[Newbies] How to get rid of objects...

Tue May 9 15:50:56 UTC 2006

Hi Cédrick, 

Welcome to the list.  This is a pretty difficult question.  There are a
number of answers and all of them have the potential of doing some damage.  

In general, if you want an object to be taken to the garbage, you need to
break all links to strong objects.  Then you can force a garbage collection
by running:  Utilities garbageCollectAndReport.

What is a strong reference you ask?  That is anything that is not a weak
reference.  You can hold onto an object using a weakArray but when the
garbage collector comes around she can pull it right outa there.  So, simply
nil-ing out references to your object and closing it (since an inspector or
even the debugger has a strong reference to it), should allow the garbage
collector to clean things up for you.

Databases are another animal all together.  If you pull something from a
database that has another object attached to it, say a person holds onto an
address, then the database can not simply pull in every connection because
in most cases that would mean pulling in the entire database on every call.
So instead databases implement something called a Proxy.  A Proxy is a
stand-in for an object and it gets de-proxified when the object is touched.
So in my example having a person object with and address ivar, if I don’t
look at the ivar, (starts to feel like quantum mechanics huh), then the
address is a proxy, which tells the database where to get the actual address
object.  Now in this case you can usually get rid of the object without
breaking the link, by sending proxyify or becomeProxy, or sometimes I’ve
seen zap.  These methods are implemented by your db provider and you will
find it on the database object that all your database objects inherit from.

Ok from your question it would seem to me that you have a connection object,
not a persistent data object that just won’t go away.  You could find the
references to that object and nil them out.  This is the preferred method.
You could look at the clean up code on that object and see if you can figure
out how the database vendor does it.  Methods like close, cleanUp,
disconnect, or something like that will give you and idea of what they
wanted to do if it hadn’t blown up on you.  Follow the path that you would
normally go down for disconnecting when you don’t blow up.

Now for the really dangerous, not for the faint of heart, back up your
entire computer before you even read this, there is become: !

(are you scared?)  Become says change this object ID into an object ID of
something else!  This is how proxies usually work.  The database grabs and
instantiates an object from disk then it tells the proxy to become the
newObject.  (aProxy become: newObject).  In this case everybody is happy
cause nobody wants a proxy they want the new object.  Now kids don’t try
this at home!  You can force a garbage collect of your connection object by
doing aConnectionObject become: String new.  The issues here are pretty
simple, you could type this wrong and blow away your whole String class.
You could leave your database objects looking for a connection on an empty
string.  But if as you say it is broken anyway, it can’t get worse!

Ok now before I go I have to tell you a story.  I worked with a lovely
object database that persisted string literals passed out of methods.  A
method that contains a string compiles that string into a literal and uses
that reference internally but you could send it out and make it persistent
on my lovely object database.  

For example try this: 

On some class create the method

Object class >> ron

            “Return a string literal”

            ^’Ron is a great guy’ 

Then do this 

Object ron at: 11 put: $e.

Object ron at: 13 put: $k.

Object ron at: 14 put: $y.

Ok so now look at the method ron it hasn’t changed, but when you print
Object ron, you get something completely different, and not very flattering
either.  This is because the method contains a string literal.  

So when my database grabbed my string literal and made it persistent it
meant that I couldn’t open the methods in a browser until connected to the
database!  Be careful of string literals.  My colleagues still make fun of
me because I always put a copy after a string literal. 

Ron

            ^’Ron is a great guy’ copy

So that I never pass out a modifiable literal!

Ok enough of that I hope that helps!

Happy Coding!!

Ron Teitelbaum

President / Principal Software Engineer

US Medical Record Specialists 

Ron at USMedRec.com 

  _____  

From: cdrick
Sent: Tuesday, May 09, 2006 10:42 AM

Hi...

I d like to know if there is a way to force garbage collection of objects
(still referenced)...
I often need that when triing to connect ot a database for instance... If it
fails, the connection object is still in memory 

Thanks

Cédrick

-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/beginners/attachments/20060509/6d483a24/attachment.htm