Re: [Boost-users] MPL iterator category

From: Aleksey Gurtovoy (agurtovoy_at_[hidden])
Date: 2005-02-18 18:17:14

Philippe Mori writes:
> In C++ Template Metaprogramming book, there is the expression
> i::category in tables 5.1, 5.2 and 5.3 that says that the result is
> convertible to mpl::forward_iterator_tag,
> mpl::bidirectional_iterator_tag and mpl::random_access_iterator_tag
> respectively.

Yep.

>
> Since a random access iterator is bidirectional iterator which is in
> turn a forward iterator, I was expecting that i::category for a
> bidirectional iterator would also be convertible to mpl::forward_tag
> which is not the case.

Correct.

>
> So if an algorithm has to works differently for random-access
> iterator but it the same for forward and bidirectional iterator, it
> seems to me that I would need some extra logic to uses forward
> iterator algorithm if I have a bidirectional iterator as theree are
> no implicit conversion.

Well, as a matter of fact this particular case can be easily handled
by explicit specialization (see
http://www.boost.org/libs/mpl/doc/refmanual/iterator-category.html#example).

Writing an algorithm for _bidirectional_ iterators and making sure
that it gets selected for random-access ones as well is somewhat more
tricky:

    template< typename Iterator > struct algorithm
        : if_c<
              iterator_category<Iterator>::type::value 
                  >= bidirectional_iterator_tag::value
            , bidirectional_algorithm<Iterator>
            , forward_algorithm<Iterator>
            >
    {
    };

but try to write this up assuming that _there is_ an implicit
conversion between 'random_access_iterator_tag' and
'bidirectional_iterator_tag', and you'll see that you'll end up with
something at least as complex as the above.

HTH,
-- 
Aleksey Gurtovoy
MetaCommunications Engineering