# Attachable A protocol describing a type whose instances can be recorded and saved as part of a test run. ``` protocol Attachable : ~Copyable ``` ## Overview To attach an attachable value to a test, pass it to [`record(_:named:sourceLocation:)`](/documentation/Testing/Attachment/record(_:named:sourceLocation:)). To further configure an attachable value before you attach it, use it to initialize an instance of [`Attachment`](/documentation/Testing/Attachment) and set its properties before passing it to [`record(_:sourceLocation:)`](/documentation/Testing/Attachment/record(_:sourceLocation:)). The testing library provides default conformances to this protocol for a variety of standard library types. Most user-defined types do not need to conform to this protocol. A type should conform to this protocol if it can be represented as a sequence of bytes that would be diagnostically useful if a test fails. If a type cannot conform directly to this protocol (such as a non-final class or a type declared in a third-party module), you can create a wrapper type that conforms to [`AttachableWrapper`](/documentation/Testing/AttachableWrapper) to act as a proxy. ## Topics ### Instance Properties [`var estimatedAttachmentByteCount: Int?`](/documentation/Testing/Attachable/estimatedAttachmentByteCount) An estimate of the number of bytes of memory needed to store this value as an attachment. ### Instance Methods [`func preferredName(for: borrowing Attachment, basedOn: String) -> String`](/documentation/Testing/Attachable/preferredName(for:basedOn:)) Generate a preferred name for the given attachment. [`func withUnsafeBytes(for: borrowing Attachment, (UnsafeRawBufferPointer) throws -> R) throws -> R`](/documentation/Testing/Attachable/withUnsafeBytes(for:_:)) Call a function and pass a buffer representing this instance to it. ## Relationships ### Inherited By [`AttachableWrapper`](/documentation/Testing/AttachableWrapper) --- 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)