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Subject: Re: [boost] Do you know of standard implementation of async() that blocks on the future-destructor?
From: Agustín K-ballo Bergé (kaballo86_at_[hidden])
Date: 2015-10-12 18:04:50

On 10/12/2015 6:42 PM, Vicente J. Botet Escriba wrote:
> Le 12/10/15 15:57, Agustín K-ballo Bergé a écrit :
>> On 10/11/2015 9:02 PM, Vicente J. Botet Escriba wrote:
>>> Hi,
>>> all is in the title.
>> Don't they all? Blocking in the future destructor is what the standard
>> mandates.
> I'm confused. Do they block on the future destructor or the destructor
> of the shared state?

They may block on the destructor of the shared state:

30.6.4 [futures.state]/5
When an asynchronous return object or an asynchronous provider is said
to release its shared state, it means:
- [...]
- these actions will not block for the shared state to become ready,
except that it may block if all of the following are true: the shared
state was created by a call to `std::async`, the shared state is not yet
ready, and this was the last reference to the shared state.

You can think of `std::async` conceptually being implemented as having
an `std::thread` within the shared state, which chooses to join on
destruction to avoid termination.

> The note of the standard says on the destructor of the returned future
> > "[ Note: If a future obtained from std::async is moved outside the
> local scope, other code that uses the future must be aware that the
> future’s destructor may block for the shared state to become ready. —
> end note ]"

Notes are not normative. A future's destructor is where you'd most
likely notice the block, but it could happen during assignment too.


Agustín K-ballo Bergé.-

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