[boost] Re: Suggested extension to the preprocessor library

From: Daniel James (daniel_at_[hidden])
Date: 2004-12-01 08:42:09

Ryan Saunders wrote:

> here's an example:
> 
>      typedef typename meta_func< typename T0::inner_type, typename 
> T1::inner_type, typename T2::inner_type>::type;
> 
> I don't see a way to do this with any of the ENUM_XXX macros.

You can do this:

     #define INNER_TYPE(z, n, data) typename T##n::inner_type

     typedef typename meta_func< BOOST_PP_ENUM(3, INNER_TYPE, _) >::type;

which is more general purpose than supplying a macro for every possibility.

 > Maybe something
 > like ENUM_SPLIT_PARAMS(COUNT, PREF, SUFF) that would generate
 > PREF ## n ## SUFF?

That wouldn't actually work in this case, since you can't concatenate a 
number onto a symbol - you probably mean PREF ## n SUFF. If you want 
this, you can do it yourself:

     #define ENUM_SPLIT_PARAMS(COUNT, PREF, SUFF) \
         BOOST_PP_ENUM(COUNT, ENUM_SPLIT_PARAMS_OP, (PREF, SUFF))
     #define ENUM_SPLIT_PARAMS_OP(z, n, data) \
         BOOST_PP_CAT(BOOST_PP_TUPLE_ELEM(2, 0, data), n) \
         BOOST_PP_TUPLE_ELEM(2, 1, data)

Daniel