Opt-in Type Conversion Method in Swift

Swift supports opt-in to convert an object to a type via the instance method __conversion(). This method can used through the as operator and is overloadable to support conversion to multiple types:
class X {
  func __conversion() -> String { return "as a string!" }
  func __conversion() -> Double { return 101.2 }

println( X() as String )  //Output: as a string!

var d : Double = X()      
println( d )              //Output: 101.2