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Section: 32.6.7.2 [thread.once.callonce] Status: C++11 Submitter: INCITS Opened: 2010-08-25 Last modified: 2016-01-28
Priority: Not Prioritized
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Discussion:
Addresses US-190
The term "are serialized" is never defined (32.6.7.2 [thread.once.callonce] p. 2).
[ Resolution proposed by ballot comment: ]
Remove the sentence with "are serialized" from
paragraph 2. Add "Calls to call_once on the same
once_flag object shall not introduce data races
(17.6.4.8)." to paragraph 3.
[ 2010-11-01 Daniel translates NB comment into wording ]
[ 2011-02-17: Hans proposes an alternative resolution ]
[ 2011-02-25: Hans, Clark, and Lawrence update the suggested wording ]
[2011-02-26 Reflector discussion]
Moved to Tentatively Ready after 5 votes.
Proposed resolution:
Change 32.6.7.2 [thread.once.callonce] p.2+3 as indicated:
template<class Callable, class ...Args> void call_once(once_flag& flag, Callable&& func, Args&&... args);[..]
2 Effects:Calls to3 Synchronization:call_onceon the sameonce_flagobject are serialized. If there has been a prior effective call tocall_onceon the sameonce_flag object, the call tocall_oncereturns without invokingfunc. If there has been no prior effective call tocall_onceon the sameonce_flagobject,INVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)is executed. The call tocall_onceis effective if and only ifINVOKE(decay_copy( std::forward<Callable>(func)), decay_copy(std::forward<Args>(args))...)returns without throwing an exception. If an exception is thrown it is propagated to the caller.The completion of an effective call tocall_onceon aonce_flagobject synchronizes with (6.10.2 [intro.multithread]) all subsequent calls tocall_onceon the sameonce_flagobject.