[Boost-users] Re: Converting from const to non-const with iterator_adaptor

From: Keith MacDonald (boost_at_[hidden])
Date: 2004-01-05 08:54:04

I may have misunderstood the purpose of boost::iterator: I was expecting it
to make it easy to add standard conforming iterators to any container, but
the complexity of the proposed solution, (using iterator_adaptor, which I
can't get to compile), is making me doubt that.  In an earlier release of
the library, I was able to declare the const_iterator using iterator_facade,
then simply adapt it to a non-const iterator using iterator_adaptor, as
shown in my original posting.  I've not followed the discussions that led up
to the current implementation, so don't know what its advantages are, but if
it can't provide a simple solution to the simplest case, has something
important been thrown out with the bath water?

My working solution is now to use iterator_facade to declare instances of
const_iterator and iterator, which are independent of each other, except
that a const_iterator can be constructed from an iterator.  Instinctively,
this does not seem to the intended way to do it, but it would probably be
quicker to write the iterators from scratch than work that out from the
documentation.  Can I make a plea for an example to be included with the
library, which shows how best to add plain vanilla iterators to a container
class?

Thanks,
Keith MacDonald

"David Abrahams" <dave_at_[hidden]> wrote in message
news:u65frftpp.fsf_at_boost-consulting.com...
>
> Looks like a job for iterator_adaptor:
>
>   // untested
>   template <class V>
>   struct iter
>     : boost::iterator_adaptor<iter<V>, V*, V,
boost::bidirectional_traversal_tag>
>   {
>       typedef
>          boost::iterator_adaptor<iter<V>, V*, V,
boost::bidirectional_traversal_tag>
>       super;
>
>       iter() {}
>
>       template <class V2>
>       iter(iter<V2> x, boost::enable_if_convertible<V2*,V*>::type* = 0)
>         : super(x.base())
>
>      friend class iterator;
>      friend class boost::iterator_core_access;
>
>    private:
>     void increment()
>      { base() = base()->next(); }
>
>     void decrement()
>      { base() = base()->prev(); }
>   };
>
>   typedef iter<Node> iterator;
>   typedef iter<Node const> const_iterator;
>
>
> HTH,
>
> -- 
> Dave Abrahams
> Boost Consulting
> www.boost-consulting.com