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### 2810. `use_count` and `unique` in `shared_ptr`

Section: 20.3.2.2 [util.smartptr.shared] Status: Resolved Submitter: Marshall Clow Opened: 2016-11-09 Last modified: 2020-09-06

Priority: Not Prioritized

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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 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.

Proposed resolution: