How do I correctly check for a successful downcast to AnyObject?

Question

I'm working with something trivialized to this:

func isAnyObject<T>(someObject: T) {
    if let object = someObject as? AnyObject {
        // do something
    } else {
        // do something else
    }
}

Which gives me the compiler warnings:

1. Conditional cast from 'T' to 'AnyObject' always succeeds
2. Non-optional expression of type 'AnyObject' used in a check for optionals

I understand why the cast always succeeds, but I don't know what to do to fix it. I tried changing line 2 to if let object = foo as AnyObject? but when I pass a struct through the function, it still gets into the 'if' block. What am I missing here?

Answer 1

Actually I can't recreate the errors you're exhibiting. However, the compiler does complain about the obvious errors in the parameter of the function.

import Foundation

func isAnyObject<T>( _ someObject: T) {
    if let object = someObject as? AnyObject {
        print("It's an AnyObject: \(object)")
    } else {
        print("It's not an AnyObject.")
    }
}

let classObject: NSString = "This is an NSString Object"
let structInstance: String = "This is a Swift String Instance"

isAnyObject(classObject) // It's an AnyObject: This is an NSString Object
isAnyObject(structInstance) // It's not an AnyObject.

You can try it online, here.

Answer 2

You can check if the type of the variable is AnyObject. If so its a reference type. If not its a value type. I also get a warning in the playground, but the warning is wrong. The code below produces false, false, false, false, true so clearly the function works and the test is not always true as the warning says. This is a bug.

    func isReferenceType<T>(someObject: T) -> Bool {
        return type(of: someObject) is AnyObject
    }

    //Values
    print(isReferenceType(someObject: "a"))
    print(isReferenceType(someObject: 1))
    print(isReferenceType(someObject: Int?(0)))
    print(isReferenceType(someObject: {print("1")}))
    //Reference
    print(isReferenceType(someObject: UIView()))