DEFCLASS

You create user-defined classes with the **DEFCLASS** macro. Because behaviors are associated with a class by defining generic functions and methods specialized on the class, **DEFCLASS** is responsible only for defining the class as a data type.

The three facets of the class as a data type are its name, its relation to other classes, and the names of the slots that make up instances of the class.2 The basic form of a **DEFCLASS** is quite simple.

(defclass name (direct-superclass-name*)
  (slot-specifier*))

What Are “User-Defined Classes”?

The term user-defined classes isn’t a term from the language standard—technically what I’m talking about when I say user-defined classes are classes that subclass **STANDARD-OBJECT** and whose metaclass is **STANDARD-CLASS**. But since I’m not going to talk about the ways you can define classes that don’t subclass **STANDARD-OBJECT** and whose metaclass isn’t **STANDARD-CLASS**, you don’t really have to worry about that. User-defined isn’t a perfect term for these classes since the implementation may define certain classes the same way. However, to call them standard classes would be even more confusing since the built-in classes, such as **INTEGER** and **STRING**, are just as standard, if not more so, because they’re defined by the language standard but they don’t extend **STANDARD-OBJECT**. To further complicate matters, it’s also possible for users to define new classes that don’t subclass **STANDARD-OBJECT**. In particular, the macro **DEFSTRUCT** also defines new classes. But that’s largely for backward compatibility—**DEFSTRUCT** predated CLOS and was retrofitted to define classes when CLOS was integrated into the language. But the classes it creates are fairly limited compared to **DEFCLASS**ed classes. So in this chapter I’ll be discussing only classes defined with **DEFCLASS** that use the default metaclass of **STANDARD-CLASS**, and I’ll refer to them as user-defined for lack of a better term.

As with functions and variables, you can use any symbol as the name of a new class.3 Class names are in a separate namespace from both functions and variables, so you can have a class, function, and variable all with the same name. You’ll use the class name as the argument to **MAKE-INSTANCE**, the function that creates new instances of user-defined classes.

The direct-superclass-names specify the classes of which the new class is a subclass. If no superclasses are listed, the new class will directly subclass **STANDARD-OBJECT**. Any classes listed must be other user-defined classes, which ensures that each new class is ultimately descended from **STANDARD-OBJECT**. **STANDARD-OBJECT** in turn subclasses **T**, so all user-defined classes are part of the single class hierarchy that also contains all the built-in classes.

Eliding the slot specifiers for a moment, the **DEFCLASS** forms of some of the classes you used in the previous chapter might look like this:

(defclass bank-account () ...)
(defclass checking-account (bank-account) ...)
(defclass savings-account (bank-account) ...)

I’ll discuss in the section “Multiple Inheritance” what it means to list more than one direct superclass in direct-superclass-names.