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Understanding Objective-C Runtime

This is an excerpt from Objective-C runtime programming guide:

When a new object is created, memory is allocated for it and its instance variables are initialized. The first among variable objects is a pointer to its class structure. This pointer, called isa, gives the object access to its class and through the class to all classes to which it inherits.

isa is declared NSObjectas follows:

Class   isa;

In turn, Classis nothing more than a pointer to a structure

typedef struct objc_class *Class;

Now look at this structure:

struct objc_class {
Class isa;

#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

}

We see that the pointer to the superclass (like all other members of the structure, except the other) isa is not available in the latest version of Objective-C.

, , , super_class ? ? ? ? - ?

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objc-class.m

Class class_getSuperclass(Class cls)
{
    return _class_getSuperclass(cls);
}

runtime-new.mm

#define newcls(cls) ((class_t *)cls)

Class 
_class_getSuperclass(Class cls)
{
    return (Class)getSuperclass(newcls(cls));
}

static class_t *
getSuperclass(class_t *cls)
{
    if (!cls) return NULL;
    return cls->superclass;
}

Class class_t

objc-runtime-new.h

typedef struct class_t {
    struct class_t *isa;
    struct class_t *superclass;
    Cache cache;
    IMP *vtable;
    uintptr_t data_NEVER_USE;  // class_rw_t * plus custom rr/alloc flags

    class_rw_t *data() const { 
        return (class_rw_t *)(data_NEVER_USE & ~(uintptr_t)3); 
    }
    void setData(class_rw_t *newData) {
        uintptr_t flags = (uintptr_t)data_NEVER_USE & (uintptr_t)3;
        data_NEVER_USE = (uintptr_t)newData | flags;
    }

    bool hasCustomRR() const {
#if CLASS_FAST_FLAGS_VIA_RW_DATA
        return data_NEVER_USE & (uintptr_t)1;
#else
        return data()->flags & RW_HAS_CUSTOM_RR;
#endif
    }
    void setHasCustomRR(bool inherited = false);

    bool hasCustomAWZ() const {
#if CLASS_FAST_FLAGS_VIA_RW_DATA
        return data_NEVER_USE & (uintptr_t)2;
#else
        return data()->flags & RW_HAS_CUSTOM_AWZ;
#endif
    }
    void setHasCustomAWZ(bool inherited = false);

    bool isRootClass() const {
        return superclass == NULL;
    }
    bool isRootMetaclass() const {
        return isa == this;
    }
} class_t;

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, , runtime.h

, , call class_getSuperclass

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