Swift: Address of static Void variable changes across multiple withUnsafePointers

Originator:jtbandes
Number:rdar://22410209 Date Originated:8/24/2015
Status:Closed Resolved:Behaves correctly
Product: Product Version:
Classification: Reproducible:Always
 
Motivation: a nice way to generate a unique "context" pointer to use for KVO.

Proposed solution: a static class variable of type Void (aka empty tuple).



class MyObject
{
    private static var KVOContext: Void
    
    init() {
        withUnsafePointer(&MyObject.KVOContext){
            print("context 1: \($0)")
        }
        withUnsafePointer(&MyObject.KVOContext){
            print("context 2: \($0)")
        }
    }
}

let o = MyObject()



Expected Results:
Address of MyObject.KVOContext is consistent across invocations



Actual Results:
Console output shows that the address varies:

context 1: 0x00007fff5d391060
context 2: 0xffffffffffffffff


Version:
Xcode 7.0 beta 6 (7A192o), OS X 10.10.5 (14F27)

Comments

"This issue behaves as intended based on the following: We don’t allocate storage for empty types, and don’t produce a stable address. If you need a unique address, you’ll need to at least use an Int8."

By jtbandes at Sept. 21, 2015, 12:42 a.m. (reply...)

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