Here is the code in question:

class Something: NSCoding {
    ...

let smth = Something() 
let data = NSKeyedArchiver.archivedDataWithRootObject(smth);

It throws this exception:

2014-09-12 16:30:00.463 MyApp[30078:60b] *** NSForwarding: warning: object 0x7a04ac70 of class '_TtC8MyApp7Something' does not implement methodSignatureForSelector: -- trouble ahead
Unrecognized selector -[MyApp.Something replacementObjectForKeyedArchiver:]

It is because Something does not inherit from NSObject