[boost] preprocessor fix

From: Arias Brent-P96059 (Brent.Arias_at_[hidden])
Date: 2004-11-17 16:25:11

I've invented a printable enum, based on the Boost preprocessor programming lib.  But although the printable enum works, I think my efforts revealed a bug in Boost.  

The BOOST_PP_SEQ_ENUM preprocessor macro doesn't work with the empty "false"
  result of BOOST_PP_EXPR_IIF.  This has forced me to do hacks like this:

#define ENUM(name, start, entries)\
BOOST_PP_SEQ_ENUM(                                       \
   BOOST_PP_IIF(ENUMS,                                   \
      (typedef enum{ BOOST_PP_SEQ_HEAD(entries) = start) \
      BOOST_PP_SEQ_TAIL(entries)                         \
      (LAST_##name } name;)                              \
      ,                                                  \
      (static void PP_NILFUNC();)                        \
   )                                                     \
) 

As you can see in the above definition, I've included an explicit "false" clause for BOOST_PP_IIF() that expands into the harmless (and utterly superfluous) expression "static void PP_NILFUNC();".  I would instead have preferred to use BOOST_PP_EXPR_IIF and thus avoid the eye-sore.  But the hack was necessary, as I said before, because BOOST_PP_SEQ_ENUM does not elogantly handle the empty "false" result of BOOST_PP_EXPR_IIF.

Can this be fixed?

-Brent