Re: [boost] [guidelines] why template errors suck

Subject: Re: [boost] [guidelines] why template errors suck
From: David Abrahams (dave_at_[hidden])
Date: 2010-10-04 06:22:26

At Mon, 04 Oct 2010 09:48:12 +0100,
John Bytheway wrote:
> 
> On 04/10/10 00:39, Mathias Gaunard wrote:
> > On 28/09/2010 19:14, David Abrahams wrote:
> >> Unfortunately, without real concepts we may not be able to do better,
> >> because the pseudo-signature approach inserts type conversions that
> >> we'd have to write by hand.  Now here's an interesting thought: write
> >> a library using TMP that generates "concept wrappers" that will force
> >> the necessary conversions.  Would require heavy use of rvalue
> >> references to avoid needless copies.  Hmm...
> > 
> > A crazy idea if you want to be TMP-heavy: maybe you could represent the
> > valid expressions with Proto and infer an archetype from these.
> > 
> > This could provide better syntax than the "operator_increment" thing you
> > suggested in some other part of the thread.
> 
> Indeed, I already had that idea, and I implemented a proof-of-concept.
> I guess my posts went unnoticed in this huge sprawling thread.  See:
> 
> http://article.gmane.org/gmane.comp.lib.boost.devel/208946
> http://article.gmane.org/gmane.comp.lib.boost.devel/208996
> 
> As you suggest, I think it should be possible to use a single definition
> to check models against concepts, make archetypes, and do concept-based
> overloading.  What I'm not sure is whether (a) it's worthwhile to do it,
> and (b) whether it would be worth attempting the more ambitious "concept
> wrappers" idea Dave suggests above.

One step at a time, maybe.  What you did looks good.  One thing I
should point out, though, is that SFINAE-controlled overloading and
good error messages from concept checks are not exactly compatible.  I
think you need to leave off the SFINAE on the overloads with the
least-refined concepts, somehow, if you want something better than a
ridiculous list of non-matching candidates.

-- 
Dave Abrahams
BoostPro Computing
http://www.boostpro.com