[boost] [preprocessor] #elif behaviour in gcc 4.4

From: Daniel James (daniel_james_at_[hidden])
Date: 2008-07-12 09:25:05

Hello,

A recent change to the development version of the gcc preprocessor
changes the way it processes '#elif' directives and breaks a common
use of the preprocessor library. We've already had reports of this
causing problems in MPL, and it might also affect: variant,
function_types, python, fusion, xpressive and phoenix. Details of the
change are at:

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=36320

With this change, the argument to an '#elif' directive is processed
even if a previous '#if'/'#elif' directive was true. This causes the
example use of BOOST_PP_ITERATION_DEPTH to break:

http://www.boost.org/doc/libs/1_35_0/libs/preprocessor/doc/ref/iteration_depth.html

In this example, the first time the header is included,
BOOST_PP_ITERATION_DEPTH is not yet defined, so it causes the
develoment gcc to fail when the #elif directives are evaluated (which
are skipped with existing versions, because the first #if directive
evaluates to true).

Looking at the standard, I think their interpretation is correct
(although I'm not familiar with it and could be missing something) but
it isn't clear that this was the intent of the authors. Any opinions
on this?

I think it's quite easy to work around, we can replace:

#if !BOOST_PP_IS_ITERATING
// ....
#elif BOOST_PP_ITERATION_DEPTH() == 1
// ....
#elif BOOST_PP_ITERATION_DEPTH() == 2
// ....
#endif

with:

#if !BOOST_PP_IS_ITERATING
// ....
#else
#  if BOOST_PP_ITERATION_DEPTH() == 1
// ....
#  elif BOOST_PP_ITERATION_DEPTH() == 2
// ....
#  endif
#endif

So that the calls to BOOST_PP_ITERATION_DEPTH are skipped - or we
could just include the necessary header before including the file. But
it does seem a pity that any code which uses this technique might need
to be changed for gcc 4.4. We should probably write to them about this
(it doesn't look like they realised this would be a problem), but I'm
writing here first in case anyone has any ideas.

Daniel