Subject: Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)
From: Jonathan Wakely (cxx_at_[hidden])
Date: 2021-01-28 04:18:31
On Thu, 28 Jan 2021 at 10:15, Ville Voutilainen <ville.voutilainen_at_[hidden]>
> 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:
> >> 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.
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