ANSI Common Lisp 4 Types and Classes 4.3 Classes
4.3.7 Integrating Types and ClassesThe object system maps the space of classes into the space of types. Every class that has a proper name has a corresponding type with the same name.
The proper name of every class is a valid type specifier. In addition, every class object is a valid type specifier. Thus the expression (typep object class) evaluates to true if the class of object is class itself or a subclass of class. The evaluation of the expression (subtypep class1 class2) returns the values true and true if class1 is a subclass of class2 or if they are the same class; otherwise it returns the values false and true. If I is an instance of some class C named S and C is an instance of standard-class, the evaluation of the expression (type-of I ) returns S if S is the proper name of C; otherwise, it returns C.
Many but not all of the predefined type specifiers have a corresponding class with the same proper name as the type. These type specifiers are listed in Figure 4.3.7 Integrating Types and Classes. For example, the type array has a corresponding class named array. No type specifier that is a list, such as (vector double-float 100), has a corresponding class. The operator deftype does not create any classes.
Each class that corresponds to a predefined type specifier can be implemented in one of three ways, at the discretion of each implementation. It can be a standard class, a structure class, or a system class.
A built-in class is one whose generalized instances have restricted capabilities or special representations. Attempting to use defclass to define subclasses of a built-in-class signals an error. Calling make-instance to create a generalized instance of a built-in class signals an error. Calling slot-value on a generalized instance of a built-in class signals an error. Redefining a built-in class or using change-class to change the class of an object to or from a built-in class signals an error. However, built-in classes can be used as parameter specializers in methods.
It is possible to determine whether a class is a built-in class by checking the metaclass. A standard class is an instance of the class standard-class, a built-in class is an instance of the class built-in-class, and a structure class is an instance of the class structure-class.
Each structure type created by defstruct without using the :type option has a corresponding class. This class is a generalized instance of the class structure-class. The :include option of defstruct creates a direct subclass of the class that corresponds to the included structure type.
It is implementation-dependent whether slots are involved in the operation of functions defined in this specification on instances of classes defined in this specification, except when slots are explicitly defined by this specification.
If in a particular implementation a class defined in this specification has slots that are not defined by this specfication, the names of these slots must not be external symbols of packages defined in this specification nor otherwise accessible in the cl-user package.
The purpose of specifying that many of the standard type specifiers have a corresponding class is to enable users to write methods that discriminate on these types. Method selection requires that a class precedence list can be determined for each class.
The class precedence list information specified in the entries for each of these classes are those that are required by the object system.
Individual implementations may be extended to define other type specifiers to have a corresponding class. Individual implementations may be extended to add other subclass relationships and to add other elements to the class precedence lists as long as they do not violate the type relationships and disjointness requirements specified by this standard. A standard class defined with no direct superclasses is guaranteed to be disjoint from all of the classes in the table, except for the class named t.