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8.11.2 Metaclasses

A metaclass is the class of an object which represents a GOOPS class. Put more succinctly, a metaclass is a class’s class.

Most GOOPS classes have the metaclass <class> and, by default, any new class that is created using define-class has the metaclass <class>.

But what does this really mean? To find out, let’s look in more detail at what happens when a new class is created using define-class: 

(define-class <my-class> (<object>) . slots)

Guile expands this to something like: 

(define <my-class> (class (<object>) . slots))

which in turn expands to: 

(define <my-class>
  (make <class> #:dsupers (list <object>) #:slots slots))

As this expansion makes clear, the resulting value of <my-class> is an instance of the class <class> with slot values specifying the superclasses and slot definitions for the class <my-class>. (#:dsupers and #:slots are initialization keywords for the dsupers and dslots slots of the <class> class.)

Now suppose that you want to define a new class with a metaclass other than the default <class>. This is done by writing: 

(define-class <my-class2> (<object>)
   slot …
   #:metaclass <my-metaclass>)

and Guile expands this to something like: 

(define <my-class2>
  (make <my-metaclass> #:dsupers (list <object>) #:slots slots))

In this case, the value of <my-class2> is an instance of the more specialized class <my-metaclass>. Note that <my-metaclass> itself must previously have been defined as a subclass of <class>. For a full discussion of when and how it is useful to define new metaclasses, see MOP Specification.

Now let’s make an instance of <my-class2>: 

(define my-object (make <my-class2> ...))

All of the following statements are correct expressions of the relationships between my-object, <my-class2>, <my-metaclass> and <class>.

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