# storeIfNil(_:) Atomically initializes this reference if its current value is nil, then returns the initialized value. If this reference is already initialized, then `storeIfNil(_:)` discards its supplied argument and returns the current value without updating it. ``` func storeIfNil(_ desired: consuming Instance) -> Instance ``` ## Parameters `desired` A value of `Instance` that we will attempt to store if the lazy reference is currently nil. ## Return Value The value of `Instance` that was successfully stored within the lazy reference. This may or may not be the same value of `Instance` that was passed to this function. ## Discussion The following example demonstrates how this can be used to implement a thread-safe lazily initialized reference: ``` class Image { let _histogram = AtomicLazyReference() // This is safe to call concurrently from multiple threads. var atomicLazyHistogram: Histogram { if let histogram = _histogram.load() { return histogram } // Note that code here may run concurrently on // multiple threads, but only one of them will get to // succeed setting the reference. let histogram = ... return _histogram.storeIfNil(histogram) } } ``` > Note: This operation uses acquiring-and-releasing memory ordering. --- 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)