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7. Objects

7.1 Object Creation and Initialization
7.2 Changing the Class of an Instance
7.3 Reinitializing an Instance
7.4 Meta-Objects
7.5 Slots
7.6 Generic Functions and Methods
Dictionary

7.1 Object Creation and Initialization#

2.5 objects

The generic function make-instance creates and returns a new instance of a class. The first argument is a class or the name of a class, and the remaining arguments form an initialization argument list.

The initialization of a new instance consists of several distinct steps, including the following: combining the explicitly supplied initialization arguments with default values for the unsupplied initialization arguments, checking the validity of the initialization arguments, allocating storage for the instance, filling slots with values, and executing user-supplied methods that perform additional initialization. Each step of make-instance is implemented by a generic function to provide a mechanism for customizing that step. In addition, make-instance is itself a generic function and thus also can be customized.

The object system specifies system-supplied primary methods for each step and thus specifies a well-defined standard behavior for the entire initialization process. The standard behavior provides four simple mechanisms for controlling initialization:

Declaring a symbol to be an initialization argument for a slot. An initialization argument is declared by using the :initarg slot option to defclass. This provides a mechanism for supplying a value for a slot in a call to make-instance.
Supplying a default value form for an initialization argument. Default value forms for initialization arguments are defined by using the :default-initargs class option to defclass. If an initialization argument is not explicitly provided as an argument to make-instance, the default value form is evaluated in the lexical environment of the defclass form that defined it, and the resulting value is used as the value of the initialization argument.
Supplying a default initial value form for a slot. A default initial value form for a slot is defined by using the :initform slot option to defclass. If no initialization argument associated with that slot is given as an argument to make-instance or is defaulted by :default-initargs, this default initial value form is evaluated in the lexical environment of the defclass form that defined it, and the resulting value is stored in the slot. The :initform form for a local slot may be used when creating an instance, when updating an instance to conform to a redefined class, or when updating an instance to conform to the definition of a different class. The :initform form for a shared slot may be used when defining or re-defining the class.
Defining methods for initialize-instance and shared-initialize. The slot-filling behavior described above is implemented by a system-supplied primary method for initialize-instance which invokes shared-initialize. The generic function shared-initialize implements the parts of initialization shared by these four situations: when making an instance, when re-initializing an instance, when updating an instance to conform to a redefined class, and when updating an instance to conform to the definition of a different class. The system-supplied primary method for shared-initialize directly implements the slot-filling behavior described above, and initialize-instance simply invokes shared-initialize.

7.1.1 Initialization Arguments#

An initialization argument controls object creation and initialization. It is often convenient to use keyword symbols to name initialization arguments, but the name of an initialization argument can be any symbol, including nil. An initialization argument can be used in two ways: to fill a slot with a value or to provide an argument for an initialization method. A single initialization argument can be used for both purposes.

An initialization argument list is a list of alternating ofproperty list of initialization argument names and values. Its structure is identical to a property list and also to the portion of an argument list processed for &key parameters. As in those lists, if an initialization argument name appears more than once in an initialization argument list, the leftmost occurrence supplies the value and the remaining occurrences are ignored. The arguments to make-instance (after the first argument) form an initialization argument list. Error-checking of initialization argument names is disabled if the keyword argument pair whose keyword is \kwd{allow-other-keys} and whose value is \term{non-nil} appears in the \term{initialization argument list}.

An initialization argument can be associated with a slot. If the initialization argument has a value in the initialization argument list, the value is stored into the slot of the newly created object, overriding any :initform form associated with the slot. A single initialization argument can initialize more than one slot. An initialization argument that initializes a shared slot stores its value into the shared slot, replacing any previous value.

An initialization argument can be associated with a method. When an object is created and a particular initialization argument is supplied, the generic functions initialize-instance, shared-initialize, and allocate-instance are called with that initialization argument's name and value as a keyword argument pair. If a value for the initialization argument is not supplied in the initialization argument list, the method's lambda list supplies a default value.

Initialization arguments are used in four situations: when making an instance, when re-initializing an instance, when updating an instance to conform to a redefined class, and when updating an instance to conform to the definition of a different class.

Because initialization arguments are used to control the creation and initialization of an instance of some particular class, we say that an initialization argument is “an initialization argument for” that class.

7.1.2 Declaring the Validity of Initialization Arguments#

Initialization arguments are checked for validity in each of the four situations that use them. An initialization argument may be valid in one situation and not another. For example, the system-supplied primary method for make-instance defined for the class standard-class checks the validity of its initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid in that situation.

There are two means for declaring initialization arguments valid.

