# SecTransformInstanceBlock A block that you return from a transform creation function. ``` typealias SecTransformInstanceBlock = () -> Unmanaged? ``` ## Return Value A an error object if an error occurred. ## Discussion You return a block of this type from the custom transform creation function of type [`SecTransformCreateFP`](/documentation/Security/SecTransformCreateFP) that you register with the [`SecTransformRegister(_:_:_:)`](/documentation/Security/SecTransformRegister(_:_:_:)) function. --- 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)