Re: [boost] [thread] terminating destructor

Subject: Re: [boost] [thread] terminating destructor
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2012-10-24 07:35:04

2012/10/14 Andrzej Krzemienski <akrzemi1_at_[hidden]>

>
>
> 2012/10/14 Vicente J. Botet Escriba <vicente.botet_at_[hidden]>
>
> Le 13/10/12 23:17, Andrzej Krzemienski a écrit :
>>
>>  2012/10/13 Vicente J. Botet Escriba <vicente.botet_at_[hidden]>
>>>
>>> I guess you are referring to the case the std::future is created by async
>>>>
>>>> 0.
>>>>
>>>>     If the implementation chooses the launch::async policy,
>>>>
>>>>       *
>>>>
>>>>         — a call to a waiting function on an asynchronous return object
>>>>         that shares the shared state created
>>>>
>>>>         by this async call shall block until the associated thread has
>>>>         completed, as if joined (30.3.1.5);
>>>>
>>>>
>>>> C++ International Standard Otherwise
>>>>
>>>> ~std::future();
>>>> Effects:
>>>> — releases any shared state (30.6.4);
>>>> — destroys *this.
>>>>
>>>> Could you explain me what waiting function is called on the future
>>>> destructor that needs to block until the completion of the associated
>>>> thread?
>>>>
>>>
>>> It is not any waiting function mentioned explicitly. It is the
>>> requirement
>>> in 30.6.8 para 5: "If the implementation chooses the launch::async
>>> policy,
>>> [...] the associated thread completion synchronizes with the return from
>>> the first function that successfully detects the ready status of the
>>> shared
>>> state or with the return from the last function
>>> that releases the shared state, whichever happens first."
>>>
>>>  Why the future destructor should be the last function that release the
>> shared state?
>
>
> Indeed, it doesn't have to be the destructor. I misinterpret this
> requirement. Sorry. I no longer claim that future's destructor blocks in
> this case. Although, others (like Herb) seem to claim that. I will
> investigate that.
>

Or perhaps future destructor IS the last function that release the shared
state. When we call async() two threads are involved: our 'master' thread
and a newly launched thread. Whatever function(s) releases the shared state
it has to do it from one of the two threads. The last release cannot be
made from the 'launched' thread because 'launched' thread completion
synchronizes with the last release. So the last release has to be performed
from the 'master' thread. And what other operation in the 'master' thread
apart from future's destructor can release the state?

Regards,
&rzej