async do if neither 'async' nor 'deferred' is set in policy?Section: 32.10.9 [futures.async] Status: C++14 Submitter: Jonathan Wakely Opened: 2012-01-01 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Implementations already disagree, one returns an invalid future with
no shared state, one chooses policy == async and one chooses policy ==
deferred, see c++std-lib-30839, c++std-lib-30840 and c++std-lib-30844.
It's not clear if returning an invalid future is allowed by the current wording.
If the intention is to allow an empty future to be returned, then 32.10.9 [futures.async] p3 and p4 should be adjusted to clarify that a shared state might not be created and an invalid future might be returned.
If the intention is that a valid future is always returned, p3 should say something about the case where none of the conditions applies.
[2012, Portland: move to Review]
We could make it undefined if no launch policy is defined.
Hans: If no launch policy is specified the behaviour is undefined
Artur: or implementation defined?
Hans: no: we don't want people to do this
[Proposed wording]
This wording is relative to N3376
Add a third bullet to the end of the list in 30.6.8p3
"if no valid launch policy is provided the behaviour is undefined"
Moved to review
[2013-04-19, Bristol]
Detlef provides new wording
Previous wording:
[This wording is relative to N3376]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– if no valid launch policy is provided the behaviour is undefined
[2013-09 Chicago]
If no policy is given, it should be undefined, so moved to Immediate.
Accept for Working Paper
Proposed resolution:
[This wording is relative to N3485]
Add a third bullet to the end of the list in 32.10.9 [futures.async]p3
– If no value is set in the launch policy, or a value is set that is neither specified in this International Standard or by the implementation, the behaviour is undefined.