Keeping a list of instances and garbage-collection

[grauzone]

Christopher Wright wrote:
> grauzone wrote:
>> Jarrett Billingsley wrote:
>>> On Sun, Mar 29, 2009 at 4:42 PM, Leandro Lucarella <llucax at gmail.com> 
>>> wrote:
>>>> This was discussed several times in the past. For example:
>>>> http://www.digitalmars.com/d/archives/digitalmars/D/learn/weak_references_13301.html 
>>>>
>>>> http://www.digitalmars.com/d/archives/digitalmars/D/learn/Soft_weak_references_8264.html 
>>>>
>>>> http://www.digitalmars.com/d/archives/digitalmars/D/announce/ANN_WeakObjectReference_-_class_to_hold_weak_references_9103.html 
>>>>
>>>> etc.
>>>>
>>>> I hope it helps.
>>>
>>> The one provided by Bill:
>>>
>>> http://www.dsource.org/projects/scrapple/browser/trunk/weakref
>>>
>>> seems to work fine, and has the advantage of working in both Phobos 
>>> and Tango.
>>
>> First, I doubt this actually works. The WeakRef stores the pointer as 
>> size_t, but the GC is conservative and will still recognize the size_t 
>> as a pointer. The unittest in the existing code only works, because he 
>> uses an explicit delete on the referenced object.
> 
> If the WeakRef is on the stack, this is true.
> 
> If the WeakRef is part of an aggregate type that contains pointers, this 
> is true.

If WeakRef is a class, this is also true. Because all objects contain a 
hidden monitor pointer, and the monitor is subject to garbage collection 
AFAIK.

> Otherwise, the GC will see that the relevant block is marked as having 
> no pointers.
> 
> True -- weakref is a difficult thing to make thread-safe.

It seems there's still work to do, and a thread-safe WeakRef can't be 
created with the current interfaces. Is this true?

I'm thinking rt_attachDisposeEvent() should take a *pointer* to the 
reference instead of the reference itself (effectively a double 
pointer), and clear this pointer during garbage collection, when all 
threads are still globally locked.