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2315. weak_ptr should be movable

Section: 20.3.2.3 [util.smartptr.weak] Status: C++14 Submitter: Stephan T. Lavavej Opened: 2013-09-21 Last modified: 2016-01-28

Priority: 2

View all other issues in [util.smartptr.weak].

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Discussion:

Like shared_ptr, weak_ptr should be movable to avoid unnecessary atomic increments/decrements of the weak refcount.

[2014-02-13 Issaquah: Move to Immediate]

Proposed resolution:

This wording is relative to N3691.

1. Edit 20.3.2.3 [util.smartptr.weak]/1, class template weak_ptr synopsis, as indicated:

   namespace std {
     template<class T> class weak_ptr {
     public:
       typedef T element_type;
   
       // 20.9.2.3.1, constructors
       constexpr weak_ptr() noexcept;
       template<class Y> weak_ptr(shared_ptr<Y> const& r) noexcept;
       weak_ptr(weak_ptr const& r) noexcept;
       template<class Y> weak_ptr(weak_ptr<Y> const& r) noexcept;
       weak_ptr(weak_ptr&& r) noexcept;
       template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;
   
       […]
   
       // 20.9.2.3.3, assignment
       weak_ptr& operator=(weak_ptr const& r) noexcept;
       template<class Y> weak_ptr& operator=(weak_ptr<Y> const& r) noexcept;
       template<class Y> weak_ptr& operator=(shared_ptr<Y> const& r) noexcept;
       weak_ptr& operator=(weak_ptr&& r) noexcept;
       template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
     };
   }
   

2. Add the following new paragraphs at the end of sub-clause 20.3.2.3.2 [util.smartptr.weak.const]:

   weak_ptr(weak_ptr&& r) noexcept;
   template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;
   

   -?- Remark: The second constructor shall not participate in overload resolution unless Y* is implicitly convertible to T*.

   -?- Effects: Move-constructs a weak_ptr instance from r.

   -?- Postconditions: *this shall contain the old value of r. r shall be empty. r.use_count() == 0.

3. Edit 20.3.2.3.4 [util.smartptr.weak.assign] as indicated:

   weak_ptr& operator=(const weak_ptr& r) noexcept;
   template<class Y> weak_ptr& operator=(const weak_ptr<Y>& r) noexcept;
   template<class Y> weak_ptr& operator=(const shared_ptr<Y>& r) noexcept;
   

   -1- Effects: […]

   -2- Remarks: […]

   -?- Returns: *this.

   weak_ptr& operator=(weak_ptr&& r) noexcept;
   template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept;
   

   -?- Effects: Equivalent to weak_ptr(std::move(r)).swap(*this).

   -?- Returns: *this.