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Section: 19.11.3 [util.smartptr.shared] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2017-02-02
Priority: Not Prioritized
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Addresses CA 14
The removal of the "debug only" restriction for use_count() and unique() in shared_ptr introduced a bug: in order for unique() to produce a useful and reliable value, it needs a synchronize clause to ensure that prior accesses through another reference are visible to the successful caller of unique(). Many current implementations use a relaxed load, and do not provide this guarantee, since it's not stated in the Standard. For debug/hint usage that was OK. Without it the specification is unclear and misleading.
Proposed change:A solution could make unique() use memory_order_acquire, and specifying that reference count decrement operations synchronize with unique(). This won't provide sequential consistency but may be useful. We could also specify use_count() as only providing an unreliable hint of the actual count, or deprecate it.
[Issues Telecon 16-Dec-2016]
Resolved by adoption of P0521R0.