on Thu Mar 13 2008, "Sebastian Theophil" <stheophil-AT-think-cell.com> wrote:

Dear David,

I stumbled upon a problem with the counting_iterator which you seem to maintain. If you prefer that I post this message to a boost mailing list, let me know and I'll do that.

I do, thanks. Please follow up there.

The problem in short is that counting_iterator<Value> returns a Value const& when it is dereferenced although the counting_iterator doesn't point to a sequence of values.

Yep.

The problem manifests itself already when trying to wrap a counting_iterator inside a reverse_iterator.

You might argue that counting_iterators are bidirectional and don't need to be reversed.

Sure they do.

The problem would arise with any application that does something like Template<class It> It::reference_type method(It it) { return *(--it); }

This returns a reference to the temporary value contained in --it.

I don't see any specific reason why the reference type must be "Value const&" and couldn't be e.g "Value". If there is, I'd be happy if you could tell me what the reasoning is and what you propose as a workaround.

It has to do with the C++03 iterator requirements. If it were not a reference, then counting_iterator wouldn't be a valid forward iterator and we'd have to call it an input iterator.

Thanks a lot and thanks for your great work on boost, Sebastian

PS: Of course just changing

typedef iterator_adaptor< counting_iterator<Incrementable, CategoryOrTraversal, Difference> // self , Incrementable // Base , Incrementable // Value (...) , traversal , Incrementable const& // reference , difference > type;

in counting_iterator.hpp

to

typedef iterator_adaptor< counting_iterator<Incrementable, CategoryOrTraversal, Difference> // self , Incrementable // Base , Incrementable // Value (...) , traversal , Incrementable // reference , difference > type;

probably doesn't do the trick. I'd like to work on a real patch unless you say it is meant to work that way.

Maybe the trick is to add a parameter to counting_iterator that allows one to optionally explicitly specify the reference type. -- Dave Abrahams Boost Consulting http://boost-consulting.com