20 Memory management library [mem]

20.3 Smart pointers [smartptr]

20.3.2 Shared-ownership pointers [util.sharedptr]

20.3.2.3 Class template weak_ptr [util.smartptr.weak]

20.3.2.3.1 General [util.smartptr.weak.general]

The weak_ptr class template stores a weak reference to an object that is already managed by a shared_ptr.
To access the object, a weak_ptr can be converted to a shared_ptr using the member function lock.
namespace std { template<class T> class weak_ptr { public: using element_type = remove_extent_t<T>; // [util.smartptr.weak.const], constructors constexpr weak_ptr() noexcept; template<class Y> weak_ptr(const shared_ptr<Y>& r) noexcept; weak_ptr(const weak_ptr& r) noexcept; template<class Y> weak_ptr(const weak_ptr<Y>& r) noexcept; weak_ptr(weak_ptr&& r) noexcept; template<class Y> weak_ptr(weak_ptr<Y>&& r) noexcept; // [util.smartptr.weak.dest], destructor ~weak_ptr(); // [util.smartptr.weak.assign], assignment 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; weak_ptr& operator=(weak_ptr&& r) noexcept; template<class Y> weak_ptr& operator=(weak_ptr<Y>&& r) noexcept; // [util.smartptr.weak.mod], modifiers void swap(weak_ptr& r) noexcept; void reset() noexcept; // [util.smartptr.weak.obs], observers long use_count() const noexcept; bool expired() const noexcept; shared_ptr<T> lock() const noexcept; template<class U> bool owner_before(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_before(const weak_ptr<U>& b) const noexcept; size_t owner_hash() const noexcept; template<class U> bool owner_equal(const shared_ptr<U>& b) const noexcept; template<class U> bool owner_equal(const weak_ptr<U>& b) const noexcept; }; template<class T> weak_ptr(shared_ptr<T>) -> weak_ptr<T>; }
Specializations of weak_ptr shall be Cpp17CopyConstructible and Cpp17CopyAssignable, allowing their use in standard containers.
The template parameter T of weak_ptr may be an incomplete type.