Le 24/10/12 13:35, Andrzej Krzemienski a écrit :
2012/10/14 Andrzej Krzemienski <akrzemi1@gmail.com>
2012/10/14 Vicente J. Botet Escriba <vicente.botet@wanadoo.fr>
Le 13/10/12 23:17, Andrzej Krzemienski a écrit :
2012/10/13 Vicente J. Botet Escriba <vicente.botet@wanadoo.fr>
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