constexpr weak_ptr() noexcept;
Effects: Constructs an empty weak_ptr object.
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;
Remark: The second and third constructors shall not participate in overload resolution unless Y* is implicitly convertible to T*.
Effects: If r is empty, constructs an empty weak_ptr object; 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;
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.