# CFStringCreateExternalRepresentation(_:_:_:_:) Creates an “external representation” of a CFString object, that is, a CFData object. ``` func CFStringCreateExternalRepresentation(_ alloc: CFAllocator!, _ theString: CFString!, _ encoding: CFStringEncoding, _ lossByte: UInt8) -> CFData! ``` ## Parameters `alloc` The allocator to use to allocate memory for the new CFData object. Pass `NULL` or [`kCFAllocatorDefault`](/documentation/CoreFoundation/kCFAllocatorDefault) to use the current default allocator. `theString` The string to convert to an external representation. `encoding` The string encoding to use for the external representation. `lossByte` The character value to assign to characters that cannot be converted to the requested encoding. Pass `0` if you want conversion to stop at the first such error; if this happens, the function returns `NULL`. ## Return Value A CFData object that stores the characters of the CFString object as an “external representation.” Returns `NULL` if no loss byte was specified and the function could not convert the characters to the specified encoding. Ownership follows the [The Create Rule](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFMemoryMgmt/Concepts/Ownership.html#//apple_ref/doc/uid/20001148-103029). ## Discussion In the CFData object form, the string can be written to disk as a file or be sent out over a network. If the encoding of the characters in the data object is Unicode, the function may insert a BOM (byte-order marker) to indicate endianness. However, representations created with encoding constants `kCFStringEncodingUTF16BE`, `kCFStringEncodingUTF16LE`, `kCFStringEncodingUTF32BE`, and `kCFStringEncodingUTF32LE` do not include a BOM because the byte order is explicitly indicated by the letters “BE” (big-endian) and “LE” (little-endian). This function allows the specification of a “loss byte” to represent characters that cannot be converted to the requested encoding. When you create an external representation from a CFMutableString object, it loses this mutability characteristic when it is converted back to a CFString object. The [`CFStringCreateFromExternalRepresentation(_:_:_:)`](/documentation/CoreFoundation/CFStringCreateFromExternalRepresentation(_:_:_:)) function complements this function by creating a CFString object from an “external representation” CFData object. --- 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)