Re: [boost] [Range] patched: boost/range/iterator.hpp

From: Thorsten Ottosen (thorsten.ottosen_at_[hidden])
Date: 2006-05-08 17:35:18

Eric Niebler wrote:

 > David Abrahams wrote:

 >>> Your patch gets everything all mixed up.
 >>
 >>
 >>
 >> That's rather a bald and sweeping statement to make without any
 >> explanation.  What do you really mean by "everything" in this case?
 >
 >
 >
 >
 > Everything, in this case, means the separate and orthogonal 
responsibilities of the three range metafunctions.
 >
 > This is a potentially breaking change. Consider a range adaptor...
 >
 > template<typename Range> struct some_adaptor
 > {
 >      typedef typename range_iterator<Range>::type iterator;
 >      typedef typename range_const_iterator<Range>::type const_iterator;
 > ....
 > };
 >
 > In this case, you really don't care about the const-ness of Range -- 
you just want the nested ::iterator type. And anyplace someone is using 
these metafunctions to perform type computations where const-ness 
doesn't matter will now be broken.

In the new version, this is how range_iterator is defined:

namespace boost
{
     template< typename C >
     struct range_iterator
     {
         typedef BOOST_RANGE_DEDUCED_TYPENAME
             mpl::if_< BOOST_DEDUCED_TYPENAME is_const<C>::type,
                       BOOST_DEDUCED_TYPENAME range_const_iterator<
                           BOOST_DEDUCED_TYPENAME remove_const<C>::type 
 >::type,
                       BOOST_DEDUCED_TYPENAME 
range_mutable_iterator<C>::type >::
type type;
     };

} // namespace boost

-Thorsten