Re: [Boost-users] Boost.Preprocessor: expansion stops, result is further expandable

Subject: Re: [Boost-users] Boost.Preprocessor: expansion stops, result is further expandable
From: Roman Neuhauser (neuhauser_at_[hidden])
Date: 2011-06-19 20:20:23

# dnljms_at_[hidden] / 2011-06-19 17:45:00 +0100:
> On 19 June 2011 01:37, Roman Neuhauser <neuhauser_at_[hidden]> wrote:
> > Why doesn't DAPHP_REGISTER_CLASSES expand all the way as
> > DAPHP_REGISTER_CLASS does?  Please enlighten me.
> 
> C macros can't be called recursively. Boost.Preprocessor uses a clever
> technique to get round that for some macros, but it doesn't seem to do
> it for BOOST_PP_SEQ_FOR_EACH, I'm not sure why, it might be too
> expensive to do for every macro.

Ah, BOOST_PP_SEQ_FOR_EACH Remarks section doesn't include the
"Previously, this macro could not be used inside ..." paragraph
that's present elsewhere.

> I think that one thing you can do (and this is pretty crummy) is to
> use BOOST_PP_SEQ_FOR_EACH_I for one of the calls and just ignore the
> index. Or alternatively you can use a macro that supports recursion, I
> think BOOST_PP_SEQ_FOLD_LEFT, BOOST_PP_SEQ_FOLD_RIGHT and BOOST_PP_FOR
> are possibilities.

I decided to abstain from using sequences inside the CXXEXT namespace.
The inner BOOST_PP_SEQ_FOR_EACH is now BOOST_PP_LIST_FOR_EACH with
input transformed by

#define CXXEXT_SEQ_TO_LIST(seq) \
  BOOST_PP_EXPAND(BOOST_PP_TUPLE_TO_LIST BOOST_PP_SEQ_TO_ARRAY(seq))

-- 
roman