Create an Array in Swift from an NSData Object

Question

I'm trying to store an array of integers to disk in swift. I can get them into an NSData object to store, but getting them back out into an array is difficult. I can get a raw COpaquePointer to the data with data.bytes but can't find a way to initialize a new swift array with that pointer. Does anyone know how to do it?

import Foundation  var arr : UInt32[] = [32,4,123,4,5,2];  let data = NSData(bytes: arr, length: arr.count * sizeof(UInt32))  println(data)  //data looks good in the inspector  // now get it back into an array? 

Answer 1

You can use the getBytes method of NSData:

// the number of elements: let count = data.length / sizeof(UInt32)  // create array of appropriate length: var array = [UInt32](count: count, repeatedValue: 0)  // copy bytes into array data.getBytes(&array, length:count * sizeof(UInt32))  print(array) // Output: [32, 4, 123, 4, 5, 2] 

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Update for Swift 3 (Xcode 8): Swift 3 has a new type struct Data which is a wrapper for NS(Mutable)Data with proper value semantics. The accessor methods are slightly different.

Array to Data:

var arr: [UInt32] = [32, 4, UInt32.max] let data = Data(buffer: UnsafeBufferPointer(start: &arr, count: arr.count)) print(data) // <20000000 04000000 ffffffff> 

Data to Array:

let arr2 = data.withUnsafeBytes {     Array(UnsafeBufferPointer<UInt32>(start: $0, count: data.count/MemoryLayout<UInt32>.stride)) } print(arr2) // [32, 4, 4294967295] 

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Update for Swift 5:

Array to Data:

let arr: [UInt32] = [32, 4, UInt32.max] let data = Data(buffer: UnsafeBufferPointer(start: arr, count: arr.count)) print(data) // <20000000 04000000 ffffffff> 

Data to Array:

var arr2 = Array<UInt32>(repeating: 0, count: data.count/MemoryLayout<UInt32>.stride) _ = arr2.withUnsafeMutableBytes { data.copyBytes(to: $0) } print(arr2) // [32, 4, 4294967295] 

Answer 2

It's also possible to do this using an UnsafeBufferPointer, which is essentially an "array pointer", as it implements the Sequence protocol:

let data = NSData(/* ... */)  // Have to cast the pointer to the right size let pointer = UnsafePointer<UInt32>(data.bytes) let count = data.length / 4  // Get our buffer pointer and make an array out of it let buffer = UnsafeBufferPointer<UInt32>(start:pointer, count:count) let array = [UInt32](buffer) 

This eliminates the need for initializing an empty array with duplicated elements first, to then overwrite it, although I have no idea if it's any faster. As it uses the Sequence protocol this implies iteration rather than fast memory copy, though I don't know if it's optimized when passed a buffer pointer. Then again, I'm not sure how fast the "create an empty array with X identical elements" initializer is either.