Re: [boost] [iterator] lvalue-ness resurrection in transform_iterator?

From: David Abrahams (dave_at_[hidden])
Date: 2007-12-10 10:21:07

on Mon Dec 10 2007, shunsuke <pstade.mb-AT-gmail.com> wrote:

> Hi,
>
>
> Assume a base iterator is an InputIterator with RandomAccessTraversalTag.
> boost::make_zip_iterator often returns such iterators.
>
> boost::transform_iterator sometimes resurrects lvalue-ness of its base iterator.
> For example,
>
>     char & to_lvalue(boost::tuple<char &, int &> const &t)
>     {
>         return boost::get<0>(t);
>     }
>
> can return a lvalue from zip_iterator dereference.
>
> Therefore, if boost::transform_iterator extracts "pure" traversal tag from its base iterator,
> it can be not an InputIterator but a RandomAccessIterator.
> The patch will be something like this:
>
>     template <class UnaryFunc, class Iterator, class Reference, class Value>
>     struct transform_iterator_base
>     {
>         // ...
>
>         typedef iterator_adaptor<
>             transform_iterator<UnaryFunc, Iterator, Reference, Value>
>           , Iterator
>           , cv_value_type
> -          , use_default    // Leave the traversal category alone
> +          , typename pure_traversal_tag<typename iterator_traversal<Iterator>::type>::type
>           , reference
>         > type;
>     };
>   }
>
>
> Is there any pitfalls?

Looks like a good idea.  Did you check to see if it breaks any tests?

-- 
Dave Abrahams
Boost Consulting
http://www.boost-consulting.com