UnsafeMutablePointer in swift as replacement for properly sized C Array in Obj-C

How can I interact with functions in swift that used to take sized C arrays?

I read through Interacting with C APIS and still can’t figure this out.

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  • The documentation for the coords parameter of func getCoordinates(_ coords:UnsafeMutablePointer<CLLocationCoordinate2D>,range range: NSRange) states: “On input, you must provide a C array of structures large enough to hold the desired number of coordinates. On output, this structure contains the requested coordinate data.”

    I tried several things, most recently:

    var coordinates: UnsafeMutablePointer<CLLocationCoordinate2D> = nil
    polyline.getCoordinates(&coordinates, range: NSMakeRange(0, polyline.pointCount))

    Would I have to use something like:

    var coordinates = UnsafeMutablePointer<CLLocationCoordinate2D>(calloc(1, UInt(polyline.pointCount)))

    Pulling my hair out here… any thoughts?

    2 Solutions Collect From Internet About “UnsafeMutablePointer in swift as replacement for properly sized C Array in Obj-C”

    Normally you can just pass an array of the required type as an in-out parameter, aka

    var coords: [CLLocationCoordinate2D] = []
    polyline.getCoordinates(&coords, range: NSMakeRange(0, polyline.pointCount))

    but that documentation makes it seem like a bad idea! Luckily, UnsafeMutablePointer provides a static alloc(num: Int) method, so you can call getCoordinates() like this:

    var coordsPointer = UnsafeMutablePointer<CLLocationCoordinate2D>.alloc(polyline.pointCount)
    polyline.getCoordinates(coordsPointer, range: NSMakeRange(0, polyline.pointCount))

    To get the actual CLLocationCoordinate2D objects out of the mutable pointer, you should be able to just loop through:

    var coords: [CLLocationCoordinate2D] = []
    for i in 0..<polyline.pointCount {

    And since you don’t want a memory leak, finish up like so:


    Just remembered Array has a reserveCapacity() instance method, so a much simpler (and probably safer) version of this would be:

    var coords: [CLLocationCoordinate2D] = []
    polyline.getCoordinates(&coords, range: NSMakeRange(0, polyline.pointCount))

    extension wrapper for @Nate Cook’s awesome answer, cannot get the reserveCapacity() version to work, it keep returning empty object.

    import MapKit
    extension MKPolyline {
        var coordinates: [CLLocationCoordinate2D] {
            get {
                let coordsPointer = UnsafeMutablePointer<CLLocationCoordinate2D>.allocate(capacity: pointCount)
                var coords: [CLLocationCoordinate2D] = []
                for i in 0..<pointCount {
                coordsPointer.deallocate(capacity: pointCount)
                return coords