Initialization arguments that fill slots are declared as valid by the :initarg slot option to defclass. The :initarg slot option is inherited from superclasses. Thus the set of valid initialization arguments that fill slots for a class is the union of the initialization arguments that fill slots declared as valid by that class and its superclasses. Initialization arguments that fill slots are valid in all four contexts.
Initialization arguments that supply arguments to methods are declared as valid by defining those methods. The keyword name of each keyword parameter specified in the method's lambda list becomes an initialization argument for all classes for which the method is applicable. The presence of &allow-other-keys in the lambda list of an applicable method disables validity checking of initialization arguments. Thus method inheritance controls the set of valid initialization arguments that supply arguments to methods. The generic functions for which method definitions serve to declare initialization arguments valid are as follows:
- – Making an instance of a class: allocate-instance, initialize-instance, and shared-initialize. Initialization arguments declared as valid by these methods are valid when making an instance of a class.
- – Re-initializing an instance: reinitialize-instance and shared-initialize. Initialization arguments declared as valid by these methods are valid when re-initializing an instance.
- – Updating an instance to conform to a redefined class: update-instance-for-redefined-class and shared-initialize. Initialization arguments declared as valid by these methods are valid when updating an instance to conform to a redefined class.
- – Updating an instance to conform to the definition of a different class: update-instance-for-different-class and shared-initialize. Initialization arguments declared as valid by these methods are valid when updating an instance to conform to the definition of a different class.

The set of valid initialization arguments for a class is the set of valid initialization arguments that either fill slots or supply arguments to methods, along with the predefined initialization argument :allow-other-keys. The default value for :allow-other-keys is nil. The meaning of \kwd{allow-other-keys} is the same as when it is passed to an ordinary \term{function}.Validity checking of initialization arguments is disabled if the value of the initialization argument :allow-other-keys is true.

7.1.3 Defaulting of Initialization Arguments#

A default value form can be supplied for an initialization argument by using the :default-initargs class option. If an initialization argument is declared valid by some particular class, its default value form might be specified by a different class. In this case :default-initargs is used to supply a default value for an inherited initialization argument.

The :default-initargs option is used only to provide default values for initialization arguments; it does not declare a symbol as a valid initialization argument name. Furthermore, the :default-initargs option is used only to provide default values for initialization arguments when making an instance.

The argument to the :default-initargs class option is a list of alternating initialization argument names and forms. Each form is the default value form for the corresponding initialization argument. The default value form of an initialization argument is used and evaluated only if that initialization argument does not appear in the arguments to make-instance and is not defaulted by a more specific class. The default value form is evaluated in the lexical environment of the defclass form that supplied it; the resulting value is used as the initialization argument's value.

The initialization arguments supplied to make-instance are combined with defaulted initialization arguments to produce a defaulted initialization argument list. A defaulted initialization argument list is a list of alternating initialization argument names and values in which unsupplied initialization arguments are defaulted and in which the explicitly supplied initialization arguments appear earlier in the list than the defaulted initialization arguments. Defaulted initialization arguments are ordered according to the order in the class precedence list of the classes that supplied the default values.

There is a distinction between the purposes of the :default-initargs and the :initform options with respect to the initialization of slots. The :default-initargs class option provides a mechanism for the user to give a default value form for an initialization argument without knowing whether the initialization argument initializes a slot or is passed to a method. If that initialization argument is not explicitly supplied in a call to make-instance, the default value form is used, just as if it had been supplied in the call. In contrast, the :initform slot option provides a mechanism for the user to give a default initial value form for a slot. An :initform form is used to initialize a slot only if no initialization argument associated with that slot is given as an argument to make-instance or is defaulted by :default-initargs.

The order of evaluation of default value forms for initialization arguments and the order of evaluation of :initform forms are undefined. If the order of evaluation is important, initialize-instance or shared-initialize methods should be used instead.

7.1.4 Rules for Initialization Arguments#

The :initarg slot option may be specified more than once for a given slot.

The following rules specify when initialization arguments may be multiply defined:

A given initialization argument can be used to initialize more than one slot if the same initialization argument name appears in more than one :initarg slot option.
A given initialization argument name can appear in the lambda list of more than one initialization method.
A given initialization argument name can appear both in an :initarg slot option and in the lambda list of an initialization method.

Reviewer: The next three paragraphs could be replaced by “If two or more initialization arguments that initialize the same slot appear in the defaulted initialization argument list, the leftmost of these supplies the value, even if they have different names.” And the rest would follow from the rules above.

If two or more initialization arguments that initialize the same slot are given in the arguments to make-instance, the leftmost of these initialization arguments in the initialization argument list supplies the value, even if the initialization arguments have different names.

If two or more different initialization arguments that initialize the same slot have default values and none is given explicitly in the arguments to make-instance, the initialization argument that appears in a :default-initargs class option in the most specific of the classes supplies the value. If a single :default-initargs class option specifies two or more initialization arguments that initialize the same slot and none is given explicitly in the arguments to make-instance, the leftmost in the :default-initargs class option supplies the value, and the values of the remaining default value forms are ignored.

Initialization arguments given explicitly in the arguments to make-instance appear to the left of defaulted initialization arguments. Suppose that the classes $C_{1}$ and $C_{2}$ supply the values of defaulted initialization arguments for different slots, and suppose that $C_{1}$ is more specific than $C_{2}$ ; then the defaulted initialization argument whose value is supplied by $C_{1}$ is to the left of the defaulted initialization argument whose value is supplied by $C_{2}$ in the defaulted initialization argument list. If a single :default-initargs class option supplies the values of initialization arguments for two different slots, the initialization argument whose value is specified farther to the left in the :default-initargs class option appears farther to the left in the defaulted initialization argument list.

Reviewer: Barmar: End of claim made three paragraphs back.

