# Object scope and cleanup

**URL:** <https://www.ruby-forum.com/t/object-scope-and-cleanup/166839>\
**Category:** Ruby\
**Created:** [April 30, 2009, 1:41am UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839 "2009-04-30T01:41:34Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![jballanc](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/97f17d/32.png) [@jballanc](https://www.ruby-forum.com/u/jballanc)\
**Post date:** [April 30, 2009, 1:41am UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839/1 "2009-04-30T01:41:34Z")

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I am having trouble understanding Ruby object cleanup wrt scope. In  
particular, I would think that an object is getting deleted, but it  
turns out that is not the case. I have an RSpec test below. The  
BinarySearchTree size is defined as:

def size  
@count = ObjectSpace.each\_object(TreeNode) {}  
end

I have the following Spec code:

describe BinarySearchTree do

it “should have size ZERO to start with” do  
@tree = BinarySearchTree.new  
@tree.size.should == 0  
end

it “should have a size of ONE when we insert an item” do  
@tree = BinarySearchTree.new  
@tree.insert(5);  
@tree.size.should == 1  
end

it “should have a size of TWO when we insert two items” do  
@tree = BinarySearchTree.new  
@tree.insert(5);  
@tree.insert(3);  
@tree.size.should == 2  
end  
end

And the problem is, that when I get to the last assertion,

> > ```
> > @tree.size.should == 2
> > 
> > ```

It fails, saying that there are 3 objects (not 2). Presumably because  
the previous BinarySearchTree didn’t delete the TreeNode.

Can you explain why?  
And what can I do about it? (including “don’t use ObjectSpace that  
way”)

---

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**Author:** ![jballanc](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/97f17d/32.png) [@jballanc](https://www.ruby-forum.com/u/jballanc)\
**Post date:** [April 30, 2009, 2:28am UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839/2 "2009-04-30T02:28:20Z")

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ball wrote:

> It fails, saying that there are 3 objects (not 2). Presumably because  
> the previous BinarySearchTree didn’t delete the TreeNode.
> 
> Can you explain why?

Because the garbace collector doesn’t destroy objects as soon as  
possible. He  
destroys them when it’s convenient or necessary.

> And what can I do about it? (including “don’t use ObjectSpace that  
> way”)

Don’t use ObjectSpace that way.  
An object will be in ObjectSpace longer than it is used (sometimes much  
longer), so the number of objects of a kind in ObjectSpace means  
nothing.

HTH,  
Sebastian

---

<div class="post-metadata">

**Author:** ![jballanc](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/97f17d/32.png) [@jballanc](https://www.ruby-forum.com/u/jballanc)\
**Post date:** [April 30, 2009, 12:08pm UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839/3 "2009-04-30T12:08:28Z")

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Joshua B. wrote:

> I am having trouble understanding Ruby object cleanup wrt scope. In  
> particular, I would think that an object is getting deleted, but it  
> turns out that is not the case.

In a language like C++,

out\_of\_scope == destruction

With garbage collectors,

out\_of\_scope == ready\_for\_destruction

When the object is destroyed and whether the object is ever destroyed,  
is out of your hands. It may be the case that the gc never sees a need  
to destroy the object, in which case the object gets wiped out of memory  
only when your program ends and the operating system reclaims the  
program’s resources(and I believe even then it’s still in memory!).

There is another current thread where the ruby gc is being discussed:

[http://www.ruby-forum.com/topic/185559#new](http://www.ruby-forum.com/topic/185559#new)

---

<div class="post-metadata">

**Author:** ![jballanc](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/97f17d/32.png) [@jballanc](https://www.ruby-forum.com/u/jballanc)\
**Post date:** [April 30, 2009, 5:58pm UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839/4 "2009-04-30T17:58:54Z")

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7stud – wrote:

> In a language like C++,
> 
> out\_of\_scope == destruction

Well if a variable containing an object goes out of scope, the object is  
destroyed, yes. But if all variables containing a pointer to an object  
(the  
only kind there is in ruby) go out of scope and you did not previously  
call  
free or destroy, the object will simply remain in memory forever.

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<div class="post-metadata">

**Author:** ![jballanc](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/j/97f17d/32.png) [@jballanc](https://www.ruby-forum.com/u/jballanc)\
**Post date:** [April 30, 2009, 1:49pm UTC](https://www.ruby-forum.com/t/object-scope-and-cleanup/166839/5 "2009-04-30T13:49:55Z")

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On Thu, Apr 30, 2009 at 6:08 AM, 7stud – [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid)  
wrote:

> With garbage collectors,
> 
> out\_of\_scope == ready\_for\_destruction

Actually, “out of scope” isn’t really the right concept here, the  
storage taken up by an object is eligible to be reclaimed by a  
properly implemented GC iff it is not reachable by transitive closure  
from one or more of a known set of root objects (like globals, and  
active stack frames).

–  
Rick DeNatale

Blog: [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)  
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