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### 1266. `shared_future::get` and deferred `async` functions

Section: 33.10.8 [futures.shared.future] Status: Resolved Submitter: Anthony Williams Opened: 2009-11-17 Last modified: 2021-06-06

Priority: Not Prioritized

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Discussion:

If a `shared_future` is constructed with the result of an `async` call with a deferred function, and two or more copies of that `shared_future` are created, with multiple threads calling `get()`, it is not clear which thread runs the deferred function. [futures.shared_future]p22 from N3000 says (minus editor's note):

> Effects: if the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready.

In the absence of wording to the contrary, this implies that every thread that calls `wait()` will execute the deferred function.

[ 2010 Pittsburgh: Moved to NAD Editorial Resolved. Rationale added below. ]

Rationale:

Solved by N3058.

Proposed resolution:

Replace [futures.shared_future]p22 with the following:

> Effects: If the associated state contains a deferred function, executes the deferred function. Otherwise, blocks until the associated state is ready. was created by a `promise` or `packaged_task` object, block until the associated state is ready. If the associated state is associated with a thread created for an `async` call (33.10.9 [futures.async]), as if `associated-thread.join()`.

> If the associated state contains a deferred function, calls to `wait()` on all `shared_future` objects that share the same associated state are serialized. The first call to `wait()` that shares a given associated state executes the deferred function and stores the return value or exception in the associated state.

> Synchronization: if the associated state was created by a `promise` object, the completion of `set_value()` or `set_exception()` to that `promise` happens before (6.9.2 [intro.multithread]) `wait()` returns. If the associated state was created by a `packaged_task` object, the completion of the associated task happens before `wait()` returns. If the associated state is associated with a thread created for an `async` call (33.10.9 [futures.async]), the completion of the associated thread happens-before `wait()` returns.

> If the associated state contained a deferred function, the invocation of the deferred function happens-before any call to `wait()` on a `future` that shares that state returns.