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2046. shared_future(future<R>&&) should be allowed to throw

Section: 30.6.8 [futures.shared_future] Status: NAD Submitter: Peter Sommerlad Opened: 2011-04-04 Last modified: 2016-02-10

Priority: Not Prioritized

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Requiring the constructor shared_future(future<R>&& rhs) not to throw is a pessimisation of the case where a future is returned from a call to async(function,launch::deferred) and possible other cases.

Such a future not dealing with multiple threads only needs to keep (a copy of) the function to be called it later. However, creating a shared_future from that future will require more infrastructure, like space for the value of type R, an exception_ptr, and a synchronized reference counter for the shared_future's instances.

Enforcing the constructor shared_future(future<R>&& rhs) not to throw, implies that any implementation of future will need to pre-allocate space for shared_future's infrastructure, that also requires an operating system resource for synchronization, regardless if is ever needed.

All this came up when discussing D/N3267 and Concurrency Working Group decided that the constructor shared_future(future<R>&& rhs) should be allowed to throw.

[ 2011 Bloomington ]

Closed as NAD. Rationale to follow by email...

Proposed resolution:

Apply the proposed resolution of n3269