On Thu, Jan 28, 2021 at 4:23 AM Jonathan Wakely via SG10 < [email protected]> wrote:
> > > On Thu, 28 Jan 2021 at 10:18, Jonathan Wakely <[email protected]> wrote: > >> >> >> On Thu, 28 Jan 2021 at 10:15, Ville Voutilainen < >> [email protected]> wrote: >> >>> On Thu, 28 Jan 2021 at 12:13, Jonathan Wakely via SG10 >>> <[email protected]> wrote: >>> > >>> > >>> > >>> > On Wed, 27 Jan 2021 at 20:54, Barry Revzin via Liaison < >>> [email protected]> 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. > > -- > SG10 mailing list > [email protected] > https://lists.isocpp.org/mailman/listinfo.cgi/sg10 So from the compilers I have available at my disposal (i.e. on compiler explorer): * gcc trunk, clang trunk, and icc 21.1.9 have __VA_OPT__ in C++20 modes * gcc even has it in C++17 * msvc 19.28 doesn't have it yet (that's the latest version up there) If you try to use #ifdef __VA_OPT__: * gcc gives you a warning, but with no label, so you can't disable it. If you compile with -Werror, you're done. * clang gives you a warning that you *can* disable * icc gives an error * msvc is fine (since it doesn't support it yet, so no special rejection) So yeah, #ifdef __VA_OPT__ probably not going to work out as a thing. But for __cpp_va_opt, is MSVC imminently going to add support for it? It may be not very informative (true means yes, false means try something else?). Barry
-- SG10 mailing list [email protected] https://lists.isocpp.org/mailman/listinfo.cgi/sg10
