# initialize(fromContentsOf:) Initializes the buffer’s memory with every element of the source. ``` func initialize(fromContentsOf source: some Collection) -> UnsafeMutableBufferPointer.Index ``` ## Parameters `source` A collection of elements to be used to initialize the buffer’s storage. ## Return Value The index one past the last element of the buffer initialized by this function. ## Discussion Prior to calling the `initialize(fromContentsOf:)` method on a buffer, the memory referenced by the buffer must be uninitialized, or the `Element` type must be a trivial type. After the call, the memory referenced by the buffer up to, but not including, the returned index is initialized. The buffer must reference enough memory to accommodate `source.count` elements. The returned index is the position of the next uninitialized element in the buffer, one past the index of the last element written. If `source` contains no elements, the returned index is equal to the buffer’s `startIndex`. If `source` contains as many elements as the buffer can hold, the returned index is equal to the buffer’s `endIndex`. > Precondition: `self.count` >= `source.count` > Note: The memory regions referenced by `source` and this buffer > must not overlap. --- 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)