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Subject: Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)
From: Ville Voutilainen (ville.voutilainen_at_[hidden])
Date: 2021-01-28 04:17:28


On Thu, 28 Jan 2021 at 12:15, 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.

Anyhoo, I forgot to say that adding the feature test macro is fine by
me. The voodoo above is a bit much, even
for those who have to fiddle with VA_OPT and its conditional availability.


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