(goops.info.gz) Metaclass

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 Metaclass
 .........
 
 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)
 
 GOOPS actually expands the `define-class' form to something like this
 
      (define <my-class> (class (<object>) . slots))
 
 and thence to
 
      (define <my-class>
        (make <class> #:supers (list <object>) #:slots slots))
 
 In other words, the value of `<my-class>' is in fact an instance of the
 class `<class>' with slot values specifying the superclasses and slot
 definitions for the class `<my-class>'.  (`#:supers' and `#:slots' are
 initialization keywords for the `dsupers' and `dslots' slots of the
 `<class>' class.)
 
 In order to take advantage of the full power of the GOOPS metaobject
 protocol ( MOP Specification), it is sometimes desirable to
 create 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>)
 
 GOOPS expands this to something like:
 
      (define <my-class2>
        (make <my-metaclass> #:supers (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>'.
 
    * `my-object' is an instance of the class `<my-class2>'.
 
    * `<my-class2>' is an instance of the class `<my-metaclass>'.
 
    * `<my-metaclass>' is an instance of the class `<class>'.
 
    * The class of `my-object' is `<my-class2>'.
 
    * The metaclass of `my-object' is `<my-metaclass>'.
 
    * The class of `<my-class2>' is `<my-metaclass>'.
 
    * The metaclass of `<my-class2>' is `<class>'.
 
    * The class of `<my-metaclass>' is `<class>'.
 
    * The metaclass of `<my-metaclass>' is `<class>'.
 
    * `<my-class2>' is not a metaclass, since it is does not inherit from
      `<class>'.
 
    * `<my-metaclass>' is a metaclass, since it inherits from `<class>'.
 

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