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Re: Object identity
From: |
Barry Margolin |
Subject: |
Re: Object identity |
Date: |
Tue, 21 Oct 2003 19:27:03 GMT |
In article <mailman.2095.1066762882.21628.help-gnu-emacs@gnu.org>,
Lars Brinkhoff <lars@nocrew.org> wrote:
>Stefan Monnier <monnier@iro.umontreal.ca> writes:
>> > Is there a function, say object-identity, in Emacs Lisp that maps an
>> > object to a unique value (other than the object itself)?
>> I'm wondering what you want it for.
>
>In the implementation of Common Lisp's print-unreadable-object when
>:identity t is supplied.
AFAIK, there's no guarantee that the same object will print identically
over time. Most implementations just display the object's current address;
if the GC moves it, the next time it's printed it will be different. And
it's also possible for two different objects to display identically; in
between the two PRINT-UNREADABLE-OBJECT calls the GC could copy one of them
into the location that the other one used to occupy.
Anyway, there's a reason why Common Lisp requires that
PRINT-UNREADABLE-OBJECT be built into the implementation. We didn't want
to expose an interface to getting these not-quite-unique identifiers.
They're not much use for anything other than this one function; if we
provided the API, users would undoubtedly misuse it.
--
Barry Margolin, barry.margolin@level3.com
Level(3), Woburn, MA
*** DON'T SEND TECHNICAL QUESTIONS DIRECTLY TO ME, post them to newsgroups.
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Re: Object identity, Barry Margolin, 2003/10/21
Re: Object identity, Lars Brinkhoff, 2003/10/21
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