Re: [boost] Do you know of standard implementation of async() that blocks on the future-destructor?

Subject: Re: [boost] Do you know of standard implementation of async() that blocks on the future-destructor?
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2015-10-18 14:18:49

Le 13/10/15 00:04, Agustín K-ballo Bergé a écrit :
> 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.
>
Thanks Agustin for clarifications,
Vicente