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Re: Possible issue in the definition of 'std::stop_source'

From: Raphael <raphael.rg91_at_[hidden]>
Date: Wed, 12 Oct 2022 13:43:51 -0300
On Wed, Oct 12, 2022 at 12:19 PM Jason McKesson via Std-Discussion <
std-discussion_at_[hidden]> wrote:

> On Wed, Oct 12, 2022 at 10:42 AM Raphael via Std-Discussion
> <std-discussion_at_[hidden]> wrote:
> >
> > Hi,
> >
> > the current definition of std::stop_source::stop_possible() [1] states
> that:
> >
> >> Returns: true if *this has ownership of a stop state; otherwise, false.
> >
> >
> > and the current definition of std::stop_source::stop_requested() [2]
> states that:
> >
> >> Returns: true if *this has ownership of a stop state that has received
> a stop request; otherwise, false.
> >
> >
> > from these requirements it appears that stop_requested() will never
> return true if stop_possible() return false, but this is exactly what is
> required as a post condition of std::stop_source::request_stop() [3], where
> it says:
> >
> >> Postconditions: stop_­possible() is false or stop_­requested() is true.
> Did you miss the "or" there? Where's the contradiction?
Oh... I did miss the "or" there. This is why I though that there was a

> > The issue here appears to be in the definition of
> std::stop_source::stop_possible() that should be made equivalent to the
> definition of std::stop_token::stop_possible() [4], something like:
> >
> >> Returns: true if *this has ownership of a stop state and a stop request
> was not made; otherwise, false.
> Perhaps you're being too literal about the words here. The question
> `stop_possible` is meant to answer is merely whether the `stop_source`
> is a valid, useful object. That is, it is an object that can feed
> `stop_token`s to request a stop.
> It does *not* answer whether such a stop has already been requested.
> That is, it does not answer whether `request_stop` will change the
> state of the `stop_source` and its attendant tokens.
This is exactly what I though that it was answering.

> So this is not a defect; it is intended behavior. If you want to ask
> whether the object controls stop tokens *and* has not yet been
> stopped, those are two separate questions according to the design.
> Probably because the combination of those two aren't terribly
> meaningful.
> `stop_possible` is a sanity check to see if the object is useful.
> `request_stop` is a directive to issue a stop to associated tokens. If
> you want to request a stop, a sanity check beforehand might be useful,
> but the check is just to see if the object is functional. Request the
> stop, and the return value will tell you if your thread is the one
> that first requested a stop.

This makes more sense than what I had in mind.
Thanks for your time and for the answer.

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Received on 2022-10-12 16:44:03