[boost] [iterator] pure_traversal in zip_iterator?

From: shunsuke (pstade.mb_at_[hidden])
Date: 2007-11-28 18:12:36

Hi,

See this example, which doesn't compile for now:

    #include <list>
    #include <string>
    #include <boost/tuple/tuple.hpp>
    #include <boost/iterator/zip_iterator.hpp>
    #include <boost/iterator/transform_iterator.hpp>

    int to_value(int const &v)
    {
        return v;
    }

    int main()
    {
        std::list<int> rng1;
        std::string rng2;

        boost::make_zip_iterator(
            boost::make_tuple(
                boost::make_transform_iterator(rng1.begin(), &to_value), // BidirectionalInput
                rng2.begin() // RandomAccess
            )
        );
    }

This incurs an error:

    boost::detail::error_not_related_by_convertibility<T1,T2>' : base class undefined
    with

T1=boost::detail::iterator_category_with_traversal<std::input_iterator_tag,boost::bidirectional_traversal_tag>,
        T2=boost::random_access_traversal_tag

in minimum_traversal_category_in_iterator_tuple.

    template<typename IteratorTuple>
    struct minimum_traversal_category_in_iterator_tuple
    {
      typedef typename tuple_impl_specific::tuple_meta_transform<
          IteratorTuple
        , iterator_traversal<> // !!!!!
      >::type tuple_of_traversal_tags;

      // ...
    };

Couldn't "iterator_pure_traversal<>" be used instead of `iterator_traversal<>` here?
The iterator_pure_traversal<> could be something like this:

    template< class Iterator = mpl::_1 >
    struct iterator_pure_traversal :
        boost::detail::pure_traversal_tag<
            typename boost::iterator_traversal<Iterator>::type
        >
    { };

Once Category is turned into pure one, the above example compiles.
Is there any pitfalls if Category is "purified"?

Regards,

-- 
Shunsuke Sogame