#### 7.7.25 defclass [Macro]

`defclass` class-name ({superclass-name}*) ({slot-specifier}*) [[!class-option]]
⇒ new-class

slot-specifier::=slot-name | (slot-name [[!slot-option]])

slot-name::= symbol

slot-option::={`:reader` reader-function-name}* |
{`:writer` writer-function-name}* |
{`:accessor` reader-function-name}* |
{`:allocation` allocation-type} |
{`:initarg` initarg-name}* |
{`:initform` form} |
{`:type` type-specifier} |
{`:documentation` string}

function-name::= {symbol | `(setf symbol)`}

class-option::=(`:default-initargs` `.` initarg-list) |
(`:documentation` string) |
(`:metaclass` class-name)

#### Arguments and Values::

Class-name—a non-nil symbol.

Superclass-name–a non-nil symbol.

Slot-name–a symbol. The slot-name argument is a symbol that is syntactically valid for use as a variable name.

Reader-function-name—a non-nil symbol. `:reader` can be supplied more than once for a given slot.

Writer-function-name—a generic function name. `:writer` can be supplied more than once for a given slot.

Reader-function-name—a non-nil symbol. `:accessor` can be supplied more than once for a given slot.

Allocation-type—(member `:instance` `:class`). `:allocation` can be supplied once at most for a given slot.

Initarg-name—a symbol. `:initarg` can be supplied more than once for a given slot.

Form—a form. `:init-form` can be supplied once at most for a given slot.

Type-specifier—a type specifier. `:type` can be supplied once at most for a given slot.

Class-option— refers to the class as a whole or to all class slots.

Initarg-list—a list of alternating initialization argument names and default initial value forms. `:default-initargs` can be supplied at most once.

Class-name—a non-nil symbol. `:metaclass` can be supplied once at most.

new-class—the new class object.

#### Description::

The macro defclass defines a new named class. It returns the new class object as its result.

The syntax of defclass provides options for specifying initialization arguments for slots, for specifying default initialization values for slots, and for requesting that methods on specified generic functions be automatically generated for reading and writing the values of slots. No reader or writer functions are defined by default; their generation must be explicitly requested. However, slots can always be accessed using slot-value.

Defining a new class also causes a type of the same name to be defined. The predicate `(typep object class-name)` returns true if the class of the given object is the class named by class-name itself or a subclass of the class class-name. A class object can be used as a type specifier. Thus `(typep object class)` returns true if the class of the object is class itself or a subclass of class.

The class-name argument specifies the proper name of the new class. If a class with the same proper name already exists and that class is an instance of standard-class, and if the defclass form for the definition of the new class specifies a class of class standard-class, the existing class is redefined, and instances of it (and its subclasses) are updated to the new definition at the time that they are next accessed. For details, see Redefining Classes.

Each superclass-name argument specifies a direct superclass of the new class. If the superclass list is empty, then the superclass defaults depending on the metaclass, with standard-object being the default for standard-class.

The new class will inherit slots and methods from each of its direct superclasses, from their direct superclasses, and so on. For a discussion of how slots and methods are inherited, see Inheritance.

The following slot options are available:

`*`

  The `:reader` slot option specifies that an unqualified method is to be defined on the generic function named reader-function-name to read the value of the given slot.

`*`

  The `:writer` slot option specifies that an unqualified method is to be defined on the generic function named writer-function-name to write the value of the slot.

`*`

  The `:accessor` slot option specifies that an unqualified method is to be defined on the generic function named reader-function-name to read the value of the given slot and that an unqualified method is to be defined on the generic function named `(setf reader-function-name)` to be used with setf to modify the value of the slot.

`*`

  The `:allocation` slot option is used to specify where storage is to be allocated for the given slot. Storage for a slot can be located in each instance or in the class object itself. The value of the allocation-type argument can be either the keyword `:instance` or the keyword `:class`. If the `:allocation` slot option is not specified, the effect is the same as specifying `:allocation :instance`.

  –

    If allocation-type is `:instance`, a local slot of the name slot-name is allocated in each instance of the class.

  –

    If allocation-type is `:class`, a shared slot of the given name is allocated in the class object created by this defclass form. The value of the slot is shared by all instances of the class. If a class C_1 defines such a shared slot, any subclass C_2 of C_1 will share this single slot unless the defclass form for C_2 specifies a slot of the same name or there is a superclass of C_2 that precedes C_1 in the class precedence list of C_2 and that defines a slot of the same name.

