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Re: Object identity

From: Stefan Monnier
Subject: Re: Object identity
Date: Tue, 21 Oct 2003 16:50:20 GMT
User-agent: Gnus/5.09 (Gnus v5.9.0) Emacs/21.3.50

>> I still have no clue what it's useful for, so I tend to find such research
>> rather pointless.  But I'm probably just missing the point.

> The OP explained what it's useful for: emulating weak hash tables in a
> system that doesn't have them built in.

That's not how I understood his article.  He seemed to say that he
currently uses an implementation of object-identity which relies
on a weak hash-table but that his Emacs's doesn't implement weakness.
I.e. he seemed to say he needed weak hash-tables to implement
oject-identity, not the other way around.

Furthermore, all the systems I know of that offer something like
`object-identity' also offer weak hash-tables.  Ah... maybe Java
uses object-identity to implement some of its hash-table libraries,
thus justifying the work done on object-identity?
Still if that's the only use, it seems stupid to work on object-identity
rather than just provide the has-table functionality directly.

> If implementing an identity function like the one the OP requests results
> in every object being larger (due to a design like the one I described),

There are various tricks possible to pay only for objects on which
object-identity was called.  It makes `object-identity' slower, of course.

> it's likely that this overhead will be much greater than the memory leak in
> his application.

If the application is long running a 50% overhead can still be
significantly better than a memory leak.


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