Int("123") // Returns 123 of Int type
Int("abcd") // Returns nil
Int("10") // Returns 10 of Int type
Int("10", radix: 2) // Returns 2 of Int type
Double("1.5") // Returns 1.5 of Double type
Double("abcd") // Returns nil

Note that doing this returns an [Optional](<http://stackoverflow.com/documentation/swift/247/optionals#t=201607230855233123287>) value, which should be unwrapped accordingly before being used.