Generic Instance Method

withUnsafeMutableBytes(_:)

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

Declaration

mutating func withUnsafeMutableBytes<R>(_ body: (UnsafeMutableRawBufferPointer) throws -> R) rethrows -> R

Parameters

body

A closure with an UnsafeMutableRawBufferPointer 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 withUnsafeMutableBytes(_:) method. The argument is valid only for the duration of the closure’s execution.

Return Value

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

Discussion

The array’s Element type must be a trivial type, which can be copied with just a bit-for-bit copy without any indirection or reference-counting operations. Generally, native Swift types that do not contain strong or weak references are trivial, as are imported C structs and enums.

The following example copies bytes from the byteValues array into numbers, an array of Int:

var numbers: [Int32] = [0, 0]
var byteValues: [UInt8] = [0x01, 0x00, 0x00, 0x00, 0x02, 0x00, 0x00, 0x00]

numbers.withUnsafeMutableBytes { destBytes in
    byteValues.withUnsafeBytes { srcBytes in
        destBytes.copyBytes(from: srcBytes)
    }
}
// numbers == [1, 2]

The pointer passed as an argument to body is valid only for the lifetime of the closure. Do not escape it from the closure for later use.

See Also

Accessing Underlying Storage

func withUnsafeBufferPointer<R>((UnsafeBufferPointer<Element>) -> R) -> R

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

func withUnsafeMutableBufferPointer<R>((inout UnsafeMutableBufferPointer<Element>) -> R) -> R

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

func withUnsafeBytes<R>((UnsafeRawBufferPointer) -> R) -> R

Calls the given closure with a pointer to the underlying bytes of the array’s contiguous storage.

func withContiguousStorageIfAvailable<R>((UnsafeBufferPointer<Element>) -> R) -> R?

Call body(p), where p is a pointer to the collection’s contiguous storage. If no such storage exists, it is first created. If the collection does not support an internal representation in a form of contiguous storage, body is not called and nil is returned.

func withContiguousMutableStorageIfAvailable<R>((inout UnsafeMutableBufferPointer<Element>) -> R) -> R?

Call body(p), where p is a pointer to the collection’s mutable contiguous storage. If no such storage exists, it is first created. If the collection does not support an internal representation in a form of mutable contiguous storage, body is not called and nil is returned.