Re: [Boost-users] [preprocessor] Argument list conversion

From: Server Levent Yilmaz (leventyilmaz_at_[hidden])
Date: 2007-06-18 12:32:37

On 6/18/07, Tobias Schwinger <tschwinger_at_[hidden]> wrote:
>
> Roman Perepelitsa wrote:
> > Tobias Schwinger <tschwinger <at> isonews2.com> writes:
> >
> >> Yep, here's another dirty trick :
> >>
> >>      #include <boost/preprocessor/detail/is_unary.hpp>
> >>
> >>      #define SPECIAL (whatever)
> >>      #define IS_SPECIAL BOOST_PP_IS_UNARY
> >>
> >>      IS_SPECIAL(a)         // 0
> >>      IS_SPECIAL(b)         // 0
> >>      IS_SPECIAL(SPECIAL)   // 1
> >>
> >> It isn't entirely portable to older preprocessors (that's why this code
> >> lives in preprocessor/detail) but should work with latest versions of
> >> the widely-used compilers.
> >
> > This also works:
> >
> > #include <boost/preprocessor/facilities/is_empty.hpp>
> > #include <boost/preprocessor/empty.hpp>
> >
> > #define SPECIAL
> > #define IS_SPECIAL(x) BOOST_PP_IS_EMPTY(BOOST_PP_EMPTY() x)
> >
>
> Yep, that's even better as it allows x to be anything. Got some idea how
> portable it is?

Well, IS_UNARY trick also works for pretty much anything except maybe
another non-SPECIAL unary.
What exactly is the offending code in BOOST_PP_IS_UNARY that makes it
non-portable?

-- 
Server Levent Yilmaz
Mechanical Engineering
University of Pittsburgh