+1 From: Liaison <[email protected]> On Behalf Of Thomas Köppe via Liaison Sent: Monday, January 25, 2021 5:21 PM To: Richard Smith <[email protected]> Cc: Thomas Köppe <[email protected]>; [email protected]; [email protected] Subject: [EXTERNAL] Re: [wg14/wg21 liaison] Feature test macro for P0306 (__VA_OPT__)
The proposed solution seems elegant to me. If you say that's implementable, I'd be in favour of that. I have very regrettably not sent an updated paper to WG14 for this feature, it's one of the things on my list. I shall prioritize getting it done! On Mon, 25 Jan 2021 at 23:02, Richard Smith <[email protected]<mailto:[email protected]>> wrote: [+WG14 liaison list] On Mon, Jan 25, 2021 at 2:59 PM Richard Smith <[email protected]<mailto:[email protected]>> wrote: Hi, SD-FeatureTest doesn't mention P0306. I think a feature test macro would be useful here, to allow projects to incrementally adopt the new functionality. Example: #if __cpp_va_opt #define FOO(a, ...) f(a __VA_OPT__(,) __VA_ARGS__) #elif __GNUC__ #define FOO(a, ...) f(a , ## __VA_ARGS__) #else // Hopefully we get the MSVC implicit comma deletion behavior. #define FOO(a, ...) f(a , __VA_ARGS__) #endif Note that the __GNUC__ extension is enabled by default, even in conforming modes, in GCC, Clang, and ICC. However, after the adoption of P0306, it's no longer a conforming extension, so presumably it will be phased out at some point, and uses of the feature-test macro, such as in the above example, are going to become necessary. Regarding the name of the macro: this functionality is shared with C, and as such, a __cpp_* name is probably not ideal. However, there's another interesting option: we could use #ifdef __VA_OPT__ ... as the feature test mechanism. This doesn't appear to conflict with anything else, and is in line with our feature test mechanism for __has_cpp_attribute and __has_include. So that's my suggestion: #ifdef/#ifndef/defined should treat `__VA_OPT__` as if it were the name of a defined macro. Thoughts? Richard
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