Inter-operability of Swift arrays with C? - objective-c

Question

How can one pass or copy the data in a C array, such as
float foo[1024];
, between C and Swift functions that use fixed size arrays, such as declared by
let foo = Float[](count: 1024, repeatedValue: 0.0)
?

Answer 1

I don't think this is easily possible. In the same way as you can't use C style arrays for parameters working with a NSArray.
All C arrays in Swift are represented by an UnsafePointer, e.g. UnsafePointer<Float>. Swift doesn't really know that the data are an array. If you want to convert them into a Swift array, you will have create a new object and copy the items there one by one.
let array: Array<Float> = [10.0, 50.0, 40.0]
// I am not sure if alloc(array.count) or alloc(array.count * sizeof(Float))
var cArray: UnsafePointer<Float> = UnsafePointer<Float>.alloc(array.count)
cArray.initializeFrom(array)
cArray.dealloc(array.count)
Edit
Just found a better solution, this could actually avoid copying.
let array: Array<Float> = [10.0, 50.0, 40.0]
// .withUnsafePointerToElements in Swift 2.x
array.withUnsafeBufferPointer() { (cArray: UnsafePointer<Float>) -> () in
// do something with the C array
}

Answer 2

As of Beta 5, one can just use pass &array
The following example passes 2 float arrays to a vDSP C function:
let logLen = 10
let len = Int(pow(2.0, Double(logLen)))
let setup : COpaquePointer = vDSP_create_fftsetup(vDSP_Length(logLen), FFTRadix(kFFTRadix2))
var myRealArray = [Float](count: len, repeatedValue: 0.0)
var myImagArray = [Float](count: len, repeatedValue: 0.0)
var cplxData = DSPSplitComplex(realp: &myRealArray, imagp: &myImagArray)
vDSP_fft_zip(setup, &cplxData, 1, vDSP_Length(logLen),FFTDirection(kFFTDirection_Forward))

Answer 3

The withUnsafePointerToElements() method was removed, now you can use the withUnsafeBufferPointer() instead, and use the baseAddress method in the block to achieve the point
let array: Array<Float> = [10.0, 50.0, 40.0]
array.withUnsafeBufferPointer { (cArray: UnsafePointer<Float>) -> () in
cArray.baseAddress
}

Answer 4

let's see what Apple do:
public struct float4 {
public var x: Float
public var y: Float
public var z: Float
public var w: Float
/// Initialize to the zero vector.
public init()
/// Initialize a vector with the specified elements.
public init(_ x: Float, _ y: Float, _ z: Float, _ w: Float)
/// Initialize a vector with the specified elements.
public init(x: Float, y: Float, z: Float, w: Float)
/// Initialize to a vector with all elements equal to `scalar`.
public init(_ scalar: Float)
/// Initialize to a vector with elements taken from `array`.
///
/// - Precondition: `array` must have exactly four elements.
public init(_ array: [Float])
/// Access individual elements of the vector via subscript.
public subscript(index: Int) -> Float
}