as! vs as operator in Xcode 6.3 in Swift

Question

Swift changed a lot with Xcode 6.3. I had to replace dozens of places in each of my app as -> as!. Why, what are the rules now?

Answer 1

Prior to Swift 1.2, the as operator could be used to carry out two different kinds of conversion, depending on the type of expression being converted and the type it was being converted to:

• Guaranteed conversion of a value of one type to another, whose success can be verified by the Swift compiler. For example, upcasting (i.e., converting from a class to one of its superclasses) or specifying the type of a literal expression, (e.g., 1 as Float).

• Forced conversion of one value to another, whose safety cannot be guaranteed by the Swift compiler and which may cause a runtime trap. For example downcasting, converting from a class to one of its subclasses.

Swift 1.2 separates the notions of guaranteed conversion and forced conversion into two distinct operators. Guaranteed conversion is still performed with the as operator, but forced conversion now uses the as! operator. The ! is meant to indicate that the conversion may fail. This way, you know at a glance which conversions may cause the program to crash.

Source: https://developer.apple.com/swift/blog/?id=23