Copy bytes from Data with Swift

README.md

Copying Bytes from Data

The Data type in Swift holds onto a raw stream of bytes. Accessing these bytes is done with the Unsafe API which provides direct access to these bytes within an instance of Data. Legacy code which uses NSData is still sometimes used because it is familiar. The copyBytes:length: function can be used to copy the bytes into an array by referencing a pointer. It is best to instead use the modern Swift API for accessing bytes.

See the included extension as well as the playground which demonstrate how the modern Swift API can be used.

Credit: sharplet

BytesPlayground.swift

import Foundation

let size = MemoryLayout<Int16>.stride
let data = Data(bytes: [1, 0, 2, 0, 3, 0]) // little endian for 16-bit values

let int16s = data.withUnsafeBytes { (bytes: UnsafePointer<Int16>) in
    Array(UnsafeBufferPointer(start: bytes, count: data.count / size))
}

let length = data.count * MemoryLayout<Int16>.stride
var bytes1 = [Int16](repeating: 0, count: data.count / size)
(data as NSData).getBytes(&bytes1, length: length)

let bytes2 = data.withUnsafeBytes {
    UnsafeBufferPointer<Int16>(start: $0, count: data.count / size).map(Int16.init(littleEndian:))
}

let bytes3 = data.withUnsafeBytes {
    Array(UnsafeBufferPointer<Int16>(start: $0, count: data.count / size))
}

// Expected output
// [1, 2, 3]

int16s
bytes1
bytes2
bytes3

DataCopyBytes.swift

extension Data {
    func copyBytes<T>(as _: T.Type) -> [T] {
        return withUnsafeBytes { (bytes: UnsafePointer<T>) in
            Array(UnsafeBufferPointer(start: bytes, count: count / MemoryLayout<T>.stride))
        }
    }
}