# About Imported Cocoa Error Parameters Learn how Cocoa error parameters are converted to Swift throwing methods. ## Overview In Cocoa, methods that produce errors take an pointer parameter as their last parameter, which populates its argument with an object if an error occurs. Swift automatically translates Objective-C methods that produce errors into methods that throw an error according to Swift’s native error handling functionality. ### Understand How Error Parameters Are Imported Swift examines Objective-C method declarations and translates them into Swift throwing methods, with shorter names when possible. For example, consider the method from . In Objective-C, it’s declared like this: ```occ - (BOOL)removeItemAtURL:(NSURL *)URL error:(NSError **)error; ``` In Swift, it’s imported like this: ```swift func removeItem(at: URL) throws ``` Notice that the method is imported by Swift with a `Void` return type, no error parameter, and a `throws` declaration. If the last non-block parameter of an Objective-C method is of type `NSError **`, Swift replaces it with the throws keyword, to indicate that the method can throw an error. If the Objective-C method’s error parameter is also its first parameter, Swift attempts to simplify the method name further, by removing the `WithError` or `AndReturnError` suffix, if present, from the first part of the selector. If another method is declared with the resulting selector, the method name is not changed. If an error producing Objective-C method returns a value to indicate the success or failure of a method call, Swift changes the return type of the function to `Void`. Similarly, if an error producing Objective-C method returns a `nil` value to indicate the failure of a method call, Swift changes the return type of the function to a nonoptional type. Otherwise, if no convention can be inferred, the method is left intact. > Note: You use the `NS_SWIFT_NOTHROW` macro on Objective-C method declarations that > produce an to prevent > it from being imported by Swift as a method that throws. --- 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)