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2434. shared_ptr::use_count() is efficient

Section: 20.3.2.2.6 [util.smartptr.shared.obs] Status: C++17 Submitter: Stephan T. Lavavej Opened: 2014-10-01 Last modified: 2017-07-30

Priority: 0

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Discussion:

shared_ptr and weak_ptr have Notes that their use_count() might be inefficient. This is an attempt to acknowledge reflinked implementations (which can be used by Loki smart pointers, for example). However, there aren't any shared_ptr implementations that use reflinking, especially after C++11 recognized the existence of multithreading. Everyone uses atomic refcounts, so use_count() is just an atomic load.

[Urbana 2014-11-07: Move to Ready]

Proposed resolution:

This wording is relative to N3936.

1. Change 20.3.2.2.6 [util.smartptr.shared.obs] p7-p10 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.

   -8- [Note: use_count() is not necessarily efficient. — end note]

   bool unique() const noexcept;
   

   -9- Returns: use_count() == 1.

   -10- [Note: unique() may be faster than use_count(). If you are using unique() to implement copy on write, do not rely on a specific value when get() == 0. — end note]

2. Change 20.3.2.3.6 [util.smartptr.weak.obs] p1-p4 as depicted:

   long use_count() const noexcept;
   

   -1- Returns: 0 if *this is empty; otherwise, the number of shared_ptr instances that share ownership with *this.

   -2- [Note: use_count() is not necessarily efficient. — end note]

   bool expired() const noexcept;
   

   -3- Returns: use_count() == 0.

   -4- [Note: expired() may be faster than use_count(). — end note]