Re: Swift enums and NSNotificationCenter
Re: Swift enums and NSNotificationCenter
- Subject: Re: Swift enums and NSNotificationCenter
- From: Jens Alfke <email@hidden>
- Date: Thu, 06 Aug 2015 09:20:24 -0700
On 06 Aug 2015, at 02:19, Rick Mann <email@hidden> wrote:
> On Aug 5, 2015, at 17:14 , Jens Alfke <email@hidden> 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
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