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 (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?
>
>
You are right, but in this case there is no problem to block as the
future is already ready.
Best,
Vicente