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

From: Server Levent Yilmaz (leventyilmaz_at_[hidden])
Date: 2007-06-18 12:30:59

On 6/18/07, Roman Perepelitsa <roman.perepelitsa_at_[hidden]> wrote:
>
> Tobias Schwinger <tschwinger <at> 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.

There is very slight usage problem though. An input such as  ( int, double&,
, char* ) where the second comma is a typo, is a valid expression and
produces a syntactically correct prototype (see my previous post where I am
using this IS_SPECIAL macro), and goes unnoticed, until crashing miserably
in run time!

L.

-- 
Server Levent Yilmaz
Mechanical Engineering
University of Pittsburgh