# isKnownUniquelyReferenced(_:) Returns a Boolean value indicating whether the given object is known to have a single strong reference. ``` func isKnownUniquelyReferenced(_ object: inout T?) -> Bool where T : AnyObject ``` ## Parameters `object` An instance of a class. This function does *not* modify `object`; the use of `inout` is an implementation artifact. ## Return Value `true` if `object` is known to have a single strong reference; otherwise, `false`. If `object` is `nil`, the return value is `false`. ## Discussion The `isKnownUniquelyReferenced(_:)` function is useful for implementing the copy-on-write optimization for the deep storage of value types: ``` mutating func update(withValue value: T) { if !isKnownUniquelyReferenced(&myStorage) { myStorage = self.copiedStorage() } myStorage.update(withValue: value) } ``` `isKnownUniquelyReferenced(_:)` checks only for strong references to the given object—if `object` has additional weak or unowned references, the result may still be `true`. Because weak and unowned references cannot be the only reference to an object, passing a weak or unowned reference as `object` always results in `false`. If the instance passed as `object` is being accessed by multiple threads simultaneously, this function may still return `true`. Therefore, you must only call this function from mutating methods with appropriate thread synchronization. That will ensure that `isKnownUniquelyReferenced(_:)` only returns `true` when there is really one accessor, or when there is a race condition, which is already undefined behavior. --- 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)