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Re: [std-proposals] std::shared_ptr resurrect

From: Marcin Jaczewski <marcinjaczewski86_at_[hidden]>
Date: Mon, 11 Dec 2023 20:42:23 +0100
pon., 11 gru 2023 o 18:26 Thiago Macieira via Std-Proposals
<std-proposals_at_[hidden]> napisaƂ(a):
> On Monday, 11 December 2023 08:45:31 PST Valentin Palade wrote:
> > Doing a local copy as Jonathan suggests, will make no difference, with
> > threads running concurrently, we can still have use_count() return anything
> > between 1 and 5.
> >
> > Or am I missing something?
> No. It can be between 1 and 5.
> But if it's 1, then you have the only copy and there is no other thread seeing
> this object. The number can't change unless the current thread changes it.

But all threads can return 5 too. As check is done before decrement
every one can observe the state before it.
And this means no thread can get 1 and acquire the last instance.
We need to guarantee that one thread will be the least instance.

Probably best will be a combination of two approaches:
move away from the old pointer and then use `resurect` on a new instance.
Then we have a guarantee that exactly one thread will get the last instance.
(and this operation can be atomic if we return this new instance of pointer)

btw small bikeshedding, I think this operation should be named
"collapse" like in wave-function where you have multiple
place where you can detect particle (pointers copies)
but when you try to detect where a particle is, only one detector
can have it (wave-function collapses to one point).

> --
> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> Software Architect - Intel DCAI Cloud Engineering
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> Std-Proposals mailing list
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Received on 2023-12-11 19:42:36