[boost] Now why doesn't pop_back work? (was: Re: How do you make MPL-compatible template meta-functions?)

9 Jul 2008

      On Jul 9, 2008, at 4:33 AM, David Abrahams wrote:
...
BOOST_MPL_ASSERT((boost::mpl::equal< ... >));
is better
I'm continuing with this code, but now it says that  
boost::mpl::pop_back can't work with my type.  Now I got:

//======================================
     typedef boost::mpl::list<unsigned char, unsigned short, unsigned,
      unsigned long long>  distinct_unsigned_types;

     //...

     template < typename T >
     struct type_to_digit_count : 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_MPL_ASSERT( (boost::mpl::equal<distinct_integral_bit_counts,
      boost::mpl::list_c<int, 8, 16, 32, 64> >) );

     // Make a list of the bit counts between each offical point
     typedef boost::mpl::pop_back<distinct_integral_bit_counts>::type
       out_shifted_dibc;
     BOOST_MPL_ASSERT( (boost::mpl::equal<out_shifted_dibc,  
boost::mpl::list_c<int,
      8, 16, 32> >) );

     typedef boost::mpl::push_front<out_shifted_dibc, integral_c<int,  
0> >::type
       shifted_dibc;
     BOOST_MPL_ASSERT( (boost::mpl::equal<shifted_dibc,  
boost::mpl::list_c<int, 0, 8,
      16, 32> >) );
//======================================

The compile chokes on the "pop_back" forming "out_shifted_dibc" in my  
altered "integer_test.cpp":

//======================================
$(MY_BOOST)/boost/mpl/pop_back.hpp: In instantiation of  
'boost::mpl::pop_back<<unnamed>::distinct_integral_bit_counts>':

$(MY_BOOST)/libs/integer/test/integer_test.cpp:108:   instantiated  
from here

$(MY_BOOST)/boost/mpl/pop_back.hpp:31: error: invalid use of  
undefined type 'struct  
boost::mpl::pop_back_impl<boost::mpl::aux::list_tag>::apply<<unnamed>::d 
istinct_integral_bit_counts>'

$(MY_BOOST)/boost/mpl/aux_/pop_back_impl.hpp:27: error: declaration  
of 'struct  
boost::mpl::pop_back_impl<boost::mpl::aux::list_tag>::apply<<unnamed>::d 
istinct_integral_bit_counts>'

$(MY_BOOST)/libs/integer/test/integer_test.cpp:108: error: 'type' in  
class 'boost::mpl::pop_back<<unnamed>::distinct_integral_bit_counts>'  
does not name a type

$(MY_BOOST)/libs/integer/test/integer_test.cpp:109: error:  
'out_shifted_dibc' was not declared in this scope
//======================================

And the errors cascade from there.  Is there some (new) compatibility  
problem with "distinct_integral_bit_counts"?  Also, I was originally  
going to elide "out_shifted_dibc" and merge it into the expression  
forming "shifted_dibc," but this decision has made debugging easier.   
Can I combine transformations, or do I have to create a new name for  
each type step?

Of course, I may be doing this the wrong way.  I have the list: {8,  
16, 32, 64}, the distinct bit lengths for the built-in integral  
types.  I want: {4, 8, 12, 16, 24, 32, 48, 64}, those lengths plus  
the mid-way lengths between them (and half of CHAR_BIT).  I want to  
test lengths that don't exactly match an integral type.  (I was  
trying to form a list of averages with "out_shifted_dibc" and  
"shifted_dibc.")

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