Re: [boost] Determining interest in combining_iterator

From: Thomas Becker (tmbecker1_at_[hidden])
Date: 2003-03-25 03:05:56

Thanks for your comments on the combining iterator.

> David Abrahams wrote:
> It's a wonderful idea, Thomas!
>
> You might look at the Boost sandbox; Jeremy Siek,
> Thomas Witt and I
> are trying to finalize the work in the 
> boost/iterator and
> libs/iterator directories to replace the current 
> iterator adaptors

Thanks for your kind words. I'll play with the sanbox
stuff. Is the main
purpose there to incorporate better iterator
categories? Anyway, I won't take any further action
until all that has settled down.

> Douglas Paul Gregor wrote:
> Sounds great. Since the functionality is a direct 
> generalization of
> transform_iterator, I'd prefer to use the name 
> transform_iterator and
> either (a) switch entirely to your 
> combining_iterator implementation or
> (b) give transform_iterator_generator the brains to 
> switch between your
> implementation and the existing implementation based

> on the number of
> iterators being adapted.

Uh uh, I was kinda hoping nobody would bring that up,
because I agree and at the same time strongly
disagree. Of course you're right, the transforming
iterator is a special case of the combining iterator,
and under any kind of theoretical point of view, it
should be treated as such. There is no way I can argue
with you on that. And yet, I am very strongly in favor
of keeping the two entirely separate. Here's why: From
a library user's point of view, the fact that the
transforming iterator is a special case of the
combining iterator is quite accidental. If someone is
looking for the functionality of the transforming
iterator, she should find just that, without being
forced to view the thing she's looking for as a
special case of something completely different. One
could in fact express this viewpoint in a somewhat
theoretical manner by saying that the transforming
iterator is not so much a *special* case of the
combining iterator than a *degenerate* case. Combining
iterators is not where one would naturally look when
in need of a transforming iterator.

> Anthony Williams wrote:
> IMHO, it seems more logical to split the concept, so

> the grouping of 
> the iterators is separated from the transformation

By "separating the grouping and the transforming," do
you mean to write an iterator that holds a tuple of
iterators and then, upon dereferencing, returns the
tuple of dereferenced values, and then use that in
conjunction with boost's transforming iterator? That
would make a lot of sense. However, I would need a
little convincing that it really adds useful
functionality that people are looking for.

> Paul A. Bristow wrote:
> I can see this VERY useful to some, but probably not

> widely useful. 

In view of the emphasis on "very," I'll count that as
a yes vote.

Thomas Becker
Zephyr Associates, Inc.
Zephyr Cove, NV

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