Generic Instance Method


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


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



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.


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