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  1. 1. as vs as! vs as?
  2. 2. Reference

as vs as! vs as?

  • as used for upcasting and type casting to bridged type
  • as? used for safe casting, return nil if failed
  • as! used to force casting, crash if failed
  • as! can’t cast raw type to optional
  • as! and as? works by inspect the source type against the destination type (if the source type or destination type is optional, inspect the type inside)
  • If succeeded, as? returns optional that contains source type casted as destination type, while as! return source type casted as destination type
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let rawString: AnyObject = "Fantageek"
    let optionalString: AnyObject? = "Fantageek"
    let nilString: AnyObject? = (nil as String?)

    let rawInt: AnyObject = Int(3)
    let optionalInt: AnyObject? = Int(3)
    let nilInt: AnyObject? = (nil as Int?)

       // as
//    let result1 = rawString as String // AnyObject is not convertible to String
//    let result2 = optionalString as String // AnyObject? is not convertible to String
//    let result3 = nilString as String // AnyObject? is not convertible to String
//    let result4 = rawInt as String // AnyObject is not convertible to String
//    let result5 = optionalInt as String // AnyObject? is not convertible to String
//    let result6 = nilInt as String // AnyObject? is not convertible to String

     // as!
    let result7 = rawString as! String // String "Fantageek"
    let result8 = optionalString as! String // String "Fantageek"
//    let result9 = nilString as! String // unexpectedly found nil while unwrapping an Optional value
//    let result10 = rawInt as! String // Could not cast value of type '__NSCFNumber' (0x107c98130) to 'NSString' (0x1082afb00).
//    let result11 = optionalInt as! String // Could not cast value of type '__NSCFNumber' (0x101e95130) to 'NSString' (0x1024acb00).
//    let result12 = nilInt as! String // unexpectedly found nil while unwrapping an Optional value

//    let result13 = rawString as! String! // Cannot downcast from AnyObject to a more optional type String!
//    let result14 = rawString as! String? // Cannot downcast from AnyObject to a more optional type String?
    let result15 = optionalString as! String! // String! "Fantageek"
    let result16 = optionalString as! String? // String? "Fantageek"

    // as?
    let result17 = rawString as? String // String? "Fantageek"
    let result18 = optionalString as? String // String? "Fantageek"
    let result19 = nilString as? String // String? nil
    let result20 = rawInt as? String // String? nil
    let result21 = optionalInt as? String // String? nil
    let result22 = nilInt as? String // String? nil
    // case 22 is interesting, because nilInt is Optiona.None while result22 is Optional.None

//    let result23 = rawString as? String! // Cannot downcast from AnyObject to a more optional type String!
//    let result24 = rawString as? String? // Cannot downcast from AnyObject to a more optional type String?
    let result25 = optionalString as? String! // String!? "Fantageek"
    let result26 = optionalString as? String? // String?? "Fantageek
    let result27 = optionalInt as? String? // String?? nil




    // Up casting
    let button = UIButton()
    let view = button as UIView



     // Brigded type casting
    let string = "3"
    let nsString = string as NSString
    let value = nsString.floatValue // 3

Reference

Contents
  1. 1. as vs as! vs as?
  2. 2. Reference