std::atomic<std::weak_ptr>

Defined in header `<memory>`
template < class T > struct std::atomic < std::weak_ptr <T>> ;		(since C++20)

The partial template specialization of std::atomic for std::weak_ptr<T>

If multiple threads of execution access the same std::weak_ptr object without synchronization and any of those accesses uses a non-const member function of weak_ptr then a data race will occur unless all such access is performed through an instance of std::atomic < std::weak_ptr >

Associated use_count increments are guaranteed to be part of the atomic operation. Associated use_count decrements are sequenced after the atomic operation, but are not required to be part of it, except for the use_count change when overriding expected

Note that the control block used by std::weak_ptr and std::shared_ptr is thread-safe: different non-atomic std::weak_ptr objects can be accessed using mutable operations, such as operator= or reset

The type T may be an incomplete type.

Member types

Member type	Definition
`value_type`	std::weak_ptr<T>

Member functions

All non-specialized std::atomic functions are also provided by this specialization, and no additional member functions.

atomic<weak_ptr<T>>::atomic

constexpr atomic( ) noexcept = default ;	(1)
atomic( std::weak_ptr <T> desired) noexcept ;	(2)
atomic(const atomic&) = delete;	(3)

1) Initializes the underlying weak_ptr<T> to default-constructed value.

2) Initializes the underlying weak_ptr<T> to a copy of desired. As with any std::atomic

3) Atomic types are not copy/move constructible.

atomic<weak_ptr<T>>::operator=

void operator= ( const atomic& ) = delete;	(1)
void operator= ( std::weak_ptr <T> desired) noexcept ;	(2)

1) Atomic types are not copy/move assignable.

2) Value assignment, equivalent to store(desired).

atomic<weak_ptr<T>>::is_lock_free

bool is_lock_free() const noexcept;

Returns true if the atomic operations on all objects of this type are lock-free, false otherwise.

atomic<weak_ptr<T>>::store

void store( std::weak_ptr <T> desired, std::memory_order order = std::memory_order_seq_cst ) noexcept ;

Atomically replaces the value of *this with the value of desired as if by p.swap(desired) where p is the underlying std::weak_ptr<T> . Memory is ordered according to order. The behavior is undefined if order is std::memory_order_consume, std::memory_order_acquire, or std::memory_order_acq_rel

atomic<weak_ptr<T>>::load

std::weak_ptr <T> load( std::memory_order order = std::memory_order_seq_cst ) const noexcept ;

Atomically returns a copy of the underlying std::weak_ptr<T> . Memory is ordered according to order. The behavior is undefined if order is std::memory_order_release or std::memory_order_acq_rel

atomic<weak_ptr<T>>::operator std::weak_ptr<T>

operator std::weak_ptr <T> ( ) const noexcept ;

Equivalent to return load();.

atomic<weak_ptr<T>>::exchange

std::weak_ptr <T> exchange( std::weak_ptr <T> desired, std::memory_order order = std::memory_order_seq_cst ) noexcept ;

Atomically replaces the underlying std::weak_ptr<T> with desired as if by p.swap(desired) where p is the underlying std::weak_ptr<T> , and returns a copy of the value that p had immediately before the swap. Memory is ordered according to order

atomic<weak_ptr<T>>::

bool compare_exchange_strong( std::weak_ptr <T> & expected, std::weak_ptr <T> desired, std::memory_order success, std::memory_order failure) noexcept ;	(1)
bool compare_exchange_weak( std::weak_ptr <T> & expected, std::weak_ptr <T> desired, std::memory_order success, std::memory_order failure) noexcept ;	(2)
bool compare_exchange_strong( std::weak_ptr <T> & expected, std::weak_ptr <T> desired, std::memory_order order = std::memory_order_seq_cst ) noexcept ;	(3)
bool compare_exchange_weak( std::weak_ptr <T> & expected, std::weak_ptr <T> desired, std::memory_order order = std::memory_order_seq_cst ) noexcept ;	(4)

1) If the underlying std::weak_ptr<T> stores the same pointer value as expected and shares ownership with it, or if both underlying and expected are empty, assigns from desired to the underlying std::weak_ptr<T>, returns true, and orders memory according to success, otherwise assigns from the underlying std::weak_ptr<T> to expected, returns false, and orders memory according to failure. The behavior is undefined if failure is std::memory_order_release or std::memory_order_acq_rel. On success, the operation is an atomic read-modify-write operation on *this and expected is not accessed after the atomic update. On failure, the operation is an atomic load operation on *this and expected is updated with the existing value read from the atomic object. This update to expected

2) Same as (1), but may also fail spuriously.

3) Equivalent to: return compare_exchange_strong(expected, desired, order, fail_order);, where fail_order is the same as order except that std::memory_order_acq_rel is replaced by std::memory_order_acquire and std::memory_order_release is replaced by std::memory_order_relaxed

4) Equivalent to: return compare_exchange_weak(expected, desired, order, fail_order);, where fail_order is the same as order except that std::memory_order_acq_rel is replaced by std::memory_order_acquire and std::memory_order_release is replaced by std::memory_order_relaxed

atomic<weak_ptr<T>>::wait

void wait( std::weak_ptr <T> old std::memory_order order = std::memory_order_seq_cst ) const noexcept ;

Performs an atomic waiting operation.

Compares load(order) with old and if they are equivalent then blocks until *this is notified by notify_one() or notify_all(). This is repeated until load(order)

Memory is ordered according to order. The behavior is undefined if order is std::memory_order_release or std::memory_order_acq_rel

Notes: two std::weak_ptrs are equivalent if they store the same pointer and either share ownership or are both empty.

atomic<weak_ptr<T>>::notify_one

void notify_one() noexcept;

Performs an atomic notifying operation.

If there is a thread blocked in atomic waiting operations (i.e. wait()) on *this

atomic<weak_ptr<T>>::notify_all

void notify_all() noexcept;