On Tue, Mar 31, 2009 at 7:37 AM, Christopher Wright <dhase...@gmail.com> wrote: > grauzone wrote: >> >> Christopher Wright wrote: >>> >>> grauzone wrote: >>>> >>>> Jarrett Billingsley wrote: >>>>> >>>>> On Sun, Mar 29, 2009 at 4:42 PM, Leandro Lucarella <llu...@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. > > Hold on, I think this isn't a real issue. > > You have essentially: > class WeakRef > { > const xor = 0x101010101; > size_t value; > bool collected; > Object toObject() > { > if (collected) return null; > auto ptr = cast(void*)(value ^ xor); > auto obj = *cast(Object*)&ptr; > if (collected) return null; > return obj; > } > } > > > After casting, you have a strong reference to the object, so it's impossible > for the object to be collected if it hasn't yet been collected. If the > object was collected between the two checks, you just have an invalid object > reference that nobody ever uses. As long as the compiler doesn't do nasty > reordering things, you should be fine.
Pardon my ignorance, but how do you know that ptr ^ xor is not a pointer to some other real object in memory? Does the language give you some guarantee that it won't ever be a valid pointer? --bb
