20 General utilities library [utilities]

20.11 Smart pointers [smartptr]

20.11.4 Class template weak_­ptr [util.smartptr.weak]

20.11.4.2 Constructors [util.smartptr.weak.const]

constexpr weak_ptr() noexcept;
Effects: Constructs an empty weak_­ptr object that stores a null pointer value.
Postconditions: use_­count() == 0.
weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept;
Constraints: For the second and third constructors, Y* is compatible with T*.
Effects: If r is empty, constructs an empty weak_­ptr object that stores a null pointer value; otherwise, constructs a weak_­ptr object that shares ownership with r and stores a copy of the pointer stored in r.
Postconditions: use_­count() == r.use_­count().
weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept;
Constraints: For the second constructor, Y* is compatible with T*.
Effects: Move constructs a weak_­ptr instance from r.
Postconditions: *this contains the old value of r.
r is empty, stores a null pointer value, and r.use_­count() == 0.