VA_OPT
Emptiness testing for C++ Preprocessor macro arguments
Copyright © 2019 Will Wray. Distributed under the Boost Software License, V1.0
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| gcc-9, clang-7 | MSVC 19.21.27702.2 |
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VA_OPT.hpp
C++ preprocessor utilities for testing emptiness of macro arguments
and for expansion conditional on the emptiness of ARGS.
- For portability
- For handy utils that wrap
__VA_OPT__
This is mostly a 'polyfill' library providing __VA_OPT__alike functionality
where support is not detected (so either not present or not selected).
The utilities are also useful __VA_OPT__ wrappers in their own right.
__VA_OPT__ details
----
C++20 introduces a special function-like macro `__VA_OPT__`
for use only within a variadic macro's replacement list, in which;
>IF `__VA_ARGS__` is empty
THEN `__VA_OPT__(X...)` 'vanishes' - i.e. expands to nothing -
ELSE it expands to `X...`
```c++
#define VARIADIC_MACRO(...) __VA_OPT__(NOT EMPTY)
VARIADIC_MACRO() // Expands to nothing
VARIADIC_MACRO(?) // Expands to NOT EMPTY
```
----
Prior to C++20's __VA_OPT__ there is no reliable test for empty ARGS.
Such 'emptiness' testing is so useful that it shouldn't be limited to C++20.
Unfortunately, some C++20 compliant compilers hide the feature behind
a std=c++2a flag and there is also no std feature-test for __VA_OPT__.
This VA_OPT library auto-detects preprocessor support for __VA_OPT__
and, if detected, uses it as a robust way to implement:
-
IS_EMPTY(...)- 1 if the ... ARGS is empty else 0.
-
IFN(...)- IF
...ARGS are not empty
THEN a trailing 'paren expression'(X)is 'deparenthesized' toX
ELSE the trailing paren expression(X)is consumed.
IFN(ARGS)is like__VA_OPT__but with explicit (ARGS)
in place of an implicit__VA_ARGS__check. - IF
-
IFE(...)- IF
...ARGS is empty
THEN expand trailing 'paren expression'(X)toX
ELSE consume the trailing paren expression.
- IF
-
IFNE(...)(N,E...)- IF
...ARGS are not empty
THEN expands toN
ELSE expands toE...
- IF
If __VA_OPT__ support is NOT detected then the above macros
are provided using a fallback method (with some failing edge cases).
Platform note:
Current Sept 2019 __VA_OPT__ support:
- Clang 6 introduced it, enabled with
-std=c++2a
(don't know how best to enable it with earlier-std) - GCC 8 has partial support (no
#__VA_OPT__)
it is enabled by default without-std=++2abut,
if an earlier-stdflag is supplied along with-pedantic
then it warns "__VA_OPT__is not available until C++2a"
(don't know how to silence that without dropping-pedantic). - MSVC's experimental preprocessor is required to compile VA_OPT;
use /experimental:preprocessor flag in vs2017 or vs2019.
There's no__VA_OPT__support yet but conformance is improving.
(I do not intend to support non-conforming preprocessors.)
Credits
Props to pre-pro pioneers, particularly Paul Mensonides.
The fallback techniques are taken from BOOST libraries:
www.boost.org/doc/libs/1_71_0/libs/vmd
www.boost.org/doc/libs/1_71_0/libs/preprocessor
The 'emptiness' methods are adapted from BOOST_VMD_IS_EMPTY
dependent on BOOST Preprocessor's BOOST_PP_IS_BEGIN_PARENS
(adapted and exposed here as undocumented IBP 'Is Begin Parens'):
__VA_OPT__ feature detection is adapted from
https://stackoverflow.com/a/48045656/7443483