Re: Andrei's "iterators must go" presentation

From:

Andrei Alexandrescu <SeeWebsiteForEmail@erdani.org>

Newsgroups:

comp.lang.c++.moderated

Date:

Tue, 12 May 2009 14:11:26 CST

Message-ID:

<KJJLzM.1H0A@beaver.cs.washington.edu>

Pete Becker wrote:

Andrei Alexandrescu wrote:

Pete Becker wrote:

george.ryan@gmail.com wrote:

Idea of 'ranges' and how they can alleviate difficulties in using
iterators isn't new. Dietmar Kuehl spoke about them almost four years
ago.

There was a lot of good information there, thanks!

That was four years ago, so presumably the people who are on the
standards committee are well aware of the problems associated with
some of the STL's design. So why no 0x changes for some of these
issues?

Yes, those of us who are on the standards committee are well aware
that different designs have different strengths and weaknesses.
Whether those constitute problems is a matter of judgment and
perspective. Also, note that Andrei's paper starts from a fundamental
misunderstanding of the STL design. It's not "iterators =
gcd(containers, algorithms)". It's "iterators = gcd(sequences,
algorithms)". Unfortunately, this is a very common mistake.

Good point. I see how the metaphor I used could have been interpreted as
a misunderstanding, but I fail to see how ranges proper stem from such a
misunderstanding. If you care to share, I'm all ears.

Sorry, that was a bit hyperbolic. There's nothing wrong with ranges.
They're more limiting than iterators, and that extra limiting can be a
good thing, but it doesn't mean we should abandon iterators. I do tend
to bristle at examples that are chosen to illustrate the weaknesses of
one approach and the strengths of others, without a similar focus on the
weaknesses of the approach being advocated.

I understand. Now, the audience of the talk was definitely an expert in
the goods and bads of iterators, so it was safe to stick with the deltas
without risking being accused of intellectual dishonesty. There's also
been criticism of iterators before (our own James Kanze has been doing
that forever in this group), but a cure was never presented convincingly.

To me the main surprise was that ranges, in spite of their narrower
interface and dirt-simple implementation, can be used exclusively to
implement a superset of <algorithm> with huge gains in generality and at
a fraction of the cost. (Yes, I've implemented <algorithm> and more
twice: once with iterators, then with ranges.)

Andrei

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