On Thu, Oct 31, 2019 at 9:28 PM Edward Diener <eldlistmailingz@xxxxxxxxxxxxxx> wrote: > > On 10/31/2019 4:20 PM, Segher Boessenkool wrote: > > On Thu, Oct 31, 2019 at 02:50:07PM -0400, Edward Diener wrote: > >> On 10/31/2019 1:14 PM, Segher Boessenkool wrote: > >>> On Thu, Oct 31, 2019 at 12:42:54PM -0400, Edward Diener wrote: > >>>> Given: > >>>> > >>>> #define NO_DATA > >>>> #define TRY_VA_OPT(...) __VA_OPT__ (0) 1 > >>>> > >>>> TRY_VA_OPT() -> expands to 1 as expected > >>>> TRY_VA_OPT(NO_DATA) -> expands to 0 1 which is not expected > >>>> > >>>> when compiled with gcc-9.2 with -std=c++2a. > >>> > >>> Why is that not expected? The variadic macro TRY_VA_OPT does get tokens > >>> in its variable argument (namely, NO_DATA), so __VA_OPT__ expands to its > >>> argument (which is 0). > >>> > >>> https://gcc.gnu.org/onlinedocs/cpp/Variadic-Macros.html > >> > >> The wording of 15.6.1 paragraph 3 is confusing to me. I had thought that > >> the arguments to the variadic ... parameter were completely macro > >> expanded before considering whether the __VA_OPT__ ( pp-tokens ) would > >> expand to 'pp_tokens' or a single placemarker token. Evidently you are > >> saying that the correct interpretation with the __VA_OPT__ construct is > >> that the variadic ... arguments are not macro expanded before > >> consideration of the __VA_OPT__ construct processing . This seems to me > >> very odd because the arguments to the variadic ... parameter are always > >> completely macro expanded before being replaced by any __VA_ARGS__ > >> parameter in the replacement list. I wonder why the C++ standard > >> committee decided to treat __VA_OPT__ differently from __VA_ARGS__ in > >> this regard ? > > > > I don't know. I don't know if the GCC implementation is correct, either. > > Clang in its C++20 mode treats __VA_OPT__ as I thought it should be, > where the determination of whether __VA_OPT__ ( pp-tokens ) expands to a > placeholder token or pp-tokens is determined by whether the arguments > for the variadic data parameter ... are empty or not after the arguments > have been fully macro replaced. In other words clang expands to 1 in > both my examples above. #define T(a) #a T(NO_DATA) Expands to "NO_DATA", which means that it is: T(NO_DATA) -> #NO_DATA -> "NO_DATA" and not: T(NO_DATA) -> T() -> <error, no argument> Which explains why __VA_OPT__ behaves this way. According to godbolt T(NO_DATA) also expands to "NO_DATA", which would mean that difference between compilers boils down to checking if list is empty - gcc checks it before macro expansion, and clang after expansion? -- Jędrzej Dudkiewicz I really hate this damn machine, I wish that they would sell it. It never does just what I want, but only what I tell it.
