Swift Allocate Array of Pointers

Question

How can I use UnsafeMutablePointer<T?> as UnsafeMutablePointer<UnsafeRawPointer?>!

e.g. Trying to allocate n blocks of memory for type T, to get values from a CFSet cfsetref:

var array = UnsafeMutableRawPointer<T?>.allocate(capacity: n)
CFSetGetValues(cfsetref, array) // error

Which gives the error

Cannot convert value of type 'UnsafeMutablePointer<T?>' to expected argument type 'UnsafeMutablePointer<UnsafeRawPointer?>!'

I tried declaring array as UnsafeMutablePointer<UnsafeRawPointer?> then doing

for i in 0..<n {
  var d = array[i]
  d?.bindMemory(to: T.self, capacity: 1)
}

But I still get EXC_BAD_INSTRUCTION errors when attempting to access array[i] (after binding the memory again to T)

OOPer · Accepted Answer · 2017-12-28 12:03:58Z

Many things depends on how you get your cfsetref and what actually is T.

But anyway, CFSetGetValues expects UnsafeMutablePointer<UnsafeRawPointer?>! as shown in the error message.

let n = CFSetGetCount(cfsetref)
let array = UnsafeMutablePointer<UnsafeRawPointer?>.allocate(capacity: n)
array.initialize(to: nil, count: n)
CFSetGetValues(cfsetref, array)

To safely access the contents of array, you need to know how T is managed by Swift ARC. For example, assuming T is NSNumber, you need to tell Swift ARC to manage the result with writing something like this:

let managedArray = UnsafeMutableBufferPointer(start: array, count: n).map{Unmanaged<NSNumber>.fromOpaque($0!).takeRetainedValue()}
print(managedArray)

But as well as other CF-collection types, the better way to handle CFSet is bridging it to Swift Set:

if let swiftSet = cfsetref as? Set<NSNumber> {
    let swiftArray = Array(swiftSet)
    print(swiftArray)
}

How do you get your cfsetref and what actually is T? With such information, I would try to tell you what sort of code you need to write in your actual case.

Bridging to Set feels right. My CFSet is coming from IOHIDManagerCopyDevices, where T is a IOHIDDevice. I was getting nowhere trying to manage my own pointers/memory, but bridging to Set seems to work out of the box (and is so much simpler). Thanks so much.

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