Proposed Resolution for CA 14 (shared_ptr use_count/unique) (original) (raw)

Published Proposal, 11 November 2016

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http://wg21.link/P0521

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Audience:

SG1, LEWG, LWG

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ISO JTC1/SC22/WG21: Programming Language C++

Table of Contents

  1. 1 CA 14
  2. 2 Proposed Wording
    1. 2.1 Part A
    2. 2.2 Part B
  3. References
    1. Informative References

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 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;

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

Add a new section:

D.� Deprecated shared_ptr observers [depr.util.smartptr.shared.obs]

The following member is defined in addition to those specified in [util.smartptr.shared]:

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

bool unique() const noexcept;

Returns: use_count() == 1.

References

Informative References

[N4604]

Richard Smith. C++17 CD Ballot Document. 12 July 2016. URL: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4604.pdf