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>.
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>.
my-object is <my-class2>.
<my-class2> is <my-metaclass>.
<my-metaclass> is <class>.