Server Levent Yilmaz wrote:
On 6/18/07, *Roman Perepelitsa* <roman.perepelitsa@db.com <mailto:roman.perepelitsa@db.com>> 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