---
title: "withUnsafeMutableBufferPointer(_:)"
framework: swift
role: symbol
role_heading: Instance Method
path: "swift/contiguousarray/withunsafemutablebufferpointer(_:)"
---

# withUnsafeMutableBufferPointer(_:)

Calls the given closure with a pointer to the array’s mutable contiguous storage.

## Declaration

```swift
mutating func withUnsafeMutableBufferPointer<R, E>(_ body: (inout UnsafeMutableBufferPointer<Element>) 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 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

Return Value The return value, if any, of the body closure parameter.

## Discussion

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.