Swift UnsafeMutablePointer<Unmanaged<CFString>?> allocation and print

I'm new to swift and I have some difficulties to deal with pointers of unmanaged CFString (or NSString). I'm working on a CoreMIDI project that implies usage of UnsafeMutablePointer?> as you can see in this function :

func MIDIObjectGetStringProperty(_ obj: MIDIObjectRef,
                           _ propertyID: CFString!,
                           _ str: UnsafeMutablePointer<Unmanaged<CFString>?>) -> OSStatus

My problem is that I want to allocate a buffer to receive the content of the property (_str) then call the function above, and finally print the content in the console by using println.

At the moment I wrote this :

// Get the first midi source (I know it exists)
var midiEndPoint : Unmanaged<MIDIEndpointRef> = MIDIGetSource(0)

//C reate a "constant" of 256
let buf = NSMutableData(capacity: 256) 

// Allocate a string buffer of 256 characters (I'm not even sure this does what I want)
var name = UnsafeMutablePointer<Unmanaged<CFString>?>(buf!.bytes)

// Call the function to fill the string buffer with the display name of the midi device
var err : OSStatus =  MIDIObjectGetStringProperty(&midiEndPoint,kMIDIPropertyDisplayName,name)

// Print the string ... here no surprises I don't know what to write to print the content of the pointer, so it prints the address for the moment
println(name)

I didn't find any sample code to use CoreMIDI functions on apple developper library not on the internet. I really confused because I come from cpp and things are a lot different in swift.

EDIT :

After Rintaro and Martin answers I still have a problem, all my test are done on iOS 8.1 and if I copy the code you brought to me the compiler tells me that I can't write :

let err = MIDIObjectGetStringProperty(midiEndPoint, kMIDIPropertyDisplayName, &property)

Results in 'Unmanaged' is not convertible to 'MIDIObjectRef'. So I added a "&" because MIDIObjectRef is a UnsafeMutablePointer<void>.

let midiEndPoint = MIDIGetSource(0)
var property : Unmanaged<CFString>?
let err = MIDIObjectGetStringProperty(&midiEndPoint, kMIDIPropertyDisplayName, &property)

Now : 'Unmanaged<MIDIEndpoint>' is not convertible to '@lvalue inout $T2'. Finally I had to change the first let to var, without understanding why ?!?

var midiEndPoint = MIDIGetSource(0)
var property : Unmanaged<CFString>?
let err = MIDIObjectGetStringProperty(&midiEndPoint, kMIDIPropertyDisplayName, &property)

The code now compiles and runs but MIDIObjectGetStringProperty returns OSStatus err -50 which corresponds to IOW or from MacErros.h :

paramErr  = -50,  /*error in user parameter list*/

So it seems that the parameters are not the ones that MIDIObjectGetStringProperty is waiting for.

The source "0" does exist on my iPad because MIDIGetNumberOfSources() returns 1. Here's the complete code :

var numDestinations: ItemCount = MIDIGetNumberOfDestinations()
    println("MIDI Destinations : " + String(numDestinations))

    for var i : ItemCount = 0 ; i < numDestinations; ++i{
        var midiEndPoint = MIDIGetDestination(i)

        var property : Unmanaged<CFString>?
        let err = MIDIObjectGetStringProperty(&midiEndPoint, kMIDIPropertyDisplayName, &property)
        if err == noErr {
            let displayName = property!.takeRetainedValue() as String
            println(displayName)
        }else{
            println("error : "+String(err))
        }
   }

Displays :

MIDI Destinations : 1
error : -50

I really don't understand anything ...

UPDATE :

Finally Martin found the solution, it seems that there are two different definitions of MIDIObjectRef in 32 and 64bits architectures. As I run the code on an old iPad 2 my code tried to compile in 32bits mode where MIDIGetSource(i) return value is not convertible into MIDIObjectRef. The solution is to "unsafe cast" the midi endpoint on 32 bits architectures :

#if arch(arm64) || arch(x86_64)
    let midiEndPoint = MIDIGetDestination(i)
#else
    let midiEndPoint = unsafeBitCast(MIDIGetDestination(i), MIDIObjectRef.self)
#endif

... Or to buy a new 64bit device ...

Thank you for the precious help

let midiEndPoint = MIDIGetSource(0) var property : Unmanaged<CFString>? let err = MIDIObjectGetStringProperty(midiEndPoint, kMIDIPropertyDisplayName, &property) if err == noErr { let displayName = property!.takeRetainedValue() as String println(displayName) }

let numSrcs = MIDIGetNumberOfSources() println("number of MIDI sources: \(numSrcs)") for srcIndex in 0 ..< numSrcs { #if arch(arm64) || arch(x86_64) let midiEndPoint = MIDIGetSource(srcIndex) #else let midiEndPoint = unsafeBitCast(MIDIGetSource(srcIndex), MIDIObjectRef.self) #endif var property : Unmanaged<CFString>? let err = MIDIObjectGetStringProperty(midiEndPoint, kMIDIPropertyDisplayName, &property) if err == noErr { let displayName = property!.takeRetainedValue() as String println("\(srcIndex): \(displayName)") } else { println("\(srcIndex): error \(err)") } }

Recommended topics

Hot tags