This page is a snapshot from the LWG issues list, see the Library Active Issues List for more information and the meaning of Resolved status.
use_count and unique in shared_ptrSection: 20.3.2.2 [util.smartptr.shared] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06
Priority: Not Prioritized
View all other issues in [util.smartptr.shared].
View all issues with Resolved status.
Discussion:
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 makeunique() 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.
Proposed resolution: