From: Robert Klemme <bob.news@...>
Date: 2003-06-10T18:03:38+09:00
Subject: Re: array.uniq does not use == ?


"Orion Hunter" <orion2480@hotmail.com> schrieb im Newsbeitrag
news:BAY2-F14247qXLFfJNw00027a03@hotmail.com...
> I have a user defined object, Agent.  And I want to pass my Agents
around in
> an Array.  I've "mixed-in" the  Comparable module, and defined <=>
within
> the class for sorting purposes.   However, I _also_ want to be able to
> perform Array.uniq/Array.uniq! as well, having the decision on the
> "uniqueness" of the object derived from one particular attribute.
>
> My first thought was that all I need to do was define == in my Agent,
and I
> had hoped/assumed that this was the operator/function that Array.uniq
looked
> at when it executed.  However, this is not the case.  My next thought
was
> that Array.uniq might use <=>, and consider them "equal" if this
returned 0,
> however this was not the case either.
>
> I have tried defining the following for my object:
>
> <=>
> ==
> ===
> eql?
> equal?
>
> but none of them seem to be used in the Array.uniq comparison.  Further
> investigation showed that a combination of overriding eql?() and hash()
will
> do the trick (are these _both_ used in uniq?).  Now, I know that I can
> create my own class which inherits from Array, and override the uniq and
> uniq! funciton to do what I want, but it seems like there is a less
clumsly
> solution (it just seems like I shouldn't have to override hash()).  Any
> suggestions?  Thanks.

Yes, you have to override method hash().  The only clean solution if you
want to implement equivalence for a given class is to implement eql?, ==
and hash the way you need it.  If you need ordering, too, you also have to
implement <=>.  Of course you can implement those in terms of each other
minimizing the effort.

See here for some remarks and examples:
http://www.rubygarden.org/ruby?EquivalenceVsIdentity

Cheers

    robert