Boost logo

Boost :

From: David Bergman (davidb_at_[hidden])
Date: 2004-02-13 00:04:50

David A wrote:

> > It is a bit confusing if we, in this new two-dimensional iterative
> > world, use "category" *both* for the combined (intersected)
> concepts,
> > being analogous to the old concepts, *and* for one of the
> dimensions.
> > In our case, Traversal.
> We are not ever doing that, AFAIK. "Category" never refers
> to "Traversal" and always refers to old-style concepts/tags.
> If you know of a place in the text where that happens, please
> point to it.

In the Design section of the proposal, you write

        "Our design reuses iterator_traits<Iter>::iterator_category to
indicate an iterator's traversal capability."

That use of iterator category does pertain to the Traversal "dimension"
(attribute, or whatever we want to call it), or?

I want there to be an explanation, such as

        "Our design reuses iterator_traits<Iter>::iterator_category to
indicate not only the old-style, combined traversal and access capabilities
of the iterator, but also the iterator's traversal capability alone. See the
category-to-traversal schema for conversion from the old-style iterator
categories to corresponding traversal capability categories. Note that the
access capability of the iterator is not incorporated in any iterator
category, but derivable from the associated value_type of the concept."

> > We have to make the meta programming facilities, such as the tags
> > used, be backward-compatible, with that I agree. So, we
> have to keep
> > the "category tag" and "iterator category" and map them to
> something
> > sensible.
> >
> > We just have to live with the categories, as realized by category
> > tags, dealing *both* with unique combinations of the two orthogonal
> > issues (backward compatibility etc.) *and* with only one of the
> > issues, Traversal, I assume.
> Now I'm lost again.

Some of of the 'iterator_trats::iterator_category' instances refer to
traversal capabilities, as in the quoted text above, and some to the
old-style concepts. I.e., there are new "iterator categories" that only
treat the Traversal attribute/dimension.

> > Could one not make that clear, such as:
> >
> > "category tags can dispatch
> Tags don't dispatch, and I consider that phrase extremely
> confusing, and possibly misleading.

I obviously meant "category tags enable dispatching on both ..."

> > based on both traversal capability and the combined capabilities of
> > traversal and access"
> Besides that, I have no clue about the intended meaning of
> the above statement, so I wouldn't make it in an effort to
> clarify anything.

I meant that one can dispatch based solely on the traversal property of the
iterator and the combined traversal and access properties.
> > I understand, and always understood, your proposal, which is very
> > welcome, but I do also understand if others might find the
> mentioning
> > of "category" a bit misleading, since we have the old,
> combined world,
> > and the new dichotomy to deal with.
> I still don't understand what you consider misleading.
> Please point to a sentence and describe how it can be misread.

See above. You still think you use "category" only for the combined
old-style concepts?

> > And... one does, unfortunately, use the word "category" in generic
> > programming for both a single concept and a family of related
> > concepts,
> I've never heard that before, and I'm pretty well-versed in
> GP. If you could point me to an example in the literature,
> I'd be indebted.

In the article, co-written by
Jeremy Siek, they use "category" very similar to how the C++ Standard deals
with iterator categories (i.e., as axiomatic sets):

        In "Each concept must have a name and optionally a

Does "category" there not refer to a qualification being isomorphic to a
family of concepts?

Also, in the same article:

        In 4.3: "A concept is described within the scope of a concept tag
which contains the concept name and the
category of the concept (in this case \Utility" for STL utility concepts)."

Thus, Utility is here an (axiomatic or not) family, or set, of concepts. A

For (unfortunate) examples where "category" is used synonymous to "concept,"
look at the eminent book "Generative Programming," by Czarnecki and
Eisenecker, where they (in A.1) state that

        "... in the context of categorization, concepts are often called
categories or classes ..."

So, "category" does carry a meaning in GP, and is not specific to the
(old-style) C++ iterators.

> > often including refinement hierarchies :-| I prefer the
> less charged
> > word "set of concepts" that you used.
> I appreciate that you're only trying to be helpful, but I
> really can't understand you. Trying to figure out what you
> mean takes a lot of time. If you won't take this suggestion,
> Quote some text from the paper, say "I think it should say
> ________" instead, and then say why you think so.
> I don't think I'm going to spend much more time trying to
> figure out what you want.

I am quoting, and I hope you see that (A) you do use "iterator category" for
both the old-style iterator classification and the pure traversal capability
classification and (B) that "category" is used in GP for sets of (related)


Boost list run by bdawes at, gregod at, cpdaniel at, john at