# CFStringCreateMutableCopy(_:_:_:) Creates a mutable copy of a string. ``` func CFStringCreateMutableCopy(_ alloc: CFAllocator!, _ maxLength: CFIndex, _ theString: CFString!) -> CFMutableString! ``` ## Parameters `alloc` The allocator to use to allocate memory for the new string. Pass `NULL` or kCFAllocatorDefault to use the current default allocator. `maxLength` The maximum number of Unicode characters that can be stored by the returned object. Pass `0` if there should be no character limit. Note that initially the returned object still has the same length as the string argument; this parameter simply specifies what the maximum size is. CFString might try to optimize its internal storage by paying attention to this value. `theString` A string to copy. ## Return Value A string that has the same contents as `theString`. Returns `NULL` if there was a problem copying the object. 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 The returned mutable string is identical to the original string except for (perhaps) the mutability attribute. You can add character data to the returned string with any of the `CFStringAppend...` functions, and you can insert, delete, replace, pad, and trim characters with the appropriate CFString functions. If the `maxLength` parameter is greater than `0`, any attempt to add characters beyond this limit results in a run-time error. --- 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)