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From: Edward Diener (eldiener_at_[hidden])
Date: 2019-10-19 17:12:19

On 10/10/2019 5:17 AM, Edward Diener via Boost wrote:
> The C++20 preprocessor construct __VA_OPT__ has the additional affect of
> providing the ability to write a general purpose macro for testing
> emptiness, ie. no preprocessor tokens, in C++ macro programming that is
> 100% reliable for any preprocessor data passed to it. Previously such a
> macro testing for emptiness always had some corner cases where the test
> might fail with a preprocessor error, but in C++20 mode at least the
> test can now be completely reliable in all cases.
> So I have added two macros along with documentation, as well as a topic
> discussing emptiness, to the Boost PP library. The first macro is called
> BOOST_PP_VARIADIC_OPT, invoked with no parameters as
> BOOST_PP_VARIADIC_OPT(), which expands to 1 if the compiler is in C++20
> mode, supports variadic macros, and supports the C++ 20 __VA_OPT__
> construct, and otherwise expands to 0. The second macro, which only
> exists if BOOST_PP_VARIADIC_OPT() expands to 1, is called
> BOOST_PP_CHECK_EMPTY. This second macro is a variadic macro which
> takes a single parameter of variadic data, and expands to 1 if the data
> is empty or expands to being empty, while otherwise expanding to 0 if
> the data is not empty.

I have changed the name of the first macro from BOOST_PP_VARIADIC_OPT to
BOOST_PP_VARIADIC_HAS_OPT on the 'develop' branch and will update the
change in the 'master' branch once 'develop' tests recycle. The new name
is more descriptive. Also since I am working on a macro called
BOOST_PP_VA_OPT, which will be a Boost PP alternative to the C++20
__VA_OPT__ construct, I did not want to cause confusion in the macro names.

> Furthermore in the Boost VMD library I have updated the
> BOOST_VMD_IS_EMPTY macro to essentially invoke the BOOST_PP_CHECK_EMPTY
> macro when BOOST_PP_VARIADIC_OPT() expands to 1, while retaining the
> current processing when that is not the case. As well I have updated the
> VMD discussion of emptiness in the documentation of Boost VMD to include
> information about the ability to provide a 100% reliable test for
> emptiness in C++20.
> Neither change in Boost PP or Boost VMD should affect any end-user or
> Boost library in any way. There are tests for all these changes and all
> tests have passed. I will update the release notes for Boost PP and
> Boost VMD for the next release of Boost with this information.
> As always any comments, questions, or criticism of these changes are
> always welcome.

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