Re: Comparing DS_HASH_TABLE objects

Hi everybody,

Had to catch up to this interesting discussion. Just want to add my 5  
cents:

Eric is perfectly right that there are many ways to compare objects  
and is_equal is just one of them. "Pure" object-orientation has just a  
single native equality and that is reference equality. All other  
equalities are artificial.

There was a discussion on removing is_equal from ANY in ECMA. The  
reason it was left in (including the cat-call prone signature) is:  
expanded types.

IMHO, 'is_equal' and 'copy' are both in ANY for the only(!) reason  
that they are needed when the semantics of ":=" and "=" changes for  
expanded types. Reference assignment and comparison are not working  
anymore in that case.

Being aware of this, using 'is_equal' and 'copy' should be avoided  
whenever you are working with reference objects. Unfortunately, this  
fact is ignored in most Eiffel libraries. That '~' is defined in terms  
of 'is_equal' in ECMA continues this mess. I think using ~ should be  
avoided, too.

Bernd

On Jun 5, 2009, at 2:35 AM, Helmut Brandl wrote:

>
>
> Peter Gummer wrote:
> > Helmut Brandl wrote:
> >> Exactly. I would like to have to most natural operator to express  
> object
> >> equality. And that is "=". I guess in the majority of the cases  
> the user
> >> wants to test object equality and not reference equality.
> >>
> >> But this is a theoretical wish. Nobody will accept such a  
> discussion.
> >> The argument with backward compatibility kills everything in that  
> direction.
> >>
> >
> > True, but it's fun to dream of what Eiffel++ might look like ;-)
>
> I am glad that there a persons out thinking about beyond ECMA.
>
> >
> > Following this logic through, we should change the assignment  
> operator
> > to perform copy semantics for reference types, so that a := b would
> > perform what we today write as a := b.twin. This is becoming a bit  
> scary.
>
> I am not sure if I would go so far. Why do you use a := b.twin?  
> Usually
> this is not necessary.
>
> But I would use is_deep_equal and deep_twin as a default for is_equal
> and twin. The flat twin is not very meaningful in most of the cases.
>
> Helmut
>
>
> 
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--
Bernd Schoeller, PhD, CTO, Partner
Comerge AG, Technoparkstrasse 1, CH-8055 Zurich, www.comerge.net





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