Stack overflow question How to compare two UIImage objects might be very good example when wrong question is highly voted up. Would be great if I find time and write longread answer to question.

Example snippet in vdsp_vsadd is not consistent as it doesn’t declare variable stride. Filed a bug with feedback id FB12116721 on 13 of April 2023. Full fixed snippet should look like:

let a: [Float] = [1, 2, 4, 5]
let stride = 1
var b: Float = 2

let n = vDSP_Length(a.count)

var c = [Float](repeating: 0,
                count: a.count)

vDSP_vsadd(a, stride,
           &c, stride,

// Prints "[3.0, 4.0, 6.0, 7.0]"

SWIFT MEETUP QUESTION: Several questions to above snippet:

  • why can’t b be declared as let if appropriate parameter of vDSP_vsadd is declared as UnsafePointer<Float>? Xcode complains something stupid Cannot pass immutable value as inout argument: 'b' is a 'let' constant. Appropriate parameter of vDSP_vsadd isn’t inout:

func vDSP_vsadd(_ __A: UnsafePointer<Float>, _ __IA: vDSP_Stride, _ __B: UnsafePointer<Float>, _ __C: UnsafeMutablePointer<Float>, _ __IC: vDSP_Stride, _ __N: vDSP_Length)

  • what happens under the hood when a which is [Float] is passed to parameter of type UnsafePointer<Float>;

  • why in last case a might be let if here parameter also has type UnsafePointer<Float>.

This week every second day I look here for memory command in Xcode. Actually memory read -t float -c50 spectrogram is what I have looked for. I tried help memory read. Firstly, remembering help whateveryouhavetroublewith I don’t have remember exact syntax anymore. Second, I am shocked how powerful command memory is. It could even store output into a file!

It’s very useful to look into Swift declaration of UnsafeMutablePointer. It has a lot of information not available anywhere else.

You declare pointer. In Swift it can’t be uninitialised. How would you initialise it, let’s say, with NULL how C understands that.

// Compiler swallows this.
// But if you chech type of p1 it's UnsafeMutablePointer<Float>? which is not what we want.
var p1 = UnsafeMutablePointer<Float>(nil)

But if you stop and think for a moment you start to suppose that probably there’s no such thing as NULL pointer in Swift.