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Subject: Re: [boost] [thread] terminating destructor
From: Vicente J. Botet Escriba (vicente.botet_at_[hidden])
Date: 2012-10-24 15:12:13

Le 24/10/12 13:35, Andrzej Krzemienski a écrit :
> 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 (;
>>>>> 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?
You are right, but in this case there is no problem to block as the
future is already ready.


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