How to Cast Self to Unsafemutablepointer≪Void≫ Type in Swift

How to cast self to UnsafeMutablePointerVoid type in swift

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.

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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

Cast a Swift struct to UnsafeMutablePointerVoid

As far as I know, the shortest way is:

var myStruct = TheStruct()
var address = withUnsafeMutablePointer(&myStruct) {UnsafeMutablePointer<Void>($0)}

But, why you need this? If you want pass it as a parameter, you can (and should):

func foo(arg:UnsafeMutablePointer<Void>) {
    //...
}

var myStruct = TheStruct()
foo(&myStruct)

Typecast UnsafeMutablePointerVoid to UnsafeMutablePointer#Struct type#

I think what you want to say is something like this:

imageData = UnsafeMutablePointer<PixelRGB>.alloc(dataLength)

Swift casting unsafemutablepointerMyStruct to C void return pointer in swift

look at this 'self explanatory' example

struct MyStruct {
    var myId: Int
    var myDouble: Double
}

var myStruct = MyStruct(myId: 1, myDouble: 1.0)

// this mimics the funcion which returns Void *myStruct
var vptr = withUnsafeMutablePointer(&myStruct) { (pStruct) -> UnsafeMutablePointer<Void> in
    return UnsafeMutablePointer<Void>(pStruct)
}
print(vptr) // 0x00000001079c7828
// mimics mPtr =  (MyStruct *) vptr
let mPtr = UnsafeMutablePointer<MyStruct>(vptr) // UnsafeMutablePointer(0x10EE37838)

mPtr.memory.myId = 10
mPtr.memory.myDouble = 10.0

print(myStruct) // MyStruct(myId: 10, myDouble: 10.0)

Casting between different UnsafePointerT in swift

struct UnsafePointer<T> has a constructor

/// Convert from a UnsafePointer of a different type.
///
/// This is a fundamentally unsafe conversion.
init<U>(_ from: UnsafePointer<U>)

which you can use here

doThingsOnRawData(UnsafePointer<UInt8>(data.bytes))

You can even omit the generic type because it is inferred from the context:

doThingsOnRawData(UnsafePointer(data.bytes))

Update for Swift 3: As of Xcode 8 beta 6, you cannot convert
directly between different unsafe pointers anymore.

For data: NSData, data.bytes is a UnsafeRawPointer which can
be converted to UnsafePointer<UInt8> with assumingMemoryBound:

doThingsOnRawData(data.bytes.assumingMemoryBound(to: UInt8.self))

For data: Data it is even simpler:

data.withUnsafeBytes { doThingsOnRawData($0) }

Swift 2 - UnsafeMutablePointerVoid to object

This should work: pass the object pointer as an opaque unmanaged pointer
to the callback:

context.info = UnsafeMutablePointer(Unmanaged.passUnretained(myObject).toOpaque())
SCNetworkReachabilitySetCallback(reachability, callback, &context) 

and retrieve in the callback via:

func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutablePointer<Void>) {

    let myObject = Unmanaged<MyObject>.fromOpaque(COpaquePointer(info)).takeUnretainedValue()

}

Of course this assumes that some strong reference to the object exists
as long as the callback is installed, so that the object is not
deallocated.

Update: Note that both conversions from object pointer to void pointer
and back can be simplified if you are willing to use "unsafe" functions:

context.info = unsafeAddressOf(myObject)
// ...
myObject = unsafeBitCast(info, MyObject.self)

The generated assembly code is – as far as I can see – identical.

Update 2: See also How to cast self to UnsafeMutablePointer<Void> type in swift for more information
about the "bridging" and some helper functions which can be used here.

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Swift 3 update (Xcode 8 beta 6):

var context = SCNetworkReachabilityContext(version: 0, info: nil, retain: nil, release: nil, copyDescription: nil)
context.info = UnsafeMutableRawPointer(Unmanaged.passUnretained(self).toOpaque())

// ...

func callback(reachability:SCNetworkReachability, flags: SCNetworkReachabilityFlags, info: UnsafeMutableRawPointer?) {
    if let info = info {
        let myObject = Unmanaged<MyObject>.fromOpaque(info).takeUnretainedValue()
        // ...
    }
}

UnsafeMutablePointerVoid to Concrete Object Type

UnsafeMutablePointer has an initializer that takes another UnsafeMutablePointer of another type, the result being the same pointer but to the new type.

So in your case:

let infoPtr = UnsafeMutablePointer<Info>(context)
let info = infoPtr.memory

Beware though this is, as the docs describe it, "a fundamentally unsafe conversion". If the pointer you have is not a pointer to the type you convert it to, or if the memory you are now accessing it is not valid in this context, you may end up accessing invalid memory and your program will get a little crashy.

How to cast back correctly a pointer (Swift & native)?

Thanks to @MartinR I found the solution here

https://stackoverflow.com/a/33310021/5709159

exactly this method helps me to solve the problem

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

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