If a slot has both an :initform form and an :initarg slot option, and the initialization argument is defaulted using :default-initargs or is supplied to make-instance, the captured :initform form is neither used nor evaluated.

The following is an example of the above rules:

 (defclass q () ((x :initarg a)))
 (defclass r (q) ((x :initarg b))
   (:default-initargs a 1 b 2))

\begin{matrix} Defaulted \\ Form & Initialization Argument List & Contents of Slot X \\ (make-instance 'r) & (a 1 b 2) & 1 \\ (make-instance 'r 'a 3) & (a 3 b 2) & 3 \\ (make-instance 'r 'b 4) & (b 4 a 1) & 4 \\ (make-instance 'r 'a 1 'a 2) & (a 1 a 2 b 2) & 1 \end{matrix}

7.1.5 Shared-Initialize#

The generic function shared-initialize is used to fill the slots of an instance using initialization arguments and :initform forms when an instance is created, when an instance is re-initialized, when an instance is updated to conform to a redefined class, and when an instance is updated to conform to a different class. It uses standard method combination. It takes the following arguments: the instance to be initialized, a specification of a set of names of slots accessible in that instance, and any number of initialization arguments. The arguments after the first two must form an initialization argument list.

The second argument to shared-initialize may be one of the following:

It can be a (possibly empty) list of slot names, which specifies the set of those slot names.
\reviewer{Barmar: This next bullet item is redundant with the previous, since NIL -is- a LIST. If there was some confusion, we could say ``(possibly empty)'' in the previous item.} \itemitem{\bull} It can be \nil, which specifies the empty set of \term{slot} names.
It can be the symbol t, which specifies the set of all of the slots.

There is a system-supplied primary method for shared-initialize whose first parameter specializer is the class standard-object. This method behaves as follows on each slot, whether shared or local:

If an initialization argument in the initialization argument list specifies a value for that slot, that value is stored into the slot, even if a value has already been stored in the slot before the method is run. The affected slots are independent of which slots are indicated by the second argument to shared-initialize.
Any slots indicated by the second argument that are still unbound at this point are initialized according to their :initform forms. For any such slot that has an :initform form, that form is evaluated in the lexical environment of its defining defclass form and the result is stored into the slot. For example, if a before method stores a value in the slot, the :initform form will not be used to supply a value for the slot. If the second argument specifies a name that does not correspond to any slots accessible in the instance, the results are unspecified.
The rules mentioned in Section 7.1.4 (Rules for Initialization Arguments) are obeyed.

The generic function shared-initialize is called by the system-supplied primary methods for reinitialize-instance, update-instance-for-different-class, update-instance-for-redefined-class, and initialize-instance. Thus, methods can be written for shared-initialize to specify actions that should be taken in all of these contexts.

7.1.6 Initialize-Instance#

The generic function initialize-instance is called by make-instance to initialize a newly created instance. It uses standard method combination. Methods for initialize-instance can be defined in order to perform any initialization that cannot be achieved This was the only case of a half-glossary-term in the entire spec. -kmp 1-Jan-91 with the simple \term{slot}-filling mechanisms.simply by supplying initial values for slots.

During initialization, initialize-instance is invoked after the following actions have been taken:

The defaulted initialization argument list has been computed by combining the supplied initialization argument list with any default initialization arguments for the class.
The validity of the defaulted initialization argument list has been checked. If any of the initialization arguments has not been declared as valid, an error is signaled.
A new instance whose slots are unbound has been created.

The generic function initialize-instance is called with the new instance and the defaulted initialization arguments. There is a system-supplied primary method for initialize-instance whose parameter specializer is the class standard-object. This method calls the generic function shared-initialize to fill in the slots according to the initialization arguments and the :initform forms for the slots; the generic function shared-initialize is called with the following arguments: the instance, t, and the defaulted initialization arguments.

Note that initialize-instance provides the defaulted initialization argument list in its call to shared-initialize, so the first step performed by the system-supplied primary method for shared-initialize takes into account both the initialization arguments provided in the call to make-instance and the defaulted initialization argument list.

Methods for initialize-instance can be defined to specify actions to be taken when an instance is initialized. If only after methods for initialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of initialize-instance.

The object system provides two functions that are useful in the bodies of initialize-instance methods. The function slot-boundp returns a generic boolean value that indicates whether a specified slot has a value; this provides a mechanism for writing after methods for initialize-instance that initialize slots only if they have not already been initialized. The function slot-makunbound causes the slot to have no value.

