The weak references is added to Objective-C since Mac OS X 10.7 and iOS 5, together with Xcode 4, and older versions of Mac OS X, iOS as well as GNUstep support only assign references as weak ones.
The weak references is added to Objective-C since Mac OS X 10.7 "Lion" and iOS 5, together with Xcode 4.1 (4.2 for iOS). Older versions of Mac OS X, iOS, and GNUstep support only assign references as weak ones.