NAME¶

std::experimental::atomic_weak_ptr::compare_exchange_strong, - std::experimental::atomic_weak_ptr::compare_exchange_strong,

Synopsis¶

bool compare_exchange_weak( std::weak_ptr<T>& expected, const std::weak_ptr<T>&
desired, (1)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_weak( std::weak_ptr<T>& expected, std::weak_ptr<T>&&
desired, (2)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_weak( std::weak_ptr<T>& expected, const std::weak_ptr<T>&
desired, (3)
std::memory_order order = std::memory_order_seq_cst
) noexcept;
bool compare_exchange_weak( std::weak_ptr<T>& expected, std::weak_ptr<T>&&
desired, (4)
std::memory_order order = std::memory_order_seq_cst
) noexcept;
bool compare_exchange_strong( std::weak_ptr<T>& expected, const
std::weak_ptr<T>& desired, (5)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_strong( std::weak_ptr<T>& expected, std::weak_ptr<T>&&
desired, (6)
std::memory_order success, std::memory_order
failure ) noexcept;
bool compare_exchange_strong( std::weak_ptr<T>& expected, const
std::weak_ptr<T>& desired, (7)
std::memory_order order =
std::memory_order_seq_cst ) noexcept;
bool compare_exchange_strong( std::weak_ptr<T>& expected, std::weak_ptr<T>&&
desired, (8)
std::memory_order order =
std::memory_order_seq_cst ) noexcept;

Atomically compares the underlying weak_ptr in *this with expected, and if they are
equivalent, replaces the former with desired (performs read-modify-write operation).
Otherwise, loads the actual value stored in *this into expected (performs load
operation). The replacement is performed as if by the copy or move assignment
operator of weak_ptr, as appropriate.

Two weak_ptrs are equivalent if and only if they store the same pointer value and
share ownership.

The memory models for the read-modify-write and load operations are success and
failure respectively. For overloads (3,4,7,8), order is used for both
read-modify-write and load operations, except that std::memory_order_acquire and
std::memory_order_relaxed are used for the load operation if order ==
std::memory_order_acq_rel, or order == std::memory_order_release respectively.

The weak versions (1-4) may fail spuriously.

Parameters¶

expected - reference to the value expected to be found in the atomic object
desired - the value to store in the atomic object if it is as expected
success - the memory synchronization ordering for the read-modify-write operation
if the comparison succeeds. All values are permitted
the memory synchronization ordering for the load operation if the
failure - comparison fails. Cannot be std::memory_order_release or
std::memory_order_acq_rel and cannot specify stronger ordering than
success
order - the memory synchronization ordering for both operations

Return value¶

true if the underlying atomic value was changed, false otherwise.

Remarks

All changes to the atomic_weak_ptr object itself, and all associated use_count
increments, are guaranteed to be performed atomically. Associated use_count
decrements take place after the atomic operation, but are not required to be part of
it. Any associated destruction or deallocation operations take place after the
atomic operation and are not part of it.

If the compare-exchange operation return true, expected is not accessed after the
atomic update step. If it returns false, expected is updated with the existing value
read from the atomic_weak_ptr object in the attempted atomic update. The use_count
update corresponding to the write to expected is part of the atomic operation, but
the write to expected itself is not required to be part of the atomic operation.

For overloads (1,3,5,7), desired is not accessed after the atomic update step.

For overloads (2,4,6,8), desired is moved from only if the compare-exchange
operation returns true; the moving occurs after the atomic update step.

Notes¶

The weak forms (1-4) of the functions are allowed to fail spuriously, that is, act
as if *this and expected are not equivalent even when they are. When a
compare-and-exchange is in a loop, the weak version will yield better performance on
some platforms.

See also¶

std::atomic_is_lock_free(std::shared_ptr)
std::atomic_load(std::shared_ptr)
std::atomic_load_explicit(std::shared_ptr)
std::atomic_store(std::shared_ptr)
std::atomic_store_explicit(std::shared_ptr) specializes atomic
std::atomic_exchange(std::shared_ptr) operations for
std::atomic_exchange_explicit(std::shared_ptr) std::shared_ptr
std::atomic_compare_exchange_weak(std::shared_ptr) (function template)
std::atomic_compare_exchange_strong(std::shared_ptr)
std::atomic_compare_exchange_weak_explicit(std::shared_ptr)
std::atomic_compare_exchange_strong_explicit(std::shared_ptr)
(deprecated in C++20)
(removed in C++26)