7.1.7 Definitions of Make-Instance and Initialize-Instance#

The generic function make-instance behaves as if it were defined as follows, except that certain optimizations are permitted:

 (defmethod make-instance ((class standard-class) &rest initargs)
   ...
   (let ((instance (apply #'allocate-instance class initargs)))
     (apply #'initialize-instance instance initargs)
     instance))

 (defmethod make-instance ((class-name symbol) &rest initargs)
   (apply #'make-instance (find-class class-name) initargs))

This is the code: (defmethod make-instance ((class standard-class) &rest initargs) (setq initargs (default-initargs class initargs)) (let* ((proto (class-prototype class)) (methods (append (compute-applicable-methods #'allocate-instance `(,class)) (compute-applicable-methods #'initialize-instance `(,proto)) (compute-applicable-methods #'shared-initialize `(,proto nil))))) (unless (subsetp (let ((keys '())) (do ((plist initargs (cddr plist))) ((null plist) keys) (push (car plist) keys))) (union (class-slot-initargs class) (reduce #'union (mapcar #'function-keywords methods)))) (error ...))) (let ((instance (apply #'allocate-instance class initargs))) (apply #'initialize-instance instance initargs) instance))

The elided code in the definition of make-instance Per X3J13. -kmp 05-Oct-93augments the initargs with any defaulted initialization arguments and checks the Per X3J13. -kmp 05-Oct-93 suppliedresulting initialization arguments to determine whether an initialization argument was supplied that neither filled a slot nor supplied an argument to an applicable method. This check could be implemented using the generic functions ???\funref{class-prototype},??? \funref{compute-applicable-methods}, \funref{function-keywords}, and ???\funref{class-slot-initargs}. ??? See Chapter~3 for a description of this initialization argument check.

The generic function initialize-instance behaves as if it were defined as follows, except that certain optimizations are permitted:

 (defmethod initialize-instance ((instance standard-object) &rest initargs)
   (apply #'shared-initialize instance t initargs)))

Barmar complains that "Programmer Interface level" is not defined. Presumably it means "this specification". Ditto "the meta-object level" is not defined. Presumably it should just be omitted as beyond the scope of this standard, or else we should define the term somewhere (e.g., the glossary). I decided to just trim it down to where glossary words weren't needed. -kmp 6-Jan-91These procedures can be customized. at either the Programmer Interface level, the meta-object level, or both.

Customizing at the Programmer Interface level includes using the :initform, :initarg, and :default-initargs options to defclass, as well as defining methods for make-instance, Per X3J13. -kmp 05-Oct-93allocate-instance, and initialize-instance. It is also possible to define methods for shared-initialize, which would be invoked by the generic functions reinitialize-instance, update-instance-for-redefined-class, update-instance-for-different-class, and initialize-instance. The meta-object level supports additional customization. by allowing methods to be defined on \funref{make-instance}, ???\b{default-initargs}???, and \funref{allocate-instance}. Chapters~2 and~3 document each of these generic functions and the system-supplied primary methods.

Implementations are permitted to make certain optimizations to initialize-instance and shared-initialize. The description of shared-initialize in Chapter 7 mentions the possible optimizations.

Because of optimization, the check for valid initialization arguments might not be implemented using the generic functions ???\funref{class-prototype},??? \funref{compute-applicable-methods}, \funref{function-keywords}, and ???\funref{class-slot-initargs}???. In addition, methods for the generic function ???\funref{default-initargs},??? and the system-supplied primary methods for ???\funref{allocate-instance}???, \funref{initialize-instance}, and \funref{shared-initialize} might not be called on every call to \funref{make-instance} or might not receive exactly the arguments that would be expected.

7.2 Changing the Class of an Instance#

The function change-class can be used to change the class of an instance from its current class, $C_{from}$ , to a different class, $C_{to}$ ; it changes the structure of the instance to conform to the definition of the class $C_{to}$ .

Note that changing the class of an instance may cause slots to be added or deleted. Changing the class of an instance does not change its identity as defined by the eq function.

When change-class is invoked on an instance, a two-step updating process takes place. The first step modifies the structure of the instance by adding new local slots and discarding local slots that are not specified in the new version of the instance. The second step initializes the newly added local slots and performs any other user-defined actions. These two steps are further described in the two following sections.

7.2.1 Modifying the Structure of the Instance#

In order to make the instance conform to the class

C_{to}

, local slots specified by the class

C_{to}

that are not specified by the class

C_{from}

are added, and local slots not specified by the class

C_{to}

that are specified by the class

C_{from}

are discarded.

The values of local slots specified by both the class $C_{to}$ and the class $C_{from}$ are retained. If such a local slot was unbound, it remains unbound.

The values of slots specified as shared in the class $C_{from}$ and as local in the class $C_{to}$ are retained.

This first step of the update does not affect the values of any shared slots.

7.2.2 Initializing Newly Added Local Slots#

The second step of the update initializes the newly added slots and performs any other user-defined actions. This step is implemented by the generic function update-instance-for-different-class. The generic function update-instance-for-different-class is invoked by change-class after the first step of the update has been completed.

The generic function update-instance-for-different-class is invoked on arguments computed by change-class. The first argument passed is a copy of the instance being updated and is an instance of the class $C_{from}$ ; this copy has dynamic extent within the generic function change-class. The second argument is the instance as updated so far by change-class and is an instance of the class $C_{to}$ . The remaining arguments are an initialization argument list.

The generic function \funref{update-instance-for-different-class} also takes any number of initialization arguments. When it is called by \funref{change-class}, no initialization arguments are provided.

There is a system-supplied primary method for update-instance-for-different-class that has two parameter specializers, each of which is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid. (For more information, see Section 7.1.2 (Declaring the Validity of Initialization Arguments).) Then it calls the generic function shared-initialize with the following arguments: the Barmar suggested we insert the word "new" here.new instance, a list of names of the newly added slots, and the initialization arguments it received.

