From: Robert Klemme <shortcutter@...>
Date: 2010-05-20T03:53:58+09:00
Subject: Re: Retrieve instance

2010/5/19 David Masover <ninja@slaphack.com>:
> On Wednesday, May 19, 2010 09:54:03 am Robert Klemme wrote:
>> 2010/5/19 Walle Wallen <walle.sthlm@gmail.com>:
>> > Quick question. Can I somehow retrieve the instance of the class Test in
>> > which the variable con exists. As in the example below, can I retrieve
>> > the instance of Test from the variable p?
>>
>> There is no general mechanism that would allow to follow an object
>> reference backwards.  If there was, that mechanism would yield
>> multiple instances because a object can be referenced by any number of
>> other objects.
>
> Well, there's ObjectSpace. Probably not general (cross-interpreter), but it
> does work if you actually need this functionality.

Even with ObjectSpace you cannot really travel a reference backwards.
You're merely forced to look at all objects (of a kind) and figure
whether any of them references (forward) the one whose parent you are
looking for as your code nicely demonstrates.  Actually I find that
approach so ridiculous for the case at hand that I didn't even want to
come up with it. :-)

> However, ObjectSpace is
> code smell -- if you're using ObjectSpace, either you _really_ know what
> you're doing, or you probably need to rethink your design.
>
> If you do need that, for some reason:
>
> def find_con con
>  ObjectSpace.each_object(Test).select{|t| t.con == con}
> end
>
> Of course, that assumes you want to test for equality. If you want to make
> sure they're actually the same object, you might do
>
> def find_con con
>  ObjectSpace.each_object(Test).select{|t| t.con.object_id == con.object_id}
> end
>
> And of course, that will return an array. It will also not be particularly
> fast, and it won't necessarily be reliable -- for example, if you don't have a
> reference to the Test object anymore, it might be garbage collected before you
> go looking for it, but it might not -- so there's all sorts of possibilities
> for subtle bugs that only show up sometimes.

Yeah, good summary of all the drawbacks of this approach. :-)

> Just do what Robert Klein said.

Erm, who?

Cheers

robert

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