[boost] How do you make MPL-compatible template meta-functions?

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-07-09 02:09:11

Here's what I'm trying (to redo integer_test.cpp):

//==============================
#include <boost/config.hpp>
#include <boost/limits.hpp>
#include <boost/integral_c.hpp>
#include <boost/mpl/list.hpp>
#include <boost/mpl/transform.hpp>
#include <boost/static_assert.hpp>
#include <boost/type_traits.hpp>

#include <climits>

namespace
{
     // List all the conventional integral types, without the ones  
that are like
     // a strong-typedef of a lower type
     typedef boost::mpl::list<unsigned char
#if USHRT_MAX > UCHAR_MAX
      , unsigned short
#endif
#if UINT_MAX > USHRT_MAX
      , unsigned int
#endif
#if ULONG_MAX > UINT_MAX
      , unsigned long
#endif
#if defined(BOOST_HAS_LONG_LONG) && ((ULLONG_MAX > ULONG_MAX) ||  
(ULONGLONG_MAX > ULONG_MAX))
      , boost::ulong_long_type
#elif defined(BOOST_HAS_MS_INT64) && (0xFFFFFFFFFFFFFFFFui64 >  
ULONG_MAX)
      , unsigned __int64
#endif
      >  distinct_unsigned_types;

     typedef boost::mpl::transform<distinct_unsigned_types,
      boost::make_signed<boost::mpl::_1> >::type  distinct_signed_types;

     // New type list: integral type -> (wrapped) number of bits
     template < typename T >
     struct type_to_digit_count
         : boost::mpl::integral_c<int, std::numeric_limits<T>::digits>
     {
     };

     typedef boost::mpl::transform<distinct_unsigned_types,
      type_to_digit_count<boost::mpl::_1> >::type   
distinct_integral_bit_counts;
     BOOST_STATIC_ASSERT 
( (boost::is_same<distinct_integral_bit_counts,  
boost::mpl::list_c<int, 8, 16, 32, 64> >) );
}
//==============================

I originally did not have that last line there with the  
"BOOST_STATIC_ASSERT," but I put it in after some errors when doing a  
compiler-sanity check while coding.  I cleaned up the other errors,  
but the compiler complains that the first argument of "is_same," i.e.  
"distinct_integral_bit_counts," is an invalid template argument.  I  
don't think my definition of "type_to_digit_count" is lambda-safe,  
like "make_signed" is.  How do I get it that way?  (Or is the problem  
in the definition of "distinct_integral_bit_counts"?)

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com