In Objective-C instance data can be
@interface Foo : NSObject
Edit: Access control mechanisms have been introduced in Xcode 6 beta 4 (6A267n):
Swift access control has three access levels:
privateentities can only be accessed from within the source file where they are defined.
internalentities can be accessed anywhere within the target where they are defined.
publicentities can be accessed from anywhere within the target and from any other context that imports the current target’s module.
Swift currently has no access control mechanisms.
However, after a couple of complaints, he states:
We don't usually promise anything for the future, but in this case we are making an exception. Swift will have access control mechanisms.
That's certainly good news. :)