Re: Promoting weak pointers

Michiel Helvensteijn <m.helvensteijn@gmail.com> writes:


> On Jun 16, 12:39 pm, Andrew Tomazos <and...@tomazos.com> wrote:
>> if (my_weak_ptr != NULL)
>> my_weak_ptr->do_something();


> [Now I'm confused, too. This is an ordinary race condition of the kind
> that every parallel program has to handle, and would present the same
> issues no matter what kind of pointer it is. -John]


It seems different, because in this case, the race condition is caused
by the implementation (i.e. the garbage collector), not by the
application.


Say Thread A is doing the above 'if', and Thread B has a pointer X that
points to the same heap object as my_weak_ptr. At the same time as
the above 'if', Thread B sets X to null, and the GC gets triggered
for whatever reason, and the heap object gets collected.


The variable my_weak_ptr is not a shared variable. Thread B didn't
touch it. Thread B didn't touch the heap object, either.
So you wouldn't normally expect to need any locking.


The weak pointer is special, in that it can asynchronously get set to
null, even though no application-level thread modified it. So the
language needs some way to deal with them atomically, and Andrew Tomazos
is suggesting an atomic "convert weak to strong" operation.


Similarly, you could have an operation like "make this weak pointer
strong during the following block of code, then it reverts to weak".


- Bob

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