Re: [boost] [preprocessor] How to prevent macro expansion with SEQ?

From: Paul Mensonides (pmenso57_at_[hidden])
Date: 2005-07-14 15:22:20

> -----Original Message-----
> From: boost-bounces_at_[hidden] 
> [mailto:boost-bounces_at_[hidden]] On Behalf Of Markus Schöpflin

> Is there a way to get this:
> 
>    scope().attr("foo") = 1; scope().attr("bar") = 2;
> 
> or this:
> 
>    scope().attr("foo") = foo; scope().attr("bar") = bar;
> 
> IOW, how can I prevent macro expansion for X when invoked from 
> PY_DEFINE_CONSTANTS()?

The only way to prevent expansion is with # and ##--and that's all about timing.

#include <boost/preprocessor/cat.hpp>
#include <boost/preprocessor/facilities/intercept.hpp>
#include <boost/preprocessor/seq/for_each.hpp>
#include <boost/preprocessor/tuple/elem.hpp>

#define PY_DEFINE_CONSTANTS_NO_EXPANSION_HELPER(r, data, x) \
        scope().attr(BOOST_PP_TUPLE_ELEM(2, 0, x)) = BOOST_PP_TUPLE_ELEM(2, 1,
x); \
        /**/
#define PY_DEFINE_CONSTANTS_NO_EXPANSION(seq) \
        BOOST_PP_SEQ_FOR_EACH( \
            PY_DEFINE_CONSTANTS_NO_EXPANSION_HELPER, ~, \
            BOOST_PP_CAT(PY_DEFINE_CONSTANTS_NO_EXPANSION_A seq ## 0, 0) \
        ) \
        /**/

#define PY_DEFINE_CONSTANTS_NO_EXPANSION_A(x) ((#x, x))
PY_DEFINE_CONSTANTS_NO_EXPANSION_B
#define PY_DEFINE_CONSTANTS_NO_EXPANSION_B(x) ((#x, x))
PY_DEFINE_CONSTANTS_NO_EXPANSION_A
#define PY_DEFINE_CONSTANTS_NO_EXPANSION_A0
#define PY_DEFINE_CONSTANTS_NO_EXPANSION_B0

#define foo 1
#define bar 2

PY_DEFINE_CONSTANTS_NO_EXPANSION((foo)(bar) BOOST_PP_INTERCEPT_)

The trick here is to get the sequence inside the machinery in a way that allows
the machinery to prevent it from expanding.  That's the trailing 'INTERCEPT_ and
the 'seq ## 0'.  From there, it can be immediately processed with custom
sequential iteration to make a new sequence with a argument/string pairs.

This is about the best way that you can do it, but I won't guarantee that it
will work on buggy preprocessors.  In fact, I know it won't work on VC.  The
timing is tricky, and it's intrusive on the interface, but that's about the only
way that it can be done.

Regards,
Paul Mensonides