The 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.
Because the names of classes
and class objects are type specifiers, they may
be used in the special form the and in type declarations.
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 4.8.
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 hierarchical relationships among the type specifiers are mirrored by relationships among the classes corresponding to those types.
Figure 4.8 lists the set of classes that correspond to predefined type specifiers.
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Figure 4.8: Classes that correspond to pre-defined type specifiers
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.