https://stackoverflow.com/questions/61236195/create-a-weak-unsafemutablerawpointer/61236529#61236529

PointerIssue.swift

import Dispatch
import Foundation

// The Weak struct is the weak wrapper
struct Weak<T: AnyObject> {
	weak var object: T?
}

// Stand-in for AUGraphAddRenderNotify
func call(
	cCallback: @escaping @convention(c) (UnsafeMutableRawPointer) -> Void,
	inRefCon: UnsafeMutableRawPointer
) {
	cCallback(inRefCon) // call once immediately
	
	DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
		cCallback(inRefCon) // call once after deallocation of the TestObject
	}
}

// A callback that's guarenteed to not capture a TestObject
let theCallBack:  @convention(c) (UnsafeMutableRawPointer) -> Void = { rawPointer in
		print("Running callback")
		let weakWrapperPointer = rawPointer.assumingMemoryBound(to: Weak<TestObject>.self)
		let weakWrapper = weakWrapperPointer.pointee
		if let object = weakWrapper.object {
			print(object.value)
		}
	}

// TestObject represents the AudioPlayer class
class TestObject {
	let value = "test"
	
	init () {
		print("init")
		
		// Create a wrapper for this object
		let weakWrapper = Weak<TestObject>(object: self)

		// Create the wrapper pointer
		let size = MemoryLayout<Weak<TestObject>>.size
		let weakWrapperPointer = UnsafeMutableRawPointer.allocate(byteCount: size, alignment: 1)
		weakWrapperPointer.storeBytes(of: weakWrapper, as: Weak<TestObject>.self)
		
		// This is the problem. I can't figure out how to get this to work without crashing
		call(cCallback: theCallBack, inRefCon: weakWrapperPointer)
	}
	
	deinit {
		print("deinit")
	}
}


// Create a test object, which immediately gets deallocated (represents the AudioPlayer)
_ = TestObject()

RunLoop.main.run(until: Date().addingTimeInterval(2))
print("The end")

Was the problem the way I was making the pointer?

Copying my answer here for passers-by:

storeBytes(of:as:) doesn't work here. Look at the documentation

The type T to be stored must be a trivial type.

struct Weak<T> isn't a trivial type, because it has an implicit deinitializer which decrements the reference count of the weak ref's side table entry.

What's happening is that the deinit runs are the end of TestObject.init, when the weakWrapper local variable goes out of scope.

But then, hen you load it from the raw pointer, you're copying the same weak ref, and tryign to deinitialize again, causing a double-free error

Instead, I move initialize the pointer's destination, destroying the weakWrapper local variable (without running its deinitializer, thus not decrementing the weak ref)

I think this may be a simpler solution:

class WeakVar<T: AnyObject> {
    weak var object: T?

    init(object: T) {
      self.object = object
    }
}

// Stand-in for AUGraphAddRenderNotify
func call(
	cCallback: @escaping @convention(c) (UnsafeMutableRawPointer) -> Void,
	inRefCon: UnsafeMutableRawPointer
) {
	cCallback(inRefCon) // call once immediately
	
	DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
		cCallback(inRefCon) // call once after deallocation of the TestObject
	}
}

// A callback that's guarenteed to not capture a TestObject
let theCallBack:  @convention(c) (UnsafeMutableRawPointer) -> Void = { rawPointer in
		print("Running callback")

		let weakWrapper = Unmanaged<WeakVar>.fromOpaque(opaque).takeRetainedValue()
		if let object = weakWrapper.object {
			print(object.value)
		}
	}

class TestObject {
    init() {
        let weakVar = Weak<TestObject>(object: self)
        let opaque = Unmanaged.passRetained(weakVar).toOpaque()
        call(cCallback: theCallBack, inRefCon: opaque)
    }
}

@mori-nobuteru if I understand correctly, you’re passing in a strong reference to a WeakVar object (now a class), whereas I was passing in a a raw pointer to the struct.

I like your approach better! It involves one more reference counted object, but it’s semantics are clearer