Question

in objc.h , there is definition for Class

typedef struct objc_class *Class;

typedef struct objc_object {

  Class isa;
} *id;

Case : use Class in NSObject :

/*************** Basic protocols  ***************/

@protocol NSObject

- (BOOL)isEqual:(id)object;
- (NSUInteger)hash;

- (Class)superclass;
- (Class)class;
- (id)self;

Hard to understand, so Class is just structure, not real class like NSObject etc.

What is the real purpose of Class ?

Answer 1

All of the dynamic behaviors of the Objective-C runtime are available because of the Class class. It’s not a class that you will ever subclass or implement. The runtime fills in metadata about your class that can be accessed for introspection or modification.

Languages such as C++ and even Java have very limited support for this sort of thing. Objective-C is closer to the Ruby or Python scripting languages in this regard. You can actually replace instance methods in existing classes with your own implementation at run-time and affect the class in other ways that most compiled languages just can’t touch.