This is a question I asked yesterday, and copy-pasted my question details and answer I get on StackOverflow topic just to have it here.

The Question

I read inline documentation of Swift and I am bit confused.

  1. Any is a protocol that all types implicitly conform.
  2. AnyObject is a protocol to which all classes implicitly conform.
  3. Int, Float, Double are structs

Here is a sample code:

import UIKit

func passAnyObject(param: AnyObject) {

class MyClass {}
struct MyStruct {}

let a: Int = 1
let b = 2.0
let c = NSObject()
let d = MyClass()
let e = MyStruct()

//passAnyObject(e) // Argument type 'MyStruct' does not conform to expected type 'AnyObject'

if a is AnyObject { // b, d, e is also AnyObject
    print("\(a.dynamicType) is AnyObject")

What I don’t understand is why Int, Double, Float are AnyObjects? Why compiler doesn’t say anything? Those types are declared as structs. Struct MyStruct cannot be passed to the method on the top because it does not conform to AnyObject.

Could you help me understand why it(…)?

The Answer

vacawama from SO answered:

Because you have Foundation imported, Int, Double, and Float get converted to NSNumber when passed to a function taking an AnyObject. This is done to make life easier when calling Cocoa and Cocoa Touch based interfaces. If you remove import UIKit (or import Cocoa for OS X), you will see: error: argument type Int does not conform to expected type 'AnyObject' when you call passAnyObject(a)`