# weakCompareExchange(expected:desired:successOrdering:failureOrdering:) Perform an atomic weak compare and exchange operation on the current value, applying the specified success/failure memory orderings. This compare-exchange variant is allowed to spuriously fail; it is designed to be called in a loop until it indicates a successful exchange has happened. ``` func weakCompareExchange(expected: consuming Value, desired: consuming Value, successOrdering: AtomicUpdateOrdering, failureOrdering: AtomicLoadOrdering) -> (exchanged: Bool, original: Value) ``` ## Parameters `expected` The expected current value. `desired` The desired new value. `successOrdering` The memory ordering to apply if this operation performs the exchange. `failureOrdering` The memory ordering to apply on this operation does not perform the exchange. ## Return Value A tuple `(exchanged, original)`, where `exchanged` is true if the exchange was successful, and `original` is the original value. ## Discussion This operation performs the following algorithm as a single atomic transaction: ``` atomic(self) { currentValue in let original = currentValue guard original == expected else { return (false, original) } currentValue = desired return (true, original) } ``` The `ordering` argument specifies the memory ordering to use when the operation manages to update the current value, while `failureOrdering` will be used when the operation leaves the value intact. > Note: The weakCompareExchange form may sometimes return false even when > the original and expected values are equal. (Such failures may happen > when some transient condition prevents the underlying operation from > succeeding – such as an incoming interrupt during a > load-link/store-conditional instruction sequence.) This variant is > designed to be called in a loop that only exits when the exchange is > successful; in such loops, using weakCompareExchange may lead to a > performance improvement by eliminating a nested loop in the regular, > “strong”, compareExchange variants. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)