`*`

  The `:initform` slot option is used to provide a default initial value form to be used in the initialization of the slot. This form is evaluated every time it is used to initialize the slot. The lexical environment in which this form is evaluated is the lexical environment in which the defclass form was evaluated. Note that the lexical environment refers both to variables and to functions. For local slots, the dynamic environment is the dynamic environment in which make-instance is called; for shared slots, the dynamic environment is the dynamic environment in which the defclass form was evaluated. See Object Creation and Initialization.

  No implementation is permitted to extend the syntax of defclass to allow `(slot-name form)` as an abbreviation for `(slot-name :initform form)`.

  [Reviewer Note by Barmar: Can you extend this to mean something else?]

`*`

  The `:initarg` slot option declares an initialization argument named initarg-name and specifies that this initialization argument initializes the given slot. If the initialization argument has a value in the call to initialize-instance, the value will be stored into the given slot, and the slot’s `:initform` slot option, if any, is not evaluated. If none of the initialization arguments specified for a given slot has a value, the slot is initialized according to the `:initform` slot option, if specified.

`*`

  The `:type` slot option specifies that the contents of the slot will always be of the specified data type. It effectively declares the result type of the reader generic function when applied to an object of this class. The consequences of attempting to store in a slot a value that does not satisfy the type of the slot are undefined. The `:type` slot option is further discussed in Inheritance of Slots and Slot Options.

`*`

  The `:documentation` slot option provides a documentation string for the slot. `:documentation` can be supplied once at most for a given slot. [Reviewer Note by Barmar: How is this retrieved?]

Each class option is an option that refers to the class as a whole. The following class options are available:

`*`

  The `:default-initargs` class option is followed by a list of alternating initialization argument names and default initial value forms. If any of these initialization arguments does not appear in the initialization argument list supplied to make-instance, the corresponding default initial value form is evaluated, and the initialization argument name and the form’s value are added to the end of the initialization argument list before the instance is created; see Object Creation and Initialization. The default initial value form is evaluated each time it is used. The lexical environment in which this form is evaluated is the lexical environment in which the defclass form was evaluated. The dynamic environment is the dynamic environment in which make-instance was called. If an initialization argument name appears more than once in a `:default-initargs` class option, an error is signaled.

`*`

  The `:documentation` class option causes a documentation string to be attached with the class object, and attached with kind type to the class-name. `:documentation` can be supplied once at most.

`*`

  The `:metaclass` class option is used to specify that instances of the class being defined are to have a different metaclass than the default provided by the system (the class standard-class).

Note the following rules of defclass for standard classes:

`*`

  It is not required that the superclasses of a class be defined before the defclass form for that class is evaluated.

`*`

  All the superclasses of a class must be defined before an instance of the class can be made.

`*`

  A class must be defined before it can be used as a parameter specializer in a defmethod form.

The object system can be extended to cover situations where these rules are not obeyed.

Some slot options are inherited by a class from its superclasses, and some can be shadowed or altered by providing a local slot description. No class options except `:default-initargs` are inherited. For a detailed description of how slots and slot options are inherited, see Inheritance of Slots and Slot Options.

The options to defclass can be extended. It is required that all implementations signal an error if they observe a class option or a slot option that is not implemented locally.

It is valid to specify more than one reader, writer, accessor, or initialization argument for a slot. No other slot option can appear more than once in a single slot description, or an error is signaled.

If no reader, writer, or accessor is specified for a slot, the slot can only be accessed by the function slot-value.

If a defclass form appears as a top level form, the compiler must make the class name be recognized as a valid type name in subsequent declarations (as for deftype) and be recognized as a valid class name for defmethod parameter specializers and for use as the `:metaclass` option of a subsequent defclass. The compiler must make the class definition available to be returned by find-class when its environment argument is a value received as the environment parameter of a macro.

#### Exceptional Situations::

If there are any duplicate slot names, an error of type program-error is signaled.

If an initialization argument name appears more than once in `:default-initargs` class option, an error of type program-error is signaled.

If any of the following slot options appears more than once in a single slot description, an error of type program-error is signaled: `:allocation`, `:initform`, `:type`, `:documentation`.

It is required that all implementations signal an error of type program-error if they observe a class option or a slot option that is not implemented locally.

#### See Also::

documentation , Initialize-Instance , make-instance , slot-value , Classes, Inheritance, Redefining Classes, Determining the Class Precedence List, Object Creation and Initialization