From: MB (mb2act_at_[hidden])
Date: 2006-01-06 19:52:32
Thorsten Ottosen wrote:
> MB wrote:
>
>>Hi,
>>
>>I think non-intrusive customization way needs four specializations;
>>'X', 'const X', 'volatile X' and 'const volatile X'.
>>At last, twenty specializations are required for five metafunctions.
>>I view that's something like the pitfall of public virtual interfaces.
>
>
> if that is true, then it is certainly bad.
>
>
>>Only range_result_iterator happens to work around by using remove_cv.
>>
>>See:
>>
>> I added followings to <libs/range/doc/example.cpp>:
>>
>> template< class Range >
>> typename boost::range_result_iterator<Range>::type
>> my_begin(Range& rng)
>> {
>> return boost::begin(rng);
>> }
>>
>> // then...
>> my_begin(cpair); // error
>
>
> right, because we instantiate range_const_iterator< const T >, which
> should never happen.
>
>
>> Why not something like:
>>
>> template< class T >
>> struct range_result_iterator :
>> mpl::eval_if< is_const<T>,
>> range_const_iterator< typename remove_cv<T>::type >,
>> range_iterator< typename remove_cv<T>::type >
>> >
>> { };
>
>
> Seems right to me.
>
> I the latest release I actually added similar stuff to begin(),end() to
> remove const qualifiers from "const T", since it lead to such "const T"
> being used to instatiate metafunctions *even though* the overload was
> never picked.
>
> So this means that begin()/end() must also remove volatile. Cute :-)
>
> Thanks for the feedback
>
> -Thorsten
I show my current private patch:
namespace boost {
namespace range_detail {
template< class T >
struct remove_cv_ :
#if !BOOST_WORKAROUND(BOOST_MSVC, == 1310)
remove_cv<T>
#else
mpl::eval_if< is_array<T>, // is_const is broken if array
remove_volatile<T>,
remove_cv<T>
>
#endif
{ };
} // namespace range_detail
//////////////////////////////////////////////////////////////////////////
// default
//////////////////////////////////////////////////////////////////////////
template< class T >
struct range_result_iterator :
mpl::eval_if< is_const<T>,
range_const_iterator< typename range_detail::remove_cv_<T>::type >,
range_iterator< typename range_detail::remove_cv_<T>::type >
>
{ };
} // namespace boost
But I guess this is not portable for broken compilers, though
eVC4, VC7.1, VC8 and GCC3.4 seem to work.
All I can say is "Be careful about 'boost::is_const'." :-)
Regards,
MB
p-stade.sourceforge.net