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

18 Jun 2007

      On 6/18/07, Tobias Schwinger  wrote:
...
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.
Take a look at this:

#define SPECIAL
#define IS_SPECIAL(x) BOOST_PP_IS_EMPTY(BOOST_PP_EMPTY() x)

#define MACRO(i,z,array) IS_SPECIAL( BOOST_PP_ARRAY_ELEM(i, array) )
#define ENUM( n, tuple ) BOOST_PP_ENUM( n, MACRO, (n, tuple) )

ENUM( 3, ( arg1,        , arg3) ) //1
ENUM( 3, ( arg1, SPECIAL, arg3) ) //2

In this context, can we safely say that 2 complies with standards more than
1?

cheers,
L.

-- 
Server Levent Yilmaz
Mechanical Engineering
University of Pittsburgh