Question

Because it is not really* possible to create abstract classes in objective-c I was wondering if anyone could tell me why the creators of objective-c didn’t add this feature to the language.

*of course there are a few workarounds to create a -kind of- abstract classes but thats not what i mean.

Answer 1

If you mean why you can’t declare a class abstract rather than just implement it as an abstract class, I suppose it’s because it doesn’t interact so well with Objective-C’s extremely dynamic, types-optional, Smalltalky message-passing style (Smalltalk doesn’t have built-in support for declaring them either). For example:

• You might not know what class you’re sending alloc or init to, so it would be a pretty weak guarantee

• Subclasses might still need to pass alloc and init up the class hierarchy, so we’d need to allow that, making it an even weaker guarantee

• Categories modify classes at runtime, making it the abstract contract weaker still

In the end, you’d wind up with a keyword that just did roughly the same thing we do now to implement abstract classes (i.e. maybe override init to bail if the receiver isn’t a subclass).

Type modifiers that just specify a contract become more useful the more you rely on the static type system — but Objective-C was designed not to rely on the type system all that much, so putting the contract in the type system is less useful than in some other languages.