How to create a static pointer variable to itself in Swift?

Question

In Objective-C I often use the pattern of using a static void* as an identification tag. At times these tags are only used within that function/method, hence it\'s

Answer 1

Unlike (Objective-)C, you cannot take the address of an uninitialized variable in Swift. Therefore creating a self-referencing pointer is a two-step process:

Swift 2:

var ptr : UnsafePointer = nil
withUnsafeMutablePointer(&ptr) { $0.memory = UnsafePointer($0) }

Swift 3:

var ptr = UnsafeRawPointer(bitPattern: 1)!
ptr = withUnsafePointer(to: &ptr) { UnsafeRawPointer($0) }

For your purpose, is it easier to use the address of a global variable with &, see for example

• Is there a way to set associated objects in Swift?.

If you want to restrict the scope of the "tag" to the function itself then you can use a static variable inside a local struct. Example:

func obscureProperty(obj : AnyObject) -> MyObscureObject {
    struct Tag {
        static var ObscurePropertyTag : Int = 0
    } 
    if let propValue = objc_getAssociatedObject(obj, &Tag.ObscurePropertyTag) as? MyObscureObject {
        return propValue
    }
    let propValue = ... // lazy instantiate property value
    objc_setAssociatedObject(obj, &Tag.ObscurePropertyTag,propValue, .OBJC_ASSOCIATION_RETAIN_NONATOMIC)
    return propValue
}