Brainstorming Objective-C runtime

Question

Every time I go back into understanding Objective-C runtime, I end up messing my brain. As far as I understand (from reading some awesome write-ups like Metaclasses, Obj-C Runtime) Objective-C classes are actually class objects e.g. NSString is itself an object to its Root Meta Class. Moreover its so called ivar "isa" pointer keeps all the basic details like super class, method lists, name etc. Below are some questions based on this theory only -

(1). Check the current declaration of objc/runtime.h and you'll find the following struct declaration:

    struct objc_class {
        Class isa  OBJC_ISA_AVAILABILITY;

>     #if !__OBJC2__
        Class super_class                                        OBJC2_UNAVAILABLE;
        const char *name                                         OBJC2_UNAVAILABLE;
        long version                                             OBJC2_UNAVAILABLE;
        long info                                                OBJC2_UNAVAILABLE;
        long instance_size                                       OBJC2_UNAVAILABLE;
        struct objc_ivar_list *ivars                             OBJC2_UNAVAILABLE;
        struct objc_method_list **methodLists                    OBJC2_UNAVAILABLE;
        struct objc_cache *cache                                 OBJC2_UNAVAILABLE;
        struct objc_protocol_list *protocols                     OBJC2_UNAVAILABLE;
> 
> **#endif**

    } OBJC2_UNAVAILABLE;

I hope this means that from Objective-C 2.0 all there is available in this struct is "isa" pointer. This seems to fade all the logic from this runtime theory. Where does these details like super class, method list etc. stored now?

---

(2). Technically isa is pointer to struct objc_class which itself has isa as its member. So when I try accessing (in AppDelegate for say):

     NSLog(@"class isa ptr %@", isa->isa);

or

NSLog(@"class isa %@", ((Class)[NSString class])->name);

I receive a compile-time error:

• Member reference base type 'Class' is not a structure or union.

Why? I think its the correct syntax to access structures members

---

(3). Printing 'Class' (I'm referring to typedef struct objc_class *Class) object on console using NSLog displays Objective-C class' name.

NSLog(@"NSObject class %@", [NSObject class]);

How is that so? class is a class method that returns 'Class' object. If class name was to be printed, it should be printed like this:

NSLog(@"NSObject class %@", ((Class)[NSObject class])->name);

Help me figure out what is there that I am missing.

Answer 1

(1) Where does these details like super class, method list etc. stored now?

In ObjC 2.0 all the class details are private implementation details. Maybe the data layout remained actually the same, maybe not.

(2) Why can't I access the field name?

The header file you quoted says that in ObjC 2.0 only the isa pointer is there, all other stuff has gone away.

(3) Why does NSLog print [NSString class] as NSString?

Because NSLog, like -[NSString stringWithFormat:] invokes the description method onto the object provided. Obviously, the method +[Class description] returns the name of the class.