On 10/31/2019 1:14 PM, Segher Boessenkool wrote:
Hi Edward,
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 ?
Segher