7.2.3 Customizing the Change of Class of an Instance#

Methods for update-instance-for-different-class may be defined to specify actions to be taken when an instance is updated. If only after methods for update-instance-for-different-class are defined, they will be run after the system-supplied primary method for initialization and will not interfere with the default behavior of update-instance-for-different-class. Removed per X3J13. -kmp 05-Oct-93 Because no initialization arguments are passed to \funref{update-instance-for-different-class} when it is called by \funref{change-class}, the \kwd{initform} forms for \term{slots} that are filled by \term{before methods} for \funref{update-instance-for-different-class} will not be evaluated by \funref{shared-initialize}.

Methods for shared-initialize may be defined to customize class redefinition. For more information, see Section 7.1.5 (Shared-Initialize).

7.3 Reinitializing an Instance#

The generic function reinitialize-instance may be used to change the values of slots according to initialization arguments.

The process of reinitialization changes the values of some slots and performs any user-defined actions. It does not modify the structure of an instance to add or delete slots, and it does not use any :initform forms to initialize slots.

The generic function reinitialize-instance may be called directly. It takes one required argument, the instance. It also takes any number of initialization arguments to be used by methods for reinitialize-instance or for shared-initialize. The arguments after the required instance must form an initialization argument list.

There is a system-supplied primary method for reinitialize-instance whose parameter specializer is the class standard-object. First this method checks the validity of initialization arguments and signals an error if an initialization argument is supplied that is not declared as valid. (For more information, see Section 7.1.2 (Declaring the Validity of Initialization Arguments).) Then it calls the generic function shared-initialize with the following arguments: the instance, nil, and the initialization arguments it received.

7.3.1 Customizing Reinitialization#

Methods for reinitialize-instance may be defined to specify actions to be taken when an instance is updated. If only after methods for reinitialize-instance are defined, they will be run after the system-supplied primary method for initialization and therefore will not interfere with the default behavior of reinitialize-instance.

Methods for shared-initialize may be defined to customize class redefinition. For more information, see Section 7.1.5 (Shared-Initialize).

7.4 Meta-Objects#

Meta-Objects

The implementation of the object system manipulates classes, methods, and generic functions. The object system contains a set of generic functions defined by methods on classes; the behavior of those generic functions defines the behavior of the object system. The instances of the classes on which those methods are defined are called meta-objects. Programming at the meta-object protocol level involves defining new classes of meta-objects along with methods specialized on these classes.

7.4.1 Standard Meta-objects#

The object system supplies a set of meta-objects, called standard meta-objects. These include the class standard-object and instances of the classes standard-method, standard-generic-function, and method-combination.

The class standard-method is the default class of methods defined by the defmethod and defgeneric forms. \macref{generic-function}, \specref{with-added-methods}, \specref{generic-flet}, and \specref{generic-labels}.
The class standard-generic-function is the default class of generic functions defined by the forms defmethod, defgeneric, \macref{generic-function}, \specref{generic-flet}, \specref{generic-labels}, \specref{with-added-methods}, and defclass.
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 standard-class except itself and structure-class.
Every method combination object is an instance of a subclass of class method-combination.

7.5 Slots#

7.5.1 Introduction to Slots#

An object of metaclass standard-class has zero or more named slots. The slots of an object are determined by the class of the object. Each slot can hold one value. Reviewer: Barmar: All symbols are valid variable names. Perhaps this means to preclude the use of named constants? We have a terminology problem to solve.!!!The name of a slot is a symbol that is syntactically valid for use as a variable name.

When a slot does not have a value, the slot is said to be unbound. When an unbound slot is read, Reviewer: Barmar: from an object whose metaclass is standard-class? the generic function slot-unbound is invoked. The system-supplied primary method for slot-unbound Barmar: on STANDARD-CLASS or T? KMP: It said T in the signature info for SLOT-UNBOUND so I copied that to here.on class t signals an error. If slot-unbound returns, its primary value is used that time as the value of the slot.

The default initial value form for a slot is defined by the :initform slot option. When the :initform form is used to supply a value, it is evaluated in the lexical environment in which the defclass form was evaluated. The :initform along with the lexical environment in which the defclass form was evaluated is called a captured initialization form. For more details, see Section 7.1 (Object Creation and Initialization).

A local slot is defined to be a slot that is Barmar says: ``Poor wording. It's "visible" to anyone calling SLOT-VALUE.'' Perhaps we mean to be saying "accessible in"? -kmp 11-Oct-90 Ok. I'll substitute accessible. -kmp 6-Jan-91 visibleaccessible to exactly one instance, namely the one in which the slot is allocated. A shared slot is defined to be a slot that is visible to more than one instance of a given class and its subclasses.

A class is said to define a slot with a given name when the defclass form for that class contains a slot specifier with that name. Defining a local slot does not immediately create a slot; it causes a slot to be created each time an instance of the class is created. Defining a shared slot immediately creates a slot.

The :allocation slot option to defclass controls the kind of slot that is defined. If the value of the :allocation slot option is :instance, a local slot is created. If the value of :allocation is :class, a shared slot is created.

