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3.1.1 Metaobjects and the Metaobject Protocol

The conceptual building blocks of GOOPS are classes, slot definitions, instances, generic functions and methods. A class is a grouping of inheritance relations and slot definitions. An instance is an object with slots that are allocated following the rules implied by its class's superclasses and slot definitions. A generic function is a collection of methods and rules for determining which of those methods to apply when the generic function is invoked. A method is a procedure and a set of specializers that specify the type of arguments to which the procedure is applicable.

Of these entities, GOOPS represents classes, generic functions and methods as "metaobjects". In other words, the values in a GOOPS program that describe classes, generic functions and methods, are themselves instances (or "objects") of special GOOPS classes that encapsulate the behaviour, respectively, of classes, generic functions, and methods.

(The other two entities are slot definitions and instances. Slot definitions are not strictly instances, but every slot definition is associated with a GOOPS class that specifies the behaviour of the slot as regards accessibility and protection from garbage collection. Instances are of course objects in the usual sense, and there is no benefit from thinking of them as metaobjects.)

The "metaobject protocol" (aka "MOP") is the specification of the generic functions which determine the behaviour of these metaobjects and the circumstances in which these generic functions are invoked.

For a concrete example of what this means, consider how GOOPS calculates the set of slots for a class that is being defined using define-class. The desired set of slots is the union of the new class's direct slots and the slots of all its superclasses. But define-class itself does not perform this calculation. Instead, there is a method of the initialize generic function that is specialized for instances of type <class>, and it is this method that performs the slot calculation.

initialize is a generic function which GOOPS calls whenever a new instance is created, immediately after allocating memory for a new instance, in order to initialize the new instance's slots. The sequence of steps is as follows.

• define-class uses make to make a new instance of the <class>, passing as initialization arguments the superclasses, slot definitions and class options that were specified in the define-class form.
• make allocates memory for the new instance, and then invokes the initialize generic function to initialize the new instance's slots.
• The initialize generic function applies the method that is specialized for instances of type <class>, and this method performs the slot calculation.

In other words, rather than being hardcoded in define-class, the behaviour of class definition is encapsulated by generic function methods that are specialized for the class <class>.

It is possible to create a new class that inherits from <class>, which is called a "metaclass", and to write a new initialize method that is specialized for instances of the new metaclass. Then, if the define-class form includes a #:metaclass class option whose value is the new metaclass, the class that is defined by the define-class form will be an instance of the new metaclass rather than of the default <class>, and will be defined in accordance with the new initialize method. Thus the default slot calculation, as well as any other aspect of the new class's relationship with its superclasses, can be modified or overridden.

In a similar way, the behaviour of generic functions can be modified or overridden by creating a new class that inherits from the standard generic function class <generic>, writing appropriate methods that are specialized to the new class, and creating new generic functions that are instances of the new class.

The same is true for method metaobjects. And the same basic mechanism allows the application class author to write an initialize method that is specialized to their application class, to initialize instances of that class.

Such is the power of the MOP. Note that initialize is just one of a large number of generic functions that can be customized to modify the behaviour of application objects and classes and of GOOPS itself. Each subsequent section of the reference manual covers a particular area of GOOPS functionality, and describes the generic functions that are relevant for customization of that area.

We conclude this subsection by emphasizing a point that may seem obvious, but contrasts with the corresponding situation in some other MOP implementations, such as CLOS. The point is simply that an identifier which represents a GOOPS class or generic function is a variable with a first-class value, the value being an instance of class <class> or <generic>. (In CLOS, on the other hand, a class identifier is a symbol that indexes the corresponding class metaobject in a separate namespace for classes.) This is, of course, simply an extension of the tendency in Scheme to avoid the unnecessary use of, on the one hand, syntactic forms that require unevaluated arguments and, on the other, separate identifier namespaces (e.g. for class names), but it is worth noting that GOOPS conforms fully to this Schemely principle.