Re: [Boost-users] EqualityComparable of unordered containers

From: Daryle Walker (darylew_at_[hidden])
Date: 2008-08-20 03:32:42

On Aug 20, 2008, at 2:20 AM, Daniel Krügler wrote:
> David Abrahams wrote:
>> on Tue Aug 19 2008, Daniel Krügler <dsp-AT-bdal.de> wrote:
[SNIP]
>>> 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 ;-)
>
> But I let it settle for a while - as I already said:
> I might have a bit too strict point of view on this
> and I would really appreciate further comments.

I don't know if what I'm going to say was what David was thinking of,  
but...

If a predicate function object has several "operator ()" overloads,  
they should be conceptually identical.  Ideally, they should be  
"const" member functions that depend only on the values of the inputs  
and any internal state data members.  Even without that, your  
predicate class should represent ONE kind of evaluation criteria to  
pass judgement on.  Your nightmare predicate would have to have all  
the "operator ()" inconsistent with each other, i.e. differing  
criteria.  That would make it useless, considering you can't choose  
which overload you get in normal use.  You can choose a specific  
version with Stupid C++ Tricks involving casting the inputs and/or  
the member function, but that won't help with generic (template- 
based) code.  Testing operations that aren't the same shouldn't be  
part of the same predicate class.

-- 
Daryle Walker
Mac, Internet, and Video Game Junkie
darylew AT hotmail DOT com