[Newbies] Re: What is a "weak" reference

Klaus D. Witzel klaus.witzel at cobss.com
Tue Jul 15 22:44:21 UTC 2008

On Tue, 15 Jul 2008 23:21:17 +0200, nicolas cellier wrote:

> While we are at it, here is a lesson i learned recently:
>      | weak obj |
>      weak := WeakIdentityKeyDictionary new.
> "Create an Object"
>      obj := Object new.
> "Add a weak reference to this Object"
>      weak at: obj put: (Array with: obj).
> "This DoIt methods points to the Object via it's temporary variable.
> Clear this pointer, so that the Object can eventually be reclaimed"
>      obj := nil.
> "Now garbageCollect to reclaim the weak references"
>      Smalltalk garbageCollect.
> "Let us see if the Object was reclaimed:"
>      ^weak size
> Why the object obj was not reclaimed?

Perhaps because WeakIdentityKeyDictionary (as well as WeakKeyDictionary)  
instance variable is not initialized/assigned, as its name suggests  
something Weak* ;)

Do (WeakIdentityKeyDictionary new inspect) and check for Weak* things:  
none here. This is so in the .image version that I'm using right now,  
'Squeak3.10.2', haven't checked others.

> Obviously, the WeakKeyAssociation value is not weak...
> It is a strong pointer and points strongly to obj through the Array...

Or it's just a Weak* instance variable bug :( anybody confirm this?

> Trivial, you might say.
> Well yes, it is just http://bugs.squeak.org/view.php?id=7119 in disguise  
> and i find it nasty.
> So be very carfull with Weak references - not only newbs - issues might  
> definitely be advanced to track down...
> Nicolas

More information about the Beginners mailing list