Re: [boost] [iterator] Idea for "range_from_ref_iterator"

From: David Abrahams (dave_at_[hidden])
Date: 2006-04-11 00:41:49

Ion Gaztañaga <igaztanaga_at_[hidden]> writes:

> Sorry, I really wanted to say "random access traversal" instead of 
> random access iterator. Taking a snippet of the proposed implementation:
>
> template <class T, class Difference = std::ptrdiff_t>
> class range_from_ref_iterator
>      :  public boost::iterator_facade
>            < range_from_ref_iterator<T, Difference>
>            , T
>            , boost::random_access_traversal_tag
>            , T &
>            , Difference>
>
>
> Correct me if I'm wrong but what I want is to simulate a source of N 
> identical objects without modifying them, so I think this is a Readable 
> Iterator and a Random Access Traversal Iterator.

Yes, but unless you take special measures, the iterator library will
notice that you're using random access traversal and your reference
type is a real reference, and it will make
iterator_traits<range_from_ref_iterator<...> >::iterator_category be
random_access_iterator_tag.

IIUC, that would be a nonconforming iterator.

24.1.3 says:

  If a and b are both dereferenceable, then a == b if and only if *a and
  *b are the same object.

I don't think your iterators meet taht criterion.

> I would now propose something like:
>
> template <class T, class Difference = std::ptrdiff_t>
> class repeat_read_iterator
>      :  public boost::iterator_facade
>            < repeat_read_iterator<T, Difference>
>            , const T
>            , boost::random_access_traversal_tag
>            , const T &
>            , Difference>
>
> so dereferencing "repeat_read_iterator<T>" would return const T & with 
> random_access_traversal capability to be able move fast inside that 
> "virtual read-only array". Am I missing something? 

Only that the library will assign the wrong category unless you do
something to prevent it.

> Do you see this read only repeated sequence simulation useful to
> include it the iterator library?

Sure.  But useful enough?  That I do not know.

-- 
Dave Abrahams
Boost Consulting
www.boost-consulting.com