Swift Type Casting

Swift Type Casting

Use as, as?, and as! to convert between types, especially with protocols and Any.

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Upcasting and Downcasting

Convert a value to a supertype (upcast) or attempt to convert back to a subtype using optional downcasting with as?.

Example

let items: [Any] = [1, "Swift"]
for item in items {
  if let i = item as? Int {
print("Int: \(i)")
  } else if let s = item as? String {
print("String: \(s)")
  }
}

This example conditionally casts values from Any to their concrete types.

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Forced Downcast

Use as! only when you are certain of the runtime type. If the cast fails, the program will crash.

Example

let value: Any = 42
let i = value as! Int   // forced downcast
print(i)

Prefer optional casting (as?) when the type may vary, and unwrap safely.