What root classes exist in iOS, and why are they needed?

Middle PRO

Essence

A root class is a class without a superclass, the top of a hierarchy. It gives all descendants the basic contract with the runtime: isa, alloc/init/dealloc, reference counting, introspection (class, isKindOfClass:, respondsToSelector:), isEqual:/hash, description, forwarding. Foundation has two root classes:

• NSObject — the root of almost everything. The class implements the NSObject protocol (NSObjectProtocol in Swift), which describes that basic contract.
• NSProxy — an abstract "stand-in object" that implements only the minimum of the NSObject protocol and has no init. Any message it lacks lands in methodSignatureForSelector: + forwardInvocation: — it exists to forward messages to a real object: lazy loading, distributed objects (NSDistantObject), mocks, logging.

Technically you can declare your own root class (__attribute__((objc_root_class)) or the NS_ROOT_CLASS macro), but then you write alloc, retain/release etc. yourself — nobody does this in practice.