How to cast self to UnsafeMutablePointer<Void> type in swift

Question

Trying to pass "self" to a C function in swift, when calling following code:

var callbackStruct : AURenderCallbackStruct = 
    AURenderCallbackStruct.init(
      inputProc: recordingCallback,
      inputProcRefCon: UnsafeMutablePointer<Void>
    )

What is the ideal way to cast "self" to a UnsafeMutablePointer type here?

Solution 1

An object pointer (i.e. an instance of a reference type) can be converted to a UnsafePointer<Void> (the Swift mapping of const void *, UnsafeRawPointer in Swift 3) and back. In Objective-C you would write

void *voidPtr = (__bridge void*)self;
// 
MyType *mySelf = (__bridge MyType *)voidPtr;

(See 3.2.4 Bridged casts in the Clang ARC documentation for the precise meaning of these casts.)

Swift has an Unmanaged type for that purpose. It is a bit cumbersome to use because it works with COpaquePointer instead of UnsafePointer<Void>. Here are two helper methods (named after the Objective-C __bridge cast):

func bridge<T : AnyObject>(obj : T) -> UnsafePointer<Void> {
    return UnsafePointer(Unmanaged.passUnretained(obj).toOpaque())
    // return unsafeAddressOf(obj) // ***
}

func bridge<T : AnyObject>(ptr : UnsafePointer<Void>) -> T {
    return Unmanaged<T>.fromOpaque(COpaquePointer(ptr)).takeUnretainedValue()
    // return unsafeBitCast(ptr, T.self) // ***
}

The "complicated" expression is only necessary to satisfy Swifts strict type system. In the compiled code this is just a cast between pointers. (It can be written shorter as indicated in the *** comments if you are willing to use "unsafe" methods, but the compiled code is identical.)

Using this helper methods you can pass self to a C function as

 let voidPtr = bridge(self)

(or UnsafeMutablePointer<Void>(bridge(self)) if the C function requires a mutable pointer), and convert it back to an object pointer – e.g. in a callback function – as

 let mySelf : MyType = bridge(voidPtr)

No transfer of ownership takes place, so you must ensure that self exists as long as the void pointer is used.

---

And for the sake of completeness, the Swift equivalent of __bridge_retained and __bridge_transfer from Objective-C would be

func bridgeRetained<T : AnyObject>(obj : T) -> UnsafePointer<Void> {
    return UnsafePointer(Unmanaged.passRetained(obj).toOpaque())
}

func bridgeTransfer<T : AnyObject>(ptr : UnsafePointer<Void>) -> T {
    return Unmanaged<T>.fromOpaque(COpaquePointer(ptr)).takeRetainedValue()
}

bridgeRetained() casts the object pointer to a void pointer and retains the object. bridgeTransfer() converts the void pointer back to an object pointer and consumes the retain.

An advantage is that the object cannot be deallocated between the calls because a strong reference is held. The disadvantage is that the calls must be properly balanced, and that it can easily cause retain cycles.

---

Update for Swift 3 (Xcode 8):

func bridge<T : AnyObject>(obj : T) -> UnsafeRawPointer {
    return UnsafeRawPointer(Unmanaged.passUnretained(obj).toOpaque())
}

func bridge<T : AnyObject>(ptr : UnsafeRawPointer) -> T {
    return Unmanaged<T>.fromOpaque(ptr).takeUnretainedValue()
}

func bridgeRetained<T : AnyObject>(obj : T) -> UnsafeRawPointer {
    return UnsafeRawPointer(Unmanaged.passRetained(obj).toOpaque())
}

func bridgeTransfer<T : AnyObject>(ptr : UnsafeRawPointer) -> T {
    return Unmanaged<T>.fromOpaque(ptr).takeRetainedValue()
}

The relevant changes to "unsafe pointers" are described in

• SE-0017 Change Unmanaged to use UnsafePointer
• SE-0107 UnsafeRawPointer API

Solution 2

It seems to me that this is what withUnsafeMutablePointer is for - to convert an arbitrary Swift pointer into a C pointer. So presumably you could do this (I have not tried it, but the code I've tested works safely):

var mself = self 
withUnsafeMutablePointer(&mself) { v in
    let v2 = UnsafeMutablePointer<Void>(v)
    myStruct.inputProcRefCon = v2
}

Solution 3

func bridge<T : AnyObject>(obj : T) -> UnsafePointer<Void> {
return UnsafePointer(Unmanaged.passUnretained(obj).toOpaque())
}


func bridge<T : AnyObject>(ptr : UnsafePointer<Void>) -> T {
return Unmanaged<T>.fromOpaque(ptr).takeUnretainedValue()
}

func bridgeRetained<T : AnyObject>(obj : T) -> UnsafePointer<Void> {
return UnsafePointer( Unmanaged.passRetained(obj).toOpaque())}

func bridgeTransfer<T : AnyObject>(ptr : UnsafePointer<Void>) -> T {
return Unmanaged<T>.fromOpaque(ptr).takeRetainedValue()}

Comments

[matt]

Note that there are going to be memory management issues; "unsafe mutable pointer" means that retaining what's at the far end of the pointer is up to you. Therefore I would think that var mself = self would need to be replaced by a persistent global.

[Martin R]

This passes the address of mself to the C function, not the address of the object. As you said, mself needs to be a global which I find quite inelegant. – The code from stackoverflow.com/a/30788165/341994 looks complicated, but actually does *nothing. It just casts the object pointer self to a mutable pointer. In fact it can be simplified to let observer = unsafeAddressOf(self), the generated assembly code seems to be identical.

[Martin R]

(Cont.) The advantage of the explicit (complicated) expression is that it matches the reverse conversion from void pointer to object pointer, and that it can be modified to retain the object.

[Martin R]

Actually the reverse conversion can be simplified as well using unsafeBitCast() ...

[Martin R]

Where would you put the mself variable? A local variable does not work as you said. A global variable means that two separate instances of the class cannot be registered. An instance variable would lead to a retain cycle. – And how would you recover self from the pointer in the callback? I wasn't able to make it work with your approach. – Sorry to keep badgering you, but you reopened the question and asked for my opinion :) Since I cannot close it again, I have now added an answer which summarizes the conversion as I see it.

[matt]

@MartinR You're not badgering me. I'm badgering you! My answer was intended to goad you into writing a real explanation, and that's exactly what you did. Thanks! :)

[Peter Peng]

Sorry for the late reply and thanks for the effort. UnsafeMutablePointer(Unmanaged.passUnretained(self).toOpaque‌​()) is the way to pass it. And nice to have those bridge methods to convert the pointer. Thank you.

[marchinram]

Thanks!! Wasted so much time on this problem

[Kirsteins]

Can this approach be used like this: 1. Pass in retained Box with weak variable to self. 2. Callback optionally bids the the value to strong. 3. If instance is deallocated and Box contains nil the callback consumes the retain of the Box?

[Martin R]

@Kirsteins: That should be possible but you still have to unregister the callback if the instance is deallocated. It might be easier to pass an unretained pointer and unregister the callback in the deinit method of the instance.

[Martin R]

@matt: Thank you.