Data vs [UInt8]

Question

Swift offers both Data and [UInt8] types, which do a very similar thing.

• What are the differences between the two?
• When designing new API's, what's the preferred type?

Answer 1

[UInt8] is essentially a byte array, a byte (as I'm sure you know), is comprised of 8 bits. Whilst NSData isn't simply a byte array, deep down it's underlying structure is based off one. You can easily convert between them using methods such as data.bytes, for example.

In terms of designing APIs, I would personally recommend you design them with NSData simply because of all the extra functionality it offers over a simple byte array. Apple has already done a lot of the legwork for you, so why do it again yourself?

Answer 2

I prefer using Data for most things, but [UInt8] has one distinct advantage: you can pass it directly to functions requiring pointers to the bytes like C functions, whereas for Data you have to do a bunch more gymnastics. The code below demonstrates the difference for both immutable and mutable arrays and Data objects.

func takesAPointer(_ p: UnsafePointer<UInt8>) {
    // ...
}

let a: [UInt8] = [1, 2, 3]
takesAPointer(a)
let d = Data([1, 2, 3])
d.withUnsafeBytes {
    let p = $0.bindMemory(to: UInt8.self).baseAddress!
    takesAPointer(p)
}

func takesAMutablePointer(_ p: UnsafeMutablePointer<UInt8>) {
    // ...
}

var b: [UInt8] = [1, 2, 3]
takesAMutablePointer(&b)
var e = Data([1, 2, 3])
e.withUnsafeMutableBytes {
    let p = $0.bindMemory(to: UInt8.self).baseAddress!
    takesAMutablePointer(p)
}