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

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-07-09 04:40:25

On Jul 9, 2008, at 3:02 AM, joaquin_at_[hidden] wrote:

> Daryle Walker escribió:
>> Here's what I'm trying (to redo integer_test.cpp)
> [...]
>> BOOST_STATIC_ASSERT
>> ( (boost::is_same<distinct_integral_bit_counts,
>> boost::mpl::list_c<int, 8, 16, 32, 64> >) );
>> }
>>
> I think there are two problems with this:
>
> 1. You're passing a *type* (boost::is_same<...>) to  
> BOOST_STATIC_ASSERT
> when you should be passing a numeric contanst. Append ::value to  
> the type.

Can't believe I missed that....

> 2. Even with the correction above the thing won't work because  
> is_same<T,Q> checks
> for strict equality of the types T and Q; in your case  
> distinct_integral_bit_counts
> is *not* the same type as boost::mpl::list_c<...>, but only has the  
> same components.
> You can use boost::mpl::equal to test this:
>
> BOOST_STATIC_ASSERT
> ( (boost::mpl::equal<distinct_integral_bit_counts,
>  boost::mpl::list_c<int, 8, 16, 32> >::value) );
> }
>
> Other than that, your type_to_digit_count is perfectly lambda- 
> cabable AFAICS. HTH,

Thanks.  I switched to mpl::equal and it compiled.  I later switched  
to BOOST_MPL_ASSERT.  (Remember to take _out_ the "::value" part!)  I  
originally had the "integral_c" part of "type_to_digit_count" as a  
member called "type," instead of as a base class, then I switched  
back and forth.  Both ways work.  Why is that?  And which way is best  
(if they're not equally good, and without a better third alternative)?

//==============================
     // New type list: integral type -> (wrapped) number of bits
     // Should the "integral_c" part be a base class or direct member?
     template < typename T >
     struct type_to_digit_count //: integral_c<int,  
std::numeric_limits<T>::digits>
     { typedef integral_c<int, std::numeric_limits<T>::digits>  type; };

     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> >) );
//==============================

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