I want to unit test some Swift classes which would take Locale as a parameter and then behave in locale-specific way. It looks like there is no easy way to provide my own locale implementation though. I'd like to avoid swizzling, so I tried to subclass NSLocale, i.e.: class MyLocale: NSLocale { init(myargs: whatever) { // initialize here super.init(localeIdentifier: "C") } } but this crashes in runtime:  -[NSLocale initWithLocaleIdentifier:]: method only defined for abstract class. So I re-define that initializer: class MyLocale: NSLocale { override init(localeIdentifier string: String) { // initialize here super.init(localeIdentifier: string) // putting super.init() here doesn't help either } } but the result is the same: but this crashes in runtime:  -[NSLocale initWithLocaleIdentifier:]: method only defined for abstract class. is there any way to properly do what I want without resorting to Objective-C runtime and method swizzling?