From: Robert Klemme <shortcutter@...>
Date: 2013-02-16T23:30:38+09:00
Subject: Re: Fun with finalizers

On Sat, Feb 16, 2013 at 12:08 PM, Garthy D
<garthy_lmkltybr@entropicsoftware.com> wrote:

> Excellent thinking. I also thought it might be something along those lines
> too.

To make it crystal clear: the reason is that there is a closure
involved. The closure will hold on to the object referenced by a on
method entry - unless, as you discovered, that reference is cleared.

Just in case and if you don't know, here's what a closure does:

irb(main):015:0> def f; x=0; lambda { x+=1 } end
=> nil
irb(main):016:0> g = f
=> #<Proc:0x802b7778@(irb):15 (lambda)>
irb(main):017:0> g.call
=> 1
irb(main):018:0> g.call
=> 2
irb(main):019:0> g.call
=> 3

The closure captures the current scope, i.e. all local variables.
This includes method arguments and "self" - and hence all member
variables of self as well:

irb(main):023:0> def f; @x = 0; lambda { @x += 1 } end
=> nil
irb(main):024:0> g = f
=> #<Proc:0x8029f934@(irb):23 (lambda)>
irb(main):025:0> g.call
=> 1
irb(main):026:0> g.call
=> 2
irb(main):027:0> g.call
=> 3
irb(main):028:0> g.call
=> 4


> I tried various combinations as well: proc, Proc.new, I think a return
> from method(), calls to a separate object; but there was no impact on the
> result. If "a" isn't cleared, the object is held.

I would be surprised if there was.  lambda and Proc both create a
closure.  Difference between lambda and Proc are in a different area:
http://stackoverflow.com/questions/1740046/whats-the-difference-between-a-proc-and-a-lambda-in-ruby

> I'm guessing that there
> might be some way to say to not touch a thing in the current scope, but I'm
> not sure *how* to specify it.

There is no way to exclude that variable from the closure - other than
not passing it. But that would be pointless here. :-)

> I also adapted the main program based on my experience with the code below,
> and suddenly the finalizers were called. So it's the same type of problem.
>
> So there's a problem, and it's avoidable. I know the "what", but don't know
> the "why". There is some subtlety I'm missing. Most interesting. :)

Now you should know.

Kind regards

robert


-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/