# update(fromContentsOf:) Updates the buffer slice’s initialized memory with every element of the source. ``` func update(fromContentsOf source: some Collection) -> Slice.Index where Base == UnsafeMutableBufferPointer ``` ## Parameters `source` A collection of elements to be used to update the buffer’s contents. ## Return Value An index one past the index of the last element updated. ## Discussion Prior to calling the `update(fromContentsOf:)` method on a buffer slice, the first `source.count` elements of the referenced memory must be initialized, or the `Element` type must be a trivial type. The buffer slice must reference enough initialized memory to accommodate `source.count` elements. The returned index is one past the index of the last element updated. If `source` contains no elements, the returned index is the buffer slice’s `startIndex`. If `source` contains as many elements as the buffer slice can hold, the returned index is the buffer slice’s `endIndex`. > Note: The memory regions referenced by `source` and this buffer slice > may overlap. > Precondition: `self.count` >= `source.count` --- 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)