# CMMetadataCreateIdentifierForKeyAndKeySpace(allocator:key:keySpace:identifierOut:) Creates a URL-like string identifier that represents a key or keyspace tuple. ``` func CMMetadataCreateIdentifierForKeyAndKeySpace(allocator: CFAllocator?, key: CFTypeRef, keySpace: CFString, identifierOut: UnsafeMutablePointer) -> OSStatus ``` ## Parameters `allocator` The allocator to use for creating the identifier. `key` Key data; may be `CFString`, `CFNumber`, or `CFData`. `keySpace` Keyspace; must be string of one to four printable ASCII characters. `identifierOut` Upon return, a pointer to the created identifier. ## Return Value If successful, a nonzero result code. See [Metadata Identifier Error Codes](/documentation/CoreMedia/metadata-identifier-errors). ## Discussion Metadata entities are identified by a key whose interpretation is defined by its keyspace. When writing metadata to a QuickTime Movie, this tuple is part of the track’s format description. See [Metadata Identifier Keyspaces](/documentation/CoreMedia/metadata-identifier-keyspaces) for the current list of supported keyspaces and the requirements for key values in each keyspace. As a matter of convenience, known keyspaces allow for a key to be passed in using a variety of `CFType`s. Note that what is returned by [`CMMetadataCreateKeyFromIdentifier(allocator:identifier:keyOut:)`](/documentation/CoreMedia/CMMetadataCreateKeyFromIdentifier(allocator:identifier:keyOut:)) depends upon the keyspace, and may be a different `CFType` than what is passed to this routine (see the discussion below for what `CFType`s are returned for known keyspaces). To get a key represented as `CFData`, call [`CMMetadataCreateKeyFromIdentifierAsCFData(allocator:identifier:keyOut:)`](/documentation/CoreMedia/CMMetadataCreateKeyFromIdentifierAsCFData(allocator:identifier:keyOut:)). Some keyspaces use `OSType`s (a.k.a. `FourCharCode`s) to define their keys, and as such their keys are four bytes in length. For these keyspaces, a key may be passed as a `CFNumber`, a `CFString`, or a `CFData`. A key passed as a `CFNumber` will have its value retrieved as comprising the four bytes of the key’s numeric value in big-endian byte order. A key passed as a `CFString` must be a valid ASCII string of four characters. A key passed as a `CFData` must be comprised of the four bytes of the key’s numeric value in big-endian byte order. All other keyspaces allow the key to be passed as a `CFString` or `CFData`. In both cases, the key will be interpreted as an ASCII string for the purposes of identifier encoding. --- 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)