A slot is said to be accessible in an instance of a class if the slot is defined by the class of the instance or is inherited from a superclass of that class. At most one slot of a given name can be accessible in an instance. A shared slot defined by a class is accessible in all instances of that class. A detailed explanation of the inheritance of slots is given in Section 7.5.3 (Inheritance of Slots and Slot Options).

7.5.2 Accessing Slots#

Slots can be accessed in two ways: by use of the primitive function slot-value and by use of generic functions generated by the defclass form.

The function slot-value can be used with any of the slot names specified in the defclass form to access a specific slot accessible in an instance of the given class.

The macro defclass provides syntax for generating methods to read and write slots. If a reader method is requested, a method is automatically generated for reading the value of the slot, but no method for storing a value into it is generated. If a writer method is requested, a method is automatically generated for storing a value into the slot, but no method for reading its value is generated. If an accessor method is requested, a method for reading the value of the slot and a method for storing a value into the slot are automatically generated. Reader and writer methods are implemented using slot-value.

When a reader or writer method is specified for a slot, the name of the generic function to which the generated method belongs is directly specified. If the name specified for the writer method is the symbol name, the name of the generic function for writing the slot is the symbol name, and the generic function takes two arguments: the new value and the instance, in that order. If the name specified for the accessor method is the symbol name, the name of the generic function for reading the slot is the symbol name, and the name of the generic function for writing the slot is the list (setf name).

A generic function created or modified by supplying :reader, :writer, or :accessor slot options can be treated exactly as an ordinary generic function.

Note that slot-value can be used to read or write the value of a slot whether or not reader or writer methods exist for that slot. When slot-value is used, no reader or writer methods are invoked.

The macro with-slots can be used to establish a lexical environment in which specified slots are lexically available as if they were variables. The macro with-slots invokes the function slot-value to access the specified slots.

The macro with-accessors can be used to establish a lexical environment in which specified slots are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the specified slots. Symbolics thinks this sentence is not meaningful: Any accessors specified by \macref{with-accessors} must already have been defined before they are used.

7.5.3 Inheritance of Slots and Slot Options#

The set of the names of all slots accessible in an instance of a class $C$ is the union of the sets of names of slots defined by $C$ and its superclasses. The structure of an instance is the set of names of local slots in that instance.

In the simplest case, only one class among $C$ and its superclasses defines a slot with a given slot name. If a slot is defined by a superclass of $C$ , the slot is said to be inherited. The characteristics of the slot are determined by the slot specifier of the defining class. Consider the defining class for a slot $S$ . If the value of the :allocation slot option is :instance, then $S$ is a local slot and each instance of $C$ has its own slot named $S$ that stores its own value. If the value of the :allocation slot option is :class, then $S$ is a shared slot, the class that defined $S$ stores the value, and all instances of $C$ can access that single slot. If the :allocation slot option is omitted, :instance is used.

In general, more than one class among $C$ and its superclasses can define a slot with a given name. In such cases, only one slot with the given name is accessible in an instance of $C$ , and the characteristics of that slot are a combination of the several slot specifiers, computed as follows:

All the slot specifiers for a given slot name are ordered from most specific to least specific, according to the order in $C$ 's class precedence list of the classes that define them. All references to the specificity of slot specifiers immediately below refers to this ordering.
The allocation of a slot is controlled by the most specific slot specifier. If the most specific slot specifier does not contain an :allocation slot option, :instance is used. Less specific slot specifiers do not affect the allocation.
The default initial value form for a slot is the value of the :initform slot option in the most specific slot specifier that contains one. If no slot specifier contains an :initform slot option, the slot has no default initial value form.
The contents of a slot will always be of type (and $T_{1}$ $\dots$ $T_{n}$ ) where $T_{1} \dots T_{n}$ are the values of the :type slot options contained in all of the slot specifiers. If no slot specifier contains the :type slot option, the contents of the slot will always be of type t. The consequences of attempting to store in a slot a value that does not satisfy the type of the slot are undefined.
The set of initialization arguments that initialize a given slot is the union of the initialization arguments declared in the :initarg slot options in all the slot specifiers.
The documentation string for a slot is the value of the :documentation slot option in the most specific slot specifier that contains one. If no slot specifier contains a :documentation slot option, the slot has no documentation string.

A consequence of the allocation rule is that a shared slot can be shadowed. For example, if a class $C_{1}$ defines a slot named $S$ whose value for the :allocation slot option is :class, that slot is accessible in instances of $C_{1}$ and all of its subclasses. However, if $C_{2}$ is a subclass of $C_{1}$ and also defines a slot named $S$ , $C_{1}$ 's slot is not shared by instances of $C_{2}$ and its subclasses. When a class $C_{1}$ defines 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 $C_{2}$ that defines a slot of the same name.

A consequence of the type rule is that the value of a slot satisfies the type constraint of each slot specifier that contributes to that slot. Because the result of attempting to store in a slot a value that does not satisfy the type constraint for the slot is undefined, the value in a slot might fail to satisfy its type constraint.

The :reader, :writer, and :accessor slot options create methods rather than define the characteristics of a slot. Reader and writer methods are inherited in the sense described in Section 7.6.7 (Inheritance of Methods).

