Converting C array ptr to native array in Swift 3

Question

I have a ptr from a C library that points to an array of Floats. Its type is UnsafeMutablePointer. How do I create a native [Float] array from this in Swift 3?

Here's what I'm trying:

var reconstructedFloats = [Float](repeatElement(0, count: size))
reconstructedFloats.withUnsafeMutableBufferPointer {
  let reconstructedFloatsPtr = $0
  print(type(of:$0)) // "UnsafeMutableBufferPointer<Float>"
  cFloatArrayPtr?.withMemoryRebound(to: [Float].self, capacity: size) {
    UnsafeMutableRawPointer(reconstructedFloatsPtr.baseAddress!).storeBytes(of: $0.pointee, as: Float.self)
  }

UnsafeMutableRawPointer(reconstructedFloatsPtr.baseAddress!).storeBytes(of: (cFloatArrayPtr?.pointee)!, as: Float.self)
}

That seems insanely overcomplicated so hopefully there's an easy way, but even this code produces a compile error: Type of expression is ambiguous without more context.

If you want to plug it into a playground, here's a complete sample that contrives the cFloatArrayPtr:

// Let's contrive a C array ptr:
var size = 3
var someFloats: [Float] = [0.1, 0.2, 0.3]
var cFloatArrayPtr: UnsafeMutablePointer<Float>?

someFloats.withUnsafeMutableBufferPointer {
    cFloatArrayPtr = $0.baseAddress
}

print(type(of:cFloatArrayPtr!)) // "UnsafeMutablePointer<Float>"

var reconstructedFloats = [Float](repeatElement(0, count: size))
reconstructedFloats.withUnsafeMutableBufferPointer {
    let reconstructedFloatsPtr = $0
    print(type(of:$0))
    cFloatArrayPtr?.withMemoryRebound(to: [Float].self, capacity: size) {
        UnsafeMutableRawPointer(reconstructedFloatsPtr.baseAddress!).storeBytes(of: $0.pointee, as: Float.self)
    }

    UnsafeMutableRawPointer(reconstructedFloatsPtr.baseAddress!).storeBytes(of: (cFloatArrayPtr?.pointee)!, as: Float.self)
}

print(reconstructedFloats)

zneak · Accepted Answer · 2016-09-23 18:00:43Z

5

You can make an UnsafeBufferPointer from your pointer. UnsafeBufferPointer is a Sequence, so you can directly make an array from it:

let buffer = UnsafeBufferPointer(start: cFloatArrayPtr, count: size)
var reconstructedFloats = Array(buffer)

Of course, this creates a copy.

answered Sep 23, 2016 at 18:00

zneak

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1 Comment

John Scalo Over a year ago

I knew there must be an easier way. Thanks!

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