUPPORTED VA_OPT_SUPPORTED_I(?)
>> >
>> >
>> > Who is going to remember that without having to look it up though?
>>
>> Is it going to be written so often that that becomes a major problem?
>>
>> > The #ifdef __VA_OPT__ solution was my first thought, it's unfortunate
>> we forbid it. If we can't have that then I think we do need a feature test
>> macro. The voodoo above will make most developers wish they were using Rust.
>>
>> If they're using VA_OPT, the cause is already lost.
>>
>
> And even if we add a feature test macro now (or allow #ifdef __VA_OPT__)
> there are still compilers that will reject it with an error (e.g. with
> -pedantic-errors in pre-C++20 modes). So maybe the ship has sailed and
> support this feature is already "untestable". You just have to know if your
> code can use it or not.
>

Actually that would be true for the #ifdef __VA_OPT__ solution (we could
say it's allowed, but if you try to use it on today's shipping compilers,
it's ill-formed) but if we add a new macro you can be conservative:

#ifdef __cpp_va_opt
// Use it.
#else
// Maybe it's actually available, but we can't be sure.
// Assume it isn't.
#endif

So the question is whether to spell it __STDC_VA_OPT for WG14 compat, or
__cpp_va_opt.



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