UntestedTypePunningExample.swift

/*
This code is completely untested but should be close enough to correct to demonstrate the concepts
*/

struct RegularMonkey {
  var bananaCount: Int
}

struct KlingonMonkey {
  var bananaCount: Int
  var opacity: Float //it has a cloaking device
}

var transparentMonkey = KlingonMonkey(bananaCount: 50, opacity: 0.5)
withUnsafePointer(to: transparentMonkey) { (basePointer:UnsafePointer<KlingonMonkey>) in
  
  //Goal: treat `basePointer` as a pointer to a `RegularMonkey` instance, since their layout in memory is compatible, but also still be able to access the "subclass" pointer
  
  // ❌ Failed attempt 1: just cast the pointer
  /* Compiler is nice and warns us:
   "warning build: 'unsafeBitCast' from 'UnsafePointer<KlingonMonkey>' to 'UnsafePointer<RegularMonkey>' changes pointee type and may lead to undefined behavior; use the 'withMemoryRebound' method on 'UnsafePointer<KlingonMonkey>' to rebind the type of memory"
  */
  let monkey = unsafeBitCast(basePointer, to: UnsafePointer<RegularMonkey>.self).pointee
  
  // ❌ Failed attempt 2: rebind the pointer
  basePointer.withMemoryRebound(to: RegularMonkey.self, capacity: 1) { monkeyPtr in
    let monkey: RegularMonkey = monkeyPtr.pointee //This is ok! We rebound the memory
    let cloakedMonkey = basePointer.pointee //NOPE, we rebound that, can't use it safely anymore
  }
  
  // ✅🤮 Successful but awful attempt 3: repeatedly rebind the pointer
  basePointer.withMemoryRebound(to: RegularMonkey.self, capacity: 1) { monkeyPtr in
    let monkey: RegularMonkey = monkeyPtr.pointee //This is ok! We rebound the memory
    monkeyPtr.withMemoryRebound(to: KlingonMonkey.self, capacity: 1) { cloakedMonkeyPtr in
      let cloakedMonkey: KlingonMonkey = cloakedMonkeyPtr.pointee //This is ok! But, ew
    }
  }
  
  // ✅ Successful attempt 4: use a raw pointer
  let rawMonkeyPtr = UnsafeRawPointer(basePointer)
  let monkey2: RegularMonkey = rawMonkeyPtr.load(as: RegularMonkey.self)
  let cloakedMonkey: KlingonMonkey = rawMonkeyPtr.load(as: KlingonMonkey.self)
}