Methods that access slots use only the name of the slot and the type of the slot's value. Suppose a superclass provides a method that expects to access a shared slot of a given name, and a subclass defines a local slot with the same name. If the method provided by the superclass is used on an instance of the subclass, the method accesses the local slot.

7.6 Generic Functions and Methods#

!!! Uses of "built-in" could be "\term{standardized}" here.

7.6.1 Introduction to Generic Functions#

A generic function is a function whose behavior depends on the classes or identities of the arguments supplied to it. A generic function object containsis associated with a set of methods, a lambda list, a method combination₂, and other information.

Like an ordinary function, a generic function takes arguments, performs a series of operations, and perhaps returns useful values. An ordinary function has a single body of code that is always executed when the function is called. A generic function has a set of bodies of code of which a subset is selected for execution. The selected bodies of code and the manner of their combination are determined by the classes or identities of one or more of the arguments to the generic function and by its method combination.

Ordinary functions and generic functions are called with identical syntax.

Generic functions are true functions that can be passed as arguments and used as the first argument to funcall and apply.

A binding of a function name to a generic function can be established in one of several ways. It can be established in the global environment by ensure-generic-function, defmethod (implicitly, due to ensure-generic-function) or defgeneric (also implicitly, due to ensure-generic-function). No standardized mechanism is provided for establishing a binding of a function name to a generic function in the lexical environment. It can be \term{established} in the \term{lexical environment} by \specref{with-added-methods}, \specref{generic-flet} or \specref{generic-labels}. The \term{name} part of such a \term{binding}, like the name of an ordinary \term{function}, can be either a \term{symbol} or a two-element \term{list} whose first element is \misc{setf} and whose second element is a \term{symbol}. This is true for both \term{lexical bindings} and \term{global bindings}. The \specref{generic-flet} special form creates new local generic functions using the set of methods specified by the method definitions in the \specref{generic-flet} form. The scoping of generic function names within a \specref{generic-flet} form is the same as for \specref{flet}. The \specref{generic-labels} special form creates a set of new mutually recursive local generic functions using the set of methods specified by the method definitions in the \specref{generic-labels} form. The scoping of generic function names within a \specref{generic-labels} form is the same as for \specref{labels}.

The \specref{with-added-methods} special form creates new local generic functions by adding the set of methods specified by the method definitions with a given name in the \specref{with-added-methods} form to copies of the methods of the lexically visible generic function of the same name. If there is a lexically visible ordinary function of the same name as one of specified generic functions, that function becomes the method function of the default method for the new generic function of that name.

The \macref{generic-function} macro creates an anonymous generic function with the set of methods specified by the method definitions in the \macref{generic-function} form.

When a defgeneric form is evaluated, one of three actions is taken (due to ensure-generic-function):

!!! Barrett observes that GENERIC-FLET and GENERIC-LABELS are not correctly spoken for here because they are classified as method-defining. But since they've been flushed, I guess it doesn't matter. -kmp 12-Feb-92

If a generic function of the given name already exists, the existing generic function object is modified. Methods specified by the current defgeneric form are added, and any methods in the existing generic function that were defined by a previous defgeneric form are removed. Methods added by the current defgeneric form might replace methods defined by defmethod, defclass, define-condition, or defstruct. No other methods in the generic function are affected or replaced.
If the given name names an ordinary function, a macro, or a special operator, an error is signaled.
Otherwise a generic function is created with the methods specified by the method definitions in the defgeneric form.

Some operators permit specification of the options of a generic function, such as the type of method combination it uses or its argument precedence order. These operators will be referred to as “operators that specify generic function options.” These \term{operators} are: \macref{defgeneric}, Reworded since there's only one.The only standardized operator in this category is defgeneric. \macref{generic-function}, \specref{with-added-methods}, \specref{generic-flet}, and \specref{generic-labels}.

Some operators define methods for a generic function. These operators will be referred to as method-defining operators; their associated forms are called method-defining forms. The standardized method-defining operators are listed in the next figure. The \term{standardized} \term{operators} in this category are: \macref{defgeneric}, \macref{defmethod}, \issue{GENERIC-FLET-POORLY-DESIGNED:DELETE} \macref{generic-function}, \specref{generic-flet}, \specref{generic-labels}, \endissue{GENERIC-FLET-POORLY-DESIGNED:DELETE} \issue{WITH-ADDED-METHODS:DELETE} \specref{with-added-methods}, \endissue{WITH-ADDED-METHODS:DELETE}% \macref{defclass}, \macref{define-condition}, and \macref{defstruct}. Removed GENERIC-FUNCTION, GENERIC-FLET, and GENERIC-LABELS. Removed WITH-ADDED-METHODS.

`defgeneric`	`defmethod`	`defclass`
`define-condition`	`defstruct`

Figure 7–1. Standardized Method-Defining Operators

Note that of the standardized method-defining operators all of the method-defining operators except \macref{defclass}, \macref{defmethod}, \macref{define-condition}, and \macref{defstruct}only defgeneric can specify generic function options. defgeneric and any implementation-defined operators that can specify generic function options are also referred to as “operators that specify generic function options.”

