[boost] Re: [iterators] request for utility iterator

From: Thomas Witt (witt_at_[hidden])
Date: 2005-02-17 17:19:54

Dave,

David Abrahams wrote:
> Thomas Witt <witt_at_[hidden]> writes:
> 
>>
>>I can see this being usefull, but deriving from an iterator is such a 
>>no-no that creating a optonally derivable iterator somehow seems wrong 
>>to me. 
> 
> 
> What you're saying is that giving people a way to do it correctly will
> encourage them to do it incorrectly, and therefore we shouldn't?

What I was trying to say was that I'd prefer to see a solution that 
provides the desired functionality without having the user derive from 
what otherwise is a valid iterator.

> 
>>We could add another level of indirection and introduce
>>xxx_iterator_base<Derived, ...> types. And no I don't consider this
>>a very bright idea either.
> 
> The problem is that the previous version of the library using the
> policy adaptor pattern _had_ this capability, because you could grab
> the policies class that each adaptor published as part of its public
> interface, and derive a new policies class from it.

Well that's actually more like deriving from xxx_iterator_base.

> 
> But you know what you _don't_ like ;-)
> 

Yep. I just try to be unhelpfull in a sophisticated way ;-).

> 
>>
>>This directly relates to what I said before. The whole area of
>>iterator categories is so murky currently. IIRC there are
>>outstanding DRs 
> 
> 
> DRs?  Do you mean against C++03 iterators in the standard?

I am mostly referring to iterator_categories here. IIRC not all issues 
that came up during the iterator adaptor discussions are resolved yet.

> 
> 
> Well, maybe we need to return to policy adaptors, with a hybrid
> approach:
> 
>    class my_iterator
>       : public iterator_facade<my_iterator, my_policies>
>    {
>        // c'tors here
>    };

Wasn't that one of the original design ideas that was ruled out due to 
... ? If only I could remember what the rationale was.

> 
> But ultimately, I think we do need some way to allow users to
> conveniently refine the existing adaptors, and we need to do it
> *before* template aliases show up in their compilers!
> 

Agreed

Thomas

-- 
Thomas Witt
witt_at_[hidden]