From: David Masover <ninja@...>
Date: 2010-05-20T01:08:00+09:00
Subject: Re: Retrieve instance

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. 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.

Just do what Robert Klein said.