Minor feature request: BOOST_PP_IGNORE or update docs for BOOST_PP_EMPTY

[hadrielk]

The current BOOST_PP_EMPTY() is not variadic, so its usefulness in BOOST_PP_IF() and the like is limited to when there are no arguments/params in the resultant call.

For example, this doesn't work:

#define X(a, b) result(a, b)
#define MACRO(a, b, c) BOOST_PP_IF(c, X, BOOST_PP_EMPTY)(a, b)

As explained in #34, changing the signature of BOOST_PP_EMPTY() is impractical, and another solution exists: BOOST_VMD_EMPTY(...) in boost/vmd/empty.hpp.

So this is a very minor issue, but it would be nice if either:

1. There were a BOOST_PP_IGNORE(...) to do this, or...
2. The reference docs for the BOOST_PP_EMPTY could also point to the VMD macro to use instead.

[andry81]

Why not just use this: https://www.boost.org/doc/libs/1_77_0/libs/preprocessor/doc/ref/identity_n.html

#define MACRO(a, b, c) BOOST_PP_IF(c, X, BOOST_PP_IDENTITY_N(BOOST_PP_EMPTY(),2))(a, b)

[hadrielk]

Oh, it's not that I can't solve it myself - I did - it's just a minor request to either document the VMD one or add one to PP.

Not a big deal; just would be nice.