From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2007-06-18 15:58:11
Server Levent Yilmaz wrote:
>
>
> On 6/18/07, *Roman Perepelitsa* <roman.perepelitsa_at_[hidden]
> <mailto:roman.perepelitsa_at_[hidden]>> 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)
>
>
>
> This is great. I am really surprised that it works in a comma separated
> argument list, and that you can pass around a funny array (4, (a1, a2,
> , a3) ), and work on it as expected.
This behavior, however, is not backed by the current standard!
That's why Roman added BOOST_PP_EMPTY(), I guess, so the argument itself
isn't empty and expands to emptiness during expansion of the invoked macro.
Regards,
Tobias