# std::experimental::atomic_weak_ptr::compare_exchange_strong, std::experimental::atomic_weak_ptr::compare_exchange_weak

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< cpp‎ | experimental‎ | atomic weak ptr

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

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_ptr`s 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.
failure | - | the memory synchronization ordering for the load operation if the 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.

### Exceptions

`noexcept` specification:
noexcept

### 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)std::atomic_exchange(std::shared_ptr)std::atomic_exchange_explicit(std::shared_ptr)std::atomic_compare_exchange_weak(std::shared_ptr)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)

specializes atomic operations for std::shared_ptr
(function template)

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