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

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