ios - Apple Swift: Type Casting Generics

Question

I'm writing some Swift code where I have an array containing a generic type:

let _data: Array<T> = T[]()

Later in my code I need to determine the type stored in the array. I tried using the type casting technique described in the documentation (although it was not used for generics).

switch self._data {
case let doubleData as Array<Double>:
  // Do something with doubleData
case let floatData as Array<Float>:
  // Do something with floatData
default:
  return nil // If the data type is unknown return nil
}

The above switch statement results in the following error upon compilation:

1. While emitting IR SIL function @_TFC19Adder_Example___Mac6Matrix9transposeUS_7Element__fGS0_Q__FT_GSqGS0_Q___ for 'transpose' at /code.viperscience/Adder/src/Adder Library/Matrix.swift:45:3 :0: error: unable to execute command: Segmentation fault: 11 :0: error: swift frontend command failed due to signal (use -v to see invocation) Command /Applications/Xcode6-Beta2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/swift failed with exit code 254

Anybody know how I can cast my generic data to its actual type in order to take specific action?

Answer 1

In swift, as operator is something like dynamic_cast in C++, which can be used to down cast an object.

Say you have an object a of type A, and you can write let a as B only when type B is identical to type A, or B is a sub-class of A.

In your case, apparently Array<T> cannot always be down cast to Array<Double> or Array<Float>, so compiler reports errors.

A simple fix is to convert to AnyObject first, and then downcast to Array<Double> or Array<Float>:

let anyData: AnyObject = self._data;
switch anyData {
case let doubleData as? Array<Double>: // use as? operator, instead of as,
                                       // to avoid runtime exception
  // Do something with doubleData
case let floatData as? Array<Float>:
  // Do something with floatData
default:
  return nil // If the data type is unknown return nil