7.6.2 Introduction to Methods#

Methods define the class-specific or identity-specific behavior and operations of a generic function.

KAB was a bit unhappy with "contains" here. He also didn't like the term "method function".A method object containsis associated with a method function, code that implements the method's behavior, a sequence of parameter specializers that specify when the given method is applicable, a lambda list, and a sequence of qualifiers that are used by the method combination facility to distinguish among methods.

A method object is not a function and cannot be invoked as a function. Various mechanisms in the object system take a method object and invoke its method function, as is the case when a generic function is invoked. When this occurs it is said that the method is invoked or called.

A method-defining form contains the code that is to be run when the arguments to the generic function cause the method that it defines to be invoked. When a method-defining form is evaluated, a method object is created and one of four actions is taken:

If a generic function of the given name already exists and if a method object already exists that agrees with the new one on parameter specializers and qualifiers, the new method object replaces the old one. For a definition of one method agreeing with another on parameter specializers and qualifiers, see Section 7.6.3 (Agreement on Parameter Specializers and Qualifiers).
If a generic function of the given name already exists and if there is no method object that agrees with the new one on parameter specializers and qualifiers, the existing generic function object is modified to contain the new method object.
If the given name names an ordinary function, a macro, or a special operator, an error is signaled.
Otherwise a generic function is created with the method specified by the method-defining form.

If the lambda list of a new method is not congruent with the lambda list of the generic function, an error is signaled. If a method-defining operator that cannot specify generic function options creates a new generic function, a lambda list for that generic function is derived from the lambda list of the method in the method-defining form in such a way as to be congruent with it. For a discussion of congruence, see Section 7.6.4 (Congruent Lambda-lists for all Methods of a Generic Function).

Each method has a specialized lambda list, which determines when that method can be applied. A specialized lambda list is like an ordinary lambda list except that a specialized parameter may occur instead of the name of a required parameter. A specialized parameter is a list (variable-name parameter-specializer-name), where parameter-specializer-name is one of the following:

a symbol
denotes a parameter specializer which is the class named by that symbol.
This was apparently introduced accidentally, but has been confirmed by X3J13 vote. -kmp 08-Apr-91
a class
denotes a parameter specializer which is the class itself.
(eql form)
denotes a parameter specializer which satisfies the type specifier (eql object), where object is the result of evaluating form. The form form is evaluated in the lexical environment in which the method-defining form is evaluated. Note that form is evaluated only once, at the time the method is defined, not each time the generic function is called.

Parameter specializer names are used in macros intended as the user-level interface (defmethod), while parameter specializers are used in the functional interface.

Only required parameters may be specialized, and there must be a parameter specializer for each required parameter. For notational simplicity, if some required parameter in a specialized lambda list in a method-defining form is simply a variable name, its parameter specializer defaults to the class t.

Given a generic function and a set of arguments, an applicable method is a method for that generic function whose parameter specializers are satisfied by their corresponding arguments. The following definition specifies what it means for a method to be applicable and for an argument to satisfy a parameter specializer.

Barmar: Review use of ``instance'' here. Also, instead of ``$C=P\sub i$ or ...'' we should refer just to ``(TYPEP Ai Pi) is true.'' Since this is a hot topic on the mail right now, I'll leave this until the dust settles. -kmp 22-Oct-90 KMP: I think this is finally fixed.Let $⟨ A_{1}, \dots, A_{n} ⟩$ be the required arguments to a generic function in order. Let $⟨ P_{1}, \dots, P_{n} ⟩$ be the parameter specializers corresponding to the required parameters of the method $M$ in order. The method $M$ is applicable when each $A\sub i$ satisfies $P\sub i$. If $P\sub i$ is a class, and if $A\sub i$ is a \term{direct instance} of a class $C$\negthinspace, then it is said that $A\sub i$ satisfies $P\sub i$ when $C=P\sub i$ or when $C$ is a subclass of $P\sub i$. If $P\sub i$ is {\tt (eql \i{object})}, then it is said that $A\sub i$ satisfies $P\sub i$ when \thefunction{eql} applied to $A\sub i$ and \i{object} \term{yields} \term{true}. $A\sub i$ is the \term{same} as \i{object} (\ie under \funref{eql}).applicable when each $A_{i}$ is of the type specified by the type specifier $P_{i}$ . Because a \term{parameter specializer} is a \term{type specifier}, \thefunction{typep} can be used during method selection to determine whether an argument satisfies a \term{parameter specializer}. For Moon In general A arbitrary \term{parameter specializer} cannot be a \term{compound type specifier}. Example moved to the glossary. such as {\tt (vector single-float)}. The only \term{list} that can be a \term{parameter specializer} is \f{(eql \i{object})}. This requires that Common Lisp be modified to include the \term{type specifier} \funref{eql} to be defined as if the following were evaluated: $$\hbox{\f{(deftype eql (object) `(member ,object))}}$$ Rewritten by KMP: This part isn't really needed because it's said just a few paragraphs before. A \term{parameter specializer} can be a \term{class}, a \term{class} \term{name}, or \f{(eql \i{object})}. I think this can stand on its own.Because every valid parameter specializer is also a valid type specifier, the function typep can be used during method selection to determine whether an