Swift CMutablePointers in factories e.g. NewMusicSequence - swift

Question

How do you use C level factory methods in Swift?
Let's try using a factory such as NewMusicSequence().
var status:OSStatus
var sequence:MusicSequence
status=NewMusicSequence(&sequence)
This errors out with "error: variable 'sequence' passed by reference before being initialized".
Set sequence to nil, and you get EXC_BAD_INSTRUCTION.
You can try being explicit like this:
var sp:CMutablePointer<MusicSequence>=nil
status=NewMusicSequence(sp)
But then you get a bad access exception when you set sp to nil. If you don't set sp, you get an "error: variable 'sp' used before being initialized"
I'd expect this to be the way to do it, but it's not:
import AudioToolbox
var sequence: MusicSequence?
var status:OSStatus = NewMusicSequence(&sequence)
error: cannot convert the expression's type 'OSStatus' to type 'inout MusicSequence?'
var status:OSStatus = NewMusicSequence(&sequence)
Here's the reference.

Answer 1

I think the issue is that you need an optional value. Optional values are always initialized to nil and only optional values can ever be nil.
var sequence: MusicSequence?
let status = NewMusicSequence(&sequence)

Answer 2

This works for me in an Xcode project. It seems to throw EXC_BAD_ACCESS in the playground for some reason.
import Foundation
import AudioToolbox
#objc class SwiftSequenceGenerator : NSObject
{
#objc func createMusicSequence() -> MusicSequence
{
var status : OSStatus = 0
var sequence : MusicSequence = MusicSequence()
status = NewMusicSequence(&sequence)
return sequence
}
}
let generator = SwiftSequenceGenerator()
let sequence = generator.createMusicSequence()
I was testing this in an Obj-C project, hence the #objc attributes. Both this and its Obj-C counterpart were able to generate a new MusicSequence.
It is a little tricky because MusicSequence is actually a struct. In Obj-C it is defined as a pointer OpaqueMusicSequence and Swift uses COpaquePointer. They are basically the same, using a void * to pass the MusicSequence struct to the factory method to be mutated.