# withUnsafeMutableBufferPointer(_:) Calls the given closure with a pointer to the array’s mutable contiguous storage. ``` mutating func withUnsafeMutableBufferPointer(_ body: (inout UnsafeMutableBufferPointer) throws(E) -> R) throws(E) -> R where E : Error ``` ## Parameters `body` A closure with an `UnsafeMutableBufferPointer` parameter that points to the contiguous storage for the array. If no such storage exists, it is created. If `body` has a return value, that value is also used as the return value for the `withUnsafeMutableBufferPointer(_:)` method. The pointer argument is valid only for the duration of the method’s execution. ## Return Value The return value, if any, of the `body` closure parameter. ## Discussion Often, the optimizer can eliminate bounds checks within an array algorithm, but when that fails, invoking the same algorithm on the buffer pointer passed into your closure lets you trade safety for speed. The following example shows how modifying the contents of the `UnsafeMutableBufferPointer` argument to `body` alters the contents of the array: ``` var numbers = [1, 2, 3, 4, 5] numbers.withUnsafeMutableBufferPointer { buffer in for i in stride(from: buffer.startIndex, to: buffer.endIndex - 1, by: 2) { buffer.swapAt(i, i + 1) } } print(numbers) // Prints "[2, 1, 4, 3, 5]" ``` The pointer passed as an argument to `body` is valid only during the execution of `withUnsafeMutableBufferPointer(_:)`. Do not store or return the pointer for later use. > Warning: Do not rely on anything about the array that is the target of > this method during execution of the `body` closure; it might not > appear to have its correct value. Instead, use only the > `UnsafeMutableBufferPointer` argument to `body`. --- 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)