Re: [Boost-users] EqualityComparable of unordered containers

From: David Abrahams (dave_at_[hidden])
Date: 2008-08-18 15:20:06

on Mon Aug 18 2008, Daniel Krügler <dsp-AT-bdal.de> wrote:

> David Abrahams wrote:
>> I suggest requiring that the predicates themselves be equality
>> comparable, and that equal predicates perform the same computation.
>> Containers with unequal predicates would not be considered equal.
>
> Yes, this proposed resolution has also be given by Pablo Halpern
> in a different mailing channnel. I totally agree that some tagging
> technique (or concepts in C++0x) would be good idea. 

I don't see what tagging has to do with my suggestion, and concepts are
certainly not applicable because we're discussing runtime tests.
  
> I had some
> resistance concerning requiring EqualityComparable for non-empty
> predicates (This was his very nice idea: Distinguish empty from
> non-empty predicates and require EqualityComparable only for non-empty
> ones), 

That's not a bad idea.

> because I thought it would be too intrusive. Maybe I was to
> strict in my position. The reason for my resistance is that in several
> scenarios an object type with overloaded operator() has *more*
> than one operator() overload and by requiring operator== this would
> have the effect that we don't know, for *which* overload of
> operator() this EqualityComparable concept would apply.

An operator== that applied for only one of many overloads would be an
abomination, especially if the class were empty.

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