P0521r0
Proposed Resolution for CA 14 (shared_ptr use_count/unique)

This paper proposes a resolution to C++17 CD comment CA 14, which identified issues with use_count and unique in shared_ptr.

1. 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.

2. Proposed Wording

The proposed changes are relative to [N4604], the Committee Draft for C++17.

The � character is used to denote a placeholder number which the editor shall determine.

2.1. Part A

Change 20.11.2.2.5 [util.smartptr.shared.obs] as depicted:

long use_count() const noexcept;

-7- Returns: The number of shared_ptr objects, *this included, that share ownership with *this, or 0 when *this is empty.

-�- Synchronization: None.

-�- [ Note: get() == nullptr does not imply a specific return value of use_count(). — end note ]

-�- [ Note: weak_ptr<T>::lock() can affect the return value of use_count(). — end note ]

-�- [ Note: When multiple threads can affect the return value of use_count(), the result should be treated as approximate. In particular, use_count() == 1 does not imply that accesses through a previously destroyed shared_ptr have in any sense completed. — end note ]

bool unique() const noexcept;

-8- Returns: use_count() == 1.

-9- [ Note: If you are using unique() to implement copy on write, do not rely on a specific value when get() == nullptr. — end note ]

2.2. Part B

Change 20.11.2.2 [util.smartptr.shared]/1 as depicted:

namespace std {
  template<class T> class shared_ptr {
  public:
    [...]
    long use_count() const noexcept;
    bool unique() const noexcept;;
    explicit operator bool() const noexcept;
    [...]
  };
  [...]
} // namespace std

Change 20.11.2.2.5 [util.smartptr.shared.obs] as depicted:

bool unique() const noexcept;

Add a new section:

namespace std {
  template<class T> class shared_ptr {
  public:
    bool unique() const noexcept;
  };
}

bool unique() const noexcept;