Proper use of swift_allocObject()?

[NSExceptional]

Given the following extension:

extension ClassMetadata {
    func createInstance<T: AnyObject>(props: [String: Any] = [:]) -> T {
        var obj = swift_allocObject(
            for: self,
            size: self.classSize,
            alignment: self.instanceAlignmentMask
        )
        
        for (key, value) in props {
            self.set(value: value, forKey: key, on: &obj)
        }
        
        return unsafeBitCast(obj, to: T.self)
    }
}

And class definition:

class JustDeallocateMe {
    var expectation: XCTestExpectation
    
    init(_ expectation: XCTestExpectation) {
        self.expectation = expectation
    }
    
    deinit {
        self.expectation.fulfill()
    }
}

I would expect the following test to pass, but it does not:

func testAllocObject() {
    let expect = XCTestExpectation(description: "deinit")
    let cls = reflectClass(JustDeallocateMe.self)!
    DispatchQueue.global().async {
        var obj: JustDeallocateMe? = cls.createInstance(props: ["expectation": expect])
        obj = nil
    }
    
    self.wait(for: [expect], timeout: 2)
}

The deinit is never executed. I have tests verifying my .set(value:forKey:on) method works. What am I doing wrong?

[Azoy]

So the correct way to call swift_allocObject is:

let object = swift_allocObject(
  for: someMetadata,
  size: someMetadata.instanceSize,
  alignment: someMetadata.instanceAlignmentMask
)

Where it will return a pointer to a heap object and the fields can be placed in the respective field offset from that pointer.

[NSExceptional]

Not really related, but do you know how to check if something given as Any is an object? Using thing as AnyObject will just bridge struct types to NSValue…

My test now working has just uncovered a bug in my set(value:forKey:on:) method. I need to retain objects when they're being set as Any.

[Azoy]

You can do something like:

let box = container(for: someAny)
print(box.type)
print(box.metadata.kind)

if box.metadata.kind == .class || box.metadata.kind == .objcClassWrapper {
  // Any is an object
}

[NSExceptional]

That works I guess, is there not a way to do it in pure swift though?

[Azoy]

thing is AnyObject this bridges to NSValue as well.. What's weird is that the following compiles:

print(thing is AnyObject) // true

but the following does not:

func test<T: AnyObject>(with: T) {}

test(with: thing) // error: some struct needs to be a class type

Difference between the compiler's type system and the runtimes...

[NSExceptional]

Yeah, the last like will work if you cast it first, weirdly