4.3.1 Introduction to Classes

A class is an object that determines the structure and behavior of a set of other objects, which are called its instances.

A class can inherit structure and behavior from other classes. A class whose definition refers to other classes for the purpose of inheriting from them is said to be a subclass of each of those classes. The classes that are designated for purposes of inheritance are said to be superclasses of the inheriting class.

A class can have a name. The function class-name takes a class object and returns its name. The name of an anonymous class is nil. A symbol can name a class. The function find-class takes a symbol and returns the class that the symbol names. A class has a proper name if the name is a symbol and if the name of the class names that class. That is, a class C has the proper name S if S= (class-name C) and C= (find-class S). Notice that it is possible for (find-class S1) = (find-class S2) and S1/=S2. If C= (find-class S), we say that C is the class named S.

A class C1 is a direct superclass of a class C2 if C2 explicitly designates C1 as a superclass in its definition. In this case C2 is a direct subclass of C1. A class Cn is a superclass of a class C1 if there exists a series of classes C2,...,Cn-1 such that Ci+1 is a direct superclass of Ci for 1 <=i<n. In this case, C1 is a subclass of Cn. A class is considered neither a superclass nor a subclass of itself. That is, if C1 is a superclass of C2, then C1 /=C2. The set of classes consisting of some given class C along with all of its superclasses is called ``C and its superclasses.''

Each class has a class precedence list, which is a total ordering on the set of the given class and its superclasses. The total ordering is expressed as a list ordered from most specific to least specific. The class precedence list is used in several ways. In general, more specific classes can shadow[1] features that would otherwise be inherited from less specific classes. The method selection and combination process uses the class precedence list to order methods from most specific to least specific.

When a class is defined, the order in which its direct superclasses are mentioned in the defining form is important. Each class has a local precedence order, which is a list consisting of the class followed by its direct superclasses in the order mentioned in the defining form.

A class precedence list is always consistent with the local precedence order of each class in the list. The classes in each local precedence order appear within the class precedence list in the same order. If the local precedence orders are inconsistent with each other, no class precedence list can be constructed, and an error is signaled. The class precedence list and its computation is discussed in Section 4.3.5 (Determining the Class Precedence List).

classes are organized into a directed acyclic graph. There are two distinguished classes, named t and standard-object. The class named t has no superclasses. It is a superclass of every class except itself. The class named standard-object is an instance of the class standard-class and is a superclass of every class that is an instance of the class standard-class except itself.

There is a mapping from the object system class space into the type space. Many of the standard types specified in this document have a corresponding class that has the same name as the type. Some types do not have a corresponding class. The integration of the type and class systems is discussed in Section 4.3.7 (Integrating Types and Classes).

Classes are represented by objects that are themselves instances of classes. The class of the class of an object is termed the metaclass of that object. When no misinterpretation is possible, the term metaclass is used to refer to a class that has instances that are themselves classes. The metaclass determines the form of inheritance used by the classes that are its instances and the representation of the instances of those classes. The object system provides a default metaclass, standard-class, that is appropriate for most programs.

Except where otherwise specified, all classes mentioned in this standard are instances of the class standard-class, all generic functions are instances of the class standard-generic-function, and all methods are instances of the class standard-method.