On 06 Aug 2015, at 02:19, Rick Mann <rm...@latencyzero.com> wrote: > On Aug 5, 2015, at 17:14 , Jens Alfke <j...@mooseyard.com> wrote: >> It’s part of the language design that only classes support inheritance, not >> structs or enums. >> >> Basically, subclassing pass-by-value types is problematic. For example, what >> happens when you assign a SubclassStruct value to a variable of type >> BaseStruct, or pass it to a function parameter of type BaseStruct? Do the >> extra instance variables get chopped off when it’s copied? What happens if >> you call a BaseStruct method that was overridden in SubclassStruct? >> >> C++ lets you do this, but it can lead to really nasty problems, so style >> guides like Effective C++ recommend avoiding it. > > But for extending the cases in an enum, it seems pretty nice.
You can't do that, because instances of the subclassed enum won’t be valid instances of the super-enum. Consider enum A which has values x and y. Now subclass it as B and add a value z. What happens if you take a variable of type B with value z and assign it to a variable (or pass it to a parameter) of type A? Either you get an A with an illegal value, or you get a runtime exception. Yuck. —Jens _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
