Class.superclass = Module, Module.class = Class?

Question

How does that compute? It's circular

Update: (in irb)

Class.superclass = Module
Module.class = Class

How is it possible to say that a Module's class is Class, when Class is Modules underclass? It's circular, chicken and egg thing.

Object: same question: Object is the root object in the object model. How can its class be Class, since Class object doesn't even exist yet?

Answer 1

Let's take a look at the class.c file of the MRI source code:

void Init_class_hierarchy(void)
{
    id_attached = rb_intern("__attached__");

    rb_cBasicObject = boot_defclass("BasicObject", 0);
    /* boot_defclass is defined as boot_defclass(const char *name, VALUE super) */
    rb_cObject = boot_defclass("Object", rb_cBasicObject);
    rb_cModule = boot_defclass("Module", rb_cObject);
    rb_cClass =  boot_defclass("Class",  rb_cModule);

    /* Very important line: */
    RBASIC(rb_cClass)->klass
          = RBASIC(rb_cModule)->klass
          = RBASIC(rb_cObject)->klass
          = RBASIC(rb_cBasicObject)->klass
          = rb_cClass;
 }

These definitions in ruby.h are very important, too:

#define R_CAST(st)   (struct st*)
#define RBASIC(obj)  (R_CAST(RBasic)(obj))
#define ROBJECT(obj) (R_CAST(RObject)(obj))
#define RCLASS(obj)  (R_CAST(RClass)(obj))
#define RMODULE(obj) RCLASS(obj)

Note that Object, Module and Class are derived from BasicObject. Indeed,

irb(main):001:0> BasicObject.superclass
=> nil

Those objects are defined simultaneously and all of them have RBASIC(*)->klass = rb_cClass.

Answer 2

x.superclass and x.class have different semantics. Observe:

irb(main):003:0> 3.superclass
NoMethodError: undefined method `superclass' for 3:Fixnum
    from (irb):3
    from :0
irb(main):004:0> 3.class
=> Fixnum

3 doesn't have a superclass because... 3 isn't a class or anything like it. But 3.class means the class that 3 is an instance of.

So the thing that should correspond to Class.superclass is not Module.class, but Module itself.