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

18 Jun 2007

      Server Levent Yilmaz wrote:
...
On 6/18/07, *Roman Perepelitsa* mailto:roman.perepelitsa@db.com> wrote:
Tobias Schwinger http://isonews2.com> writes:
> Yep, here's another dirty trick :
     >
     >      #include 
     >
     >      #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 
    #include 
#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