`VA_OPT`

Emptiness testing for C++ Preprocessor macro arguments

Boost Software License - Version 1.0 - August 17th, 2003

Permission is hereby granted, free of charge, to any person or organization
obtaining a copy of the software and accompanying documentation covered by
this license (the "Software") to use, reproduce, display, distribute,
execute, and transmit the Software, and to prepare derivative works of the
Software, and to permit third-parties to whom the Software is furnished to
do so, all subject to the following:

The copyright notices in the Software and this entire statement, including
the above license grant, this restriction and the following disclaimer,
must be included in all copies of the Software, in whole or in part, and
all derivative works of the Software, unless such copies or derivative
works are solely in the form of machine-executable object code generated by
a source language processor.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE, TITLE AND NON-INFRINGEMENT. IN NO EVENT
SHALL THE COPYRIGHT HOLDERS OR ANYONE DISTRIBUTING THE SOFTWARE BE LIABLE
FOR ANY DAMAGES OR OTHER LIABILITY, WHETHER IN CONTRACT, TORT OR OTHERWISE,
ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
DEALINGS IN THE SOFTWARE.

Also at boost.org and accompanying file LICENSE_1_0.txt

Linux Travis	Windows Appveyor
gcc-9, clang-7	MSVC 19.21.27702.2

`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...

#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' to X
  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.
IFE(...)
- IF ... ARGS is empty
  THEN expand trailing 'paren expression' (X) to X
  ELSE consume the trailing paren expression.
IFNE(...)(N,E...)
- IF ... ARGS are not empty
  THEN expands to N
  ELSE expands to E...

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=++2a but,
if an earlier -std flag 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

willwray/VA_OPT