CLHS: Section 4.3.7

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## 4.3.7 Integrating Types and Classes

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.

    arithmetic-error                  generic-function    simple-error
    array                             hash-table          simple-type-error
    bit-vector                        integer             simple-warning
    broadcast-stream                  list                standard-class
    built-in-class                    logical-pathname    standard-generic-function
    cell-error                        method              standard-method
    character                         method-combination  standard-object
    class                             null                storage-condition
    complex                           number              stream
    concatenated-stream               package             stream-error
    condition                         package-error       string
    cons                              parse-error         string-stream
    control-error                     pathname            structure-class
    division-by-zero                  print-not-readable  structure-object
    echo-stream                       program-error       style-warning
    end-of-file                       random-state        symbol
    error                             ratio               synonym-stream
    file-error                        rational            t
    file-stream                       reader-error        two-way-stream
    float                             readtable           type-error
    floating-point-inexact            real                unbound-slot
    floating-point-invalid-operation  restart             unbound-variable
    floating-point-overflow           sequence            undefined-function
    floating-point-underflow          serious-condition   vector
    function                          simple-condition    warning

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.

The following X3J13 cleanup issues, not part of the specification, apply to this section:

- REAL-NUMBER-TYPE:X3J13-MAR-89
- CONDITION-SLOTS:HIDDEN
- METACLASS-OF-SYSTEM-CLASS:UNSPECIFIED

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