[boost] Macro which can determine size of tuple passed as argument

From: 6orkup (6orkup_at_[hidden])
Date: 2008-05-04 16:59:28

Hi,

I am trying to implement a macro which can determine size of tuple passed as
argument.
My first idea was to
1) remove parentheses from tuple using variadic macro (which I can afford
for)
2) complement tuple elements list with "filler and marker" sequence
3) scan resulting arguments list right-to-left.

Straightforward 1-3-arguments version using variadic macros and
boost.preprocessor looks like:

#include <boost/preprocessor/control/if.hpp>

#define M4(a1,a2,a3,a4,...) BOOST_PP_IF(a4,3,M3(a1,a2,a3))
#define M3(a1,a2,a3)        BOOST_PP_IF(a3,2,M2(a1,a2))
#define M2(a1,a2)           BOOST_PP_IF(a2,1,M1(a1))
#define M1(a1)              0
#define M(...)              M4(__VA_ARGS__,1,0,0,0)
#define T(t)                M t

After preprocessing these invocations

  T((a,b,c))
  T((a,b))
  T((a))

expand to:
  3
  2
  1

Unfortunately these macros do not allow to discriminate between 1 and
0 -element tuples: both T((a)) and T(()) invocations expand to 1 instead of
1 and 0 correspondingly. It seems that chosen approach might work if I had
any way to append different filler to non-empty and empty tuple: with and
without leading comma separator.

Could you please advise me if it is possible to implement a macro which can
determine size of tuple passed as argument ?
Does boost.preprocessor allow this ? Can variadic macros be of any help ?

Cheers,
Yuri