typedef struct from C to Swift - ios

Question

I'm working on a Swift app that accesses a C library.
The .h file contains a typedef struct:
typedef struct _DATA_KEY_ * CURRENT_DATA_KEY;
And there a method in another class:
-(int) initWithKey(CURRENT_DATA_KEY * key);
I need to create a CURRENT_DATA_KEY in my code. Not sure how I can achieve that in Swift.
There is some old Objective-C code that uses:
CURRENT_DATA_KEY key = NULL;
initWithKey(key)
I've tried:
let myKey = UnsafeMutablePointer<CURRENT_DATA_KEY>.allocate(capacity: 1)
But when I try to use it later as an argument in the function, I get the error:
Cannot convert value of type 'UnsafeMutablePointer<_DATA_KEY_>' (aka 'UnsafeMutablePointer(<OpaquePointer>)') to expected argument type 'UnsafeMutablePointer(<_DATA_KEY_?>!)'
Which looks like the function is expecting an optional value?
I also tried:
let myKey: CURRENT_DATA_KEY? = nil
let myKey: CURRENT_DATA_KEY = NSNull()
But those get similar type errors.
How do I create: UnsafeMutablePointer(<_DATA_KEY_?>!)

Answer 1

It's been a bit since I've done this but IIRC this:
typedef struct _DATA_KEY_ * CURRENT_DATA_KEY;
-(int) initWithKey(CURRENT_DATA_KEY * key);
Is actually equivalent to:
-(int) initWithKey(struct _DATA_KEY_ ** key);
So if we look up Interacting with C APIs: Pointers it falls under:
Type **
Which becomes:
AutoreleasingUnsafeMutablePointer<Type>
If you know the members of the C struct then you may be able to treat it like a Swift struct and use the synthesized init method. So if it is defined like this:
struct _DATA_KEY_ {
int foo
};
It becomes like this in Swift:
public struct _DATA_KEY_ {
var foo: Int
init()
init(foo: Int)
}
And you call the init method like:
// no parameter
let myKey = AutoreleasingUnsafeMutablePointer<_DATA_KEY_>(&_DATA_KEY_())
// parameter
let myKey = AutoreleasingUnsafeMutablePointer<_DATA_KEY_>(&_DATA_KEY_(foo: 12))
I have not tried this code but I've followed the advice in the Apple document before and it's worked out well.