Retrieve instance

Ruby
1

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?

class Test
attr_accessor :con

def initialize()
@con = Hash.new
end

end

p = Test.new.con

2

2010/5/19 Walle W. [email protected]:

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.

Bottom line: you can only follow object references from the container
to the contained so you need to make sure you have the reference you
need (see below).

class Test
attr_accessor :con

def initialize()
@con = Hash.new
end

end

p = Test.new.con

t = Test.new
p = t.com

Btw, p might not be a good name for a local variable as it may lead to
confusion with method p.

Kind regards

robert

3

Ah, I see.
Thank you for the enlightenment

4

On Wed, May 19, 2010 at 4:59 PM, Walle W. [email protected]
wrote:

Ah, I see.
Thank you for the enlightenment

You can also explicitly pass the container object into the contained:

class Contained
attr_reader :container
def initialize container
@container = container
end
end

class Test
attr_reader :con
def initialize
@con = Contained.new self
end
end

contained = Test.new.con
p contained.container

Jesus.

5

2010/5/19 David M. [email protected]:

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. :slight_smile:

Of course, that assumes you want to test for equality. If you want to make
for subtle bugs that only show up sometimes.
Yeah, good summary of all the drawbacks of this approach. :slight_smile:

Just do what Robert Klein said.

Erm, who?

Cheers

robert

6

On Wednesday, May 19, 2010 09:54:03 am Robert K. wrote:

2010/5/19 Walle W. [email protected]:

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.

7

On Wednesday, May 19, 2010 01:53:58 pm Robert K. wrote:

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.

I suppose. I still tend to think of things like this, because the
Internet is
actually eroding that kind of assumption. Once upon a time, I might have
considered having another identity online, but occasionally linking back
to my
“real” identity. Basically, people who know me as SecretGuy could find
out who
I actually am, but not the other way around, with the assumption that
friends
and family from reality wouldn’t be likely to stumble on SecretGuy.

But the idea of one-way links on the Web is just as absurd as the idea
of one-
way links in SQL. The assumption that no one would ever look through the
entire Internet for my alter-egos, or stumble on one by chance, is
destroyed
by the fact that Google already indexes the entire Internet, turning
forward
references into backward references.

Something similar could be done with ObjectSpace, but it’d be even more
ridiculous:

Thread do
loop do
ObjectSpace.each_object(Test) do |t|
con.instance_variable_set :@test, t
end
sleep 30
end
end

def find_con con
con.instance_variable_get :@test
end

I’ll leave it as an exercise to the reader why that’s so bad, and how it
might
be improved on.

But this can’t be good for my sanity.

Just do what Robert Klein said.

Erm, who?

Whoops… Actually, what is the etiquette on a forum like this? Should I
just
say “Robert”?

8

On Wed, May 19, 2010 at 3:19 PM, David M. [email protected]
wrote:

On Wednesday, May 19, 2010 01:53:58 pm Robert K. wrote:

Just do what Robert Klein said.

Erm, who?

Whoops… Actually, what is the etiquette on a forum like this? Should I just
say “Robert”?

Unlike Robert Klein - Wikipedia who I can’t recall
ever seeing around these parts, Robert K. CAN stop his leg, as far
as I know.

–
Rick DeNatale

Blog: http://talklikeaduck.denhaven2.com/
Github: rubyredrick (Rick DeNatale) · GitHub
Twitter: @RickDeNatale
WWR: http://www.workingwithrails.com/person/9021-rick-denatale
LinkedIn: http://www.linkedin.com/in/rickdenatale

9

2010/5/19 Rick DeNatale [email protected]:

On Wed, May 19, 2010 at 3:19 PM, David M. [email protected] wrote:

On Wednesday, May 19, 2010 01:53:58 pm Robert K. wrote:

Just do what Robert Klein said.

Erm, who?

Whoops… Actually, what is the etiquette on a forum like this? Should I just
say “Robert”?

That’s perfectly OK for me.

Btw, funny thing is that “klein” is actually also a German word which
can be directly translated to “small”. “Klemme” is a tad more
difficult, you can see translations here:

Unlike Robert Klein - Wikipedia who I can’t recall
ever seeing around these parts, Robert K. CAN stop his leg, as far
as I know.

Actually I had to look around a bit since I never heard of this
namesake of mine. I found

Hehe. Movit, movit… I…I…I…can’t stop - aaargh :wink:

Cheers

robert

10

On Wed, May 19, 2010 at 9:48 PM, Robert K.
[email protected]
wrote:

Btw, funny thing is that “klein” is actually also a German word which
can be directly translated to “small”. “Klemme” is a tad more
difficult, you can see translations here:
klemme - LEO: Übersetzung im ­Englisch ⇔ Deutsch Wörterbuch
Google Translate seems quite close to that, giving several
possibilities:
http://translate.google.co.uk/?&hl=en&lr=&tab=wT#de|en|klemmehttp://translate.google.co.uk/?&hl=en&lr=&tab=wT#de|en|klemme
but http://babelfish.yahoo.com gives just a one word answer “wedges”!
I rather like the phrase “in der Klemme sitzen” in
“http://dict.leo.org”,
and that “leo” reminds me of the following “quiz” question:
Was the world’s first business computer manufactured by:
(a) IBM,
(b) another American business machines company, or
(c) a chain of British tea-shops and cafes?
The answer is of course (c)!
LEO (computer) - Wikipediahttp://en.wikipedia.org/wiki/LEO_(computer)
http://www.leo-computers.org.uk
http://www.kzwp.com/lyons/index.htm http://www.kzwp.com/lyons
http://www.kzwp.com/lyons/leo.htm
http://www.economicexpert.com/a/LEO:computer.html
Amazon.co.uk
Amazon.co.uk
… When I joined the computer industry in 1965, a friend in IBM
confided in
me that the Leo (Lyons Electronic Office) series of computers (already
in
there last days) were IBM’s most serious rival in commercial computing.
…

11

2010/5/20 Colin B. [email protected]:

On Wed, May 19, 2010 at 9:48 PM, Robert K. [email protected]
wrote:

Btw, funny thing is that “klein” is actually also a German word which
can be directly translated to “small”. “Klemme” is a tad more
difficult, you can see translations here:
klemme - LEO: Übersetzung im ­Englisch ⇔ Deutsch Wörterbuch
Google Translate seems quite close to that, giving several possibilities:
http://translate.google.co.uk/?&hl=en&lr=&tab=wT#de|en|klemmehttp://translate.google.co.uk/?&hl=en&lr=&tab=wT#de|en|klemme
but http://babelfish.yahoo.com gives just a one word answer “wedges”!
I rather like the phrase “in der Klemme sitzen” in “http://dict.leo.org”,

Hehe. :slight_smile: You know what? That’s the typical phrase I use when
someone does not know how to spell my surname.

http://www.economicexpert.com/a/LEO:computer.html
Amazon.co.uk
Amazon.co.uk
… When I joined the computer industry in 1965, a friend in IBM confided in
me that the Leo (Lyons Electronic Office) series of computers (already in
there last days) were IBM’s most serious rival in commercial computing. …

Amazing! Learn something new every day. Thanks for that!

Kind regards

robert