# init(_:) Creates an array containing the elements of `s`. ``` init(_ s: some Sequence) ``` ## Discussion The backing C++ storage is reserved up front from the sequence’s `underestimatedCount`, so the common case allocates once rather than reallocating repeatedly while appending. `underestimatedCount` is exact and `O(1)` for collections such as `Array`; for sequences whose count isn’t known it is a lower bound (possibly zero) used purely as a reservation hint, which avoids a second traversal just to 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)