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Section: 33.6 [futures] Status: Resolved Submitter: Alisdair Meredith Opened: 2009-03-12 Last modified: 2017-02-03
Priority: Not Prioritized
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Addresses UK 329 [CD1]
future, promise and packaged_task provide a framework for creating future values, but a simple function to tie all three components together is missing. Note that we only need a simple facility for C++0x. Advanced thread pools are to be left for TR2.
Provide a simple function along the lines of:
template< typename F, typename ... Args > requires Callable< F, Args... > future< Callable::result_type > async( F&& f, Args && ... );
Semantics are similar to creating a thread object with a packaged_task invoking f with forward<Args>(args...) but details are left unspecified to allow different scheduling and thread spawning implementations.
It is unspecified whether a task submitted to async is run on its own thread or a thread previously used for another async task. If a call to async succeeds, it shall be safe to wait for it from any thread.
The state of thread_local variables shall be preserved during async calls.
No two incomplete async tasks shall see the same value of this_thread::get_id().
[Note: this effectively forces new tasks to be run on a new thread, or a fixed-size pool with no queue. If the library is unable to spawn a new thread or there are no free worker threads then the async call should fail. --end note]
[ Summit: ]
The concurrency subgroup has revisited this issue and decided that it could be considered a defect according to the Kona compromise. A task group was formed lead by Lawrence Crowl and Bjarne Stroustrup to write a paper for Frankfort proposing a simple asynchronous launch facility returning a future. It was agreed that the callable must be run on a separate thread from the caller, but not necessarily a brand-new thread. The proposal might or might not allow for an implementation that uses fixed-size or unlimited thread pools.
Bjarne in c++std-lib-23121: I think that what we agreed was that to avoid deadlock async() would almost certainly be specified to launch in a different thread from the thread that executed async(), but I don't think it was a specific design constraint.
[ 2009-10 Santa Cruz: ]
Proposed resolution: see N2996 (Herb's and Lawrence's paper on Async). Move state to