I don't *know*, but it does seem possible that a garbage collector could 
follow the class field of every marked object and mark those classes as 
well. Maybe VW does that? Not necessary, of course, in ordinary 
Smalltalk as classes are generally strongly held by the eponymous root.


On 3/27/18 6:33 AM, Nicolas Cellier wrote:
> Hi Bob,
> yes, you force me to rethink. The way mark and sweep GC works, cycles 
> in object graphs do not matter.
> As long as the objects in the cycles are only pointed to weakly from 
> roots,
>
> | value weak array2 |
> value := Array new.
> weak := WeakArray with: value.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 1. "Not yet reclaimed, 
> thisContext points to it thru value variable"
> value := nil.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 0. "This time, it is reclaimed"
>
> "Example of cycle"
> value := Array new: 1.
> array2 := Array with: value.
> value at: 1 put: array2.
> weak := WeakArray with: value.
> array2 := nil.
> value := nil.
> 3 timesRepeat: [Smalltalk garbageCollect].
> self assert: (weak count: #notNil) = 0. "The value is reclaimed 
> despite the cycle"
>
> It's then VW that behaves strangely: I did not find any pointer to the 
> class nor its metaclass apart themselves
> (I took care to not add the metaclass to metaclass superclass 
> subclasses, because those are not weak).
> This has obscured my mind.
> Sorry for the noise.
>
> 2018-03-26 23:15 GMT+02:00 Bob Arning <arning315 at comcast.net 
> <mailto:arning315 at comcast.net>>:
>
>     Other that the WeakValueDictionary, is there anything referencing
>     either the class or metaclass? If not, then they would be
>     collected. Perhaps VW is saving a reference elsewhere?
>
>
>     On 3/26/18 3:07 PM, Nicolas Cellier wrote:
>>     Hi all,
>>     while porting MatFileReader to Squeak, I encountered this
>>     questionable behaviour:
>>
>>     I create classes on the fly for holding Matlab struct().
>>     I want those classes to be garbage collected when no more used.
>>
>>     If I store the classes in a WeakArray, then this should normally
>>     not work:
>>     I mean that the classes should not be reclaimed.
>>     Why?
>>     Because the metaclass points strongly to the class (via thisClass
>>     variable).
>>     And the metaclass can't be reclaimed either, because it still has
>>     an instance.
>>     That's what happens in Visualworks, classes put in a WeakArray
>>     are never reclaimed.
>>     Fortunately, there is some Ephemeron support in VW to work around
>>     this (such loop are detected by ephemerons, and strong references
>>     by some object ephemerely pointed to does not count).
>>
>>     In Squeak, I used a WeakValueDictionary, and to my surprise,
>>     discovered that the classes were reclaimed.
>>     Is this a bug?
>>
>>     Nicolas
>>
>>
>>
>>
>
>
>
>
>
>
>

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