Re: [Boost-users] EqualityComparable of unordered containers

From: David Abrahams (dave_at_[hidden])
Date: 2008-08-21 21:23:49

on Tue Aug 19 2008, Daniel Krügler <dsp-AT-bdal.de> wrote:

> David Abrahams wrote:
>> on Tue Aug 19 2008, Daniel Krügler <dsp-AT-bdal.de> wrote:
>>
>>
>>>>An operator== that applied for only one of many overloads would be an
>>>>abomination, especially if the class were empty.
>>>
>>>Right, if the class is empty, all operator() overloads
>>> are stateless, 
>>
>> What about random number generators (or anything else that uses global
>> state?)
>
> Note that I'm restricting on EqualityComparable for predicates
> (therefore I carefully choose the name comparable_predicate).
>
> Can you think of any reasonable *predicate* with global state?

  has_this_time_been_reached( sometime )

>>>Or to express the problem in different words: In general
>>>there exists a one-to-many relation between a class
>>>type and it's operator() overloads [acting as predicates],
>>>so the class-type alone is not sufficient to define
>>>an equality of *one* special operator() overload, which
>>>we are interested in. Therefore the predicate equality needs
>>>to be  restricted to a given predicate (a given operator()
>>>overload).
>>
>> I think you're over-engineering this.  It's not unreasonable to
>> require operator== to make sense in this context.
>
> This answer is a bit too short for me and has not
> the convincing power which we usually get from your
> contributions ;-)

Thanks; I've been under the weather.  Still, I don't know how to say it
much better.

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