# CFMutableData ``` class CFMutableData ``` ## Overview CFMutableData manages dynamic binary data. The basic interface for managing binary data is provided by [`CFData`](/documentation/CoreFoundation/CFData). CFMutableData adds functions to modify the contents of a binary data object. You create a mutable data object using either the [`CFDataCreateMutable(_:_:)`](/documentation/CoreFoundation/CFDataCreateMutable(_:_:)) or [`CFDataCreateMutableCopy(_:_:_:)`](/documentation/CoreFoundation/CFDataCreateMutableCopy(_:_:_:)) function. Bytes are added to a data object with the [`CFDataAppendBytes(_:_:_:)`](/documentation/CoreFoundation/CFDataAppendBytes(_:_:_:)) function. Bytes are removed from a data object with the [`CFDataDeleteBytes(_:_:)`](/documentation/CoreFoundation/CFDataDeleteBytes(_:_:)) function. > Important: > Many of the CFMutableData functions take a ``doc://com.apple.corefoundation/documentation/CoreFoundation/CFIndex`` `length` or `capacity` argument. You must not pass a negative number for such values—this may introduce a security risk. CFMutableData is “toll-free bridged” with its Cocoa Foundation counterpart, . What this means is that the Core Foundation type is interchangeable in function or method calls with the bridged Foundation object. In other words, in a method where you see an `NSMutableData *` parameter, you can pass in a `CFMutableDataRef`, and in a function where you see a `CFMutableDataRef` parameter, you can pass in an `NSMutableData` instance. This also applies to concrete subclasses of `NSMutableData`. See [Toll-Free Bridged Types](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFDesignConcepts/Articles/tollFreeBridgedTypes.html#//apple_ref/doc/uid/TP40010677) for more information on toll-free bridging. ## Topics ### Creating a Mutable Data Object [`CFDataCreateMutable(_:_:)`](/documentation/CoreFoundation/CFDataCreateMutable(_:_:)) Creates an empty CFMutableData object. [`CFDataCreateMutableCopy(_:_:_:)`](/documentation/CoreFoundation/CFDataCreateMutableCopy(_:_:_:)) Creates a CFMutableData object by copying another CFData object. ### Accessing Data [`CFDataGetMutableBytePtr(_:)`](/documentation/CoreFoundation/CFDataGetMutableBytePtr(_:)) Returns a pointer to a mutable byte buffer of a CFMutableData object. ### Modifying a Mutable Data Object [`CFDataAppendBytes(_:_:_:)`](/documentation/CoreFoundation/CFDataAppendBytes(_:_:_:)) Appends the bytes from a byte buffer to the contents of a CFData object. [`CFDataDeleteBytes(_:_:)`](/documentation/CoreFoundation/CFDataDeleteBytes(_:_:)) Deletes the bytes in a CFMutableData object within a specified range. [`CFDataReplaceBytes(_:_:_:_:)`](/documentation/CoreFoundation/CFDataReplaceBytes(_:_:_:_:)) Replaces those bytes in a CFMutableData object that fall within a specified range with other bytes. [`CFDataIncreaseLength(_:_:)`](/documentation/CoreFoundation/CFDataIncreaseLength(_:_:)) Increases the length of a CFMutableData object’s internal byte buffer, zero-filling the extension to the buffer. [`CFDataSetLength(_:_:)`](/documentation/CoreFoundation/CFDataSetLength(_:_:)) Resets the length of a CFMutableData object’s internal byte buffer. ## See Also [Property List Programming Topics for Core Foundation](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFPropertyLists/CFPropertyLists.html#//apple_ref/doc/uid/10000130i) [Binary Data Programming Guide for Core Foundation](https://developer.apple.com/library/archive/documentation/CoreFoundation/Conceptual/CFBinaryData/CFBinaryData.html#//apple_ref/doc/uid/10000144i) --- 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)