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/iteratio n_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.
Hmm, seems they flushed out the baby with the bath (as we say in Germany). Adding a diagnostic message if the expression in #elif is empty is a valid concern, but requiring this expression to be fully defined and meaningful is just nonsense, IMHO. FWIW, Wave makes sure the expression is not empty, but skips evaluating it if a previous #if/#elif was true. Regards Hartmut