Why subclass NSObject?

Question

What is the purpose/use of NSObject in Objective-C? I see classes that extend NSObject like this:

@interface Fraction : NSObject 

In C++ or

Answer 1

NSObject

The root class of most Objective-C class hierarchies, from which subclasses inherit a basic interface to the runtime system and the ability to behave as Objective-C objects.

From Apple documentation - https://developer.apple.com/documentation/objectivec/nsobject.

Basically, most of OOP programming languages explicitly or implicitly specify base class or base functionality. Otherwise you cannot build system where objects communicate with each other. Properties, memory management, message sending mechanism are partly or completely provided or supported by NSObject. Apple provide parts of the Objective-C implementation - https://opensource.apple.com/source/objc4/objc4-723/runtime/NSObject.mm.auto.html, where it's possible to see what is actually inside NSObject.

Also because Objective-C is a language from C-family, so compiler and linker needs to calculate how to layout object in memory and where put and find methods, that's only possible if you know how each of the classes/instances lays in memory and where. In case of Objective-C all base classes (NSObject, NSProxy, etc) have specification of that, so it's possible to calculate their size and add on top all inherited stuff - https://clang.llvm.org/compatibility.html#objective-c.

Consequently compiler don't let to leave a class without base class. So in the end class inheritance should lead to one of the root classes. Here is the error that appears if you don't specify it (from Xcode):

Class 'ClassWithoutBaseClass' defined without specifying a base class