From: Eric Hodel <drbrain@...7.net>
Date: 2004-04-07T13:58:28-07:00
Subject: Re: IdentityHash, like Hash but with keys using equals? vs eql?

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Jean-Hugues ROBERT (jean_hugues_robert@yahoo.com) wrote:

> I just encountered a rare case where I need a Hash where keys
> are object id. Class Hash compares keys by value. As a result
> two different String with the same value are considered the
> same key. This is fine in most cases. But right now I need to
> be more restrictive and I need two String objects to be treated
> as two keys even if they have the same value.

Not entirely true.

Hash uses #hash and #eql? to decide how to store entries.  Some objects
(String) implement #hash, but by default Ruby uses #object_id for #hash:

irb(main):001:0> f =3D "foo"
irb(main):002:0> f.hash
=3D> 876516207
irb(main):003:0> f.id
=3D> 67718468
irb(main):004:0> o =3D Object.new
=3D> #<Object:0x81277c4>
irb(main):005:0> o.hash
=3D> 67714018
irb(main):006:0> o.id
=3D> 67714018

> I think IdentityHash is a probable name for such a beast, yet
> I could not find one in Ruby and I don't see any method in
> class Hash that I could redefine in a derived class to change
> the way keys are compared.

How about instead extending Strings you want stored uniquely to return
#object_id for #hash?

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Eric Hodel - drbrain@segment7.net - http://segment7.net
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