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

From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-06-18 16:03:24

Server Levent Yilmaz wrote:
> 
> 
> On 6/18/07, *Tobias Schwinger* <tschwinger_at_[hidden] 
> <mailto:tschwinger_at_[hidden]>> wrote:
> 
>     Roman Perepelitsa wrote:
>      > Tobias Schwinger <tschwinger <at> isonews2.com
>     <http://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.

Or a non-SPECIAL non-unary tuple, e.g. (foo,bar).

> What exactly is the offending code in BOOST_PP_IS_UNARY that makes it 
> non-portable?

IIRC, there is no offending code - just some deficient preprocessor 
implementations around that can't handle it...

Regards,
Tobias