Question

I noticed that Cocoa/Objective-C classes inherit and conform from other classes. I understand what inheritance is, but not conformance. What’s the difference?

Also, is a class like UIView a Cocoa class or an Objective-C class?

Answer 1

Classes don’t conform to classes, they conform to protocols, which are basically lists of messages that a class can respond to. If a class conforms to a protocol, it must respond to all the non-optional messages listed in the protocol interface.

Also, there isn’t really a difference between a Cocoa class and an Objective-C class. Cocoa is an Objective-C framework, so all of its classes are Objective-C classes. If you’re asking whether classes like UIView are part of the language or the framework: Every class you normally interact with in a Cocoa/Cocoa Touch app comes from the framework. The native classes the standard Objective-C runtime provides are:

• Object
• Protocol
• List

You can go to /usr/include/objc if you want to see the headers for yourself. None of these are used in a normal Cocoa or Cocoa Touch program. You do use Protocols (as mentioned above), but don’t normally refer to the class directly.