What is the recommended way to call APIs that have a `NSError **` argument?

# ios

[Jacob Rhoda]

What is the recommended way to call APIs that have a NSError ** argument?

[Jeff Lockhart]

The general pattern is something like:

memScoped {
    val error = alloc<ObjCObjectVar<NSError?>>()
    val result = objcFun(params, error.ptr)
    val nsError = error.value
    if (nsError != null) {
        // throw exception, return error type, etc.
    }
    // return result, success type, etc.
}

You can wrap this code in a reusable function that aligns with your app's error handling pattern.

[Jeff Lockhart]

Note, I've also found if you are using Objective-C optional constructors, that may return nil on error, since Kotlin constructors can't have null return types the interop header can't represent the constructor with a nullable type, and so it will throw a NullPointerException when an error occurs. In this case I catch the NPE and handle the NSError instead.

[Jacob Rhoda]

That’s helpful, thanks!

[Jacob Rhoda]

I found this example elsewhere on the web. Is yours substantially different from this?

val nsError: NSError? = null
val errorPtr = ObjCObjectVar<NSError?>(nsError.objcPtr()).ptr

[Jeff Lockhart]

Have you tried this code? It won't compile. Native memory has to be allocated on the heap: alloc<ObjCObjectVar<NSError?>>()

[Jacob Rhoda]

No, it seems to not give errors in the IDE but I am still working on the tests.

[Jacob Rhoda]

I suppose my question is, is yours just heap allocated and “mine” is on the stack? Or is getting stack pointers in Kotlin just unsupported?

[Jacob Rhoda]

Lol okay it doesn’t compile

[Jeff Lockhart]

There are occasional subtle differences between the IDE and the compiler with KMP that still need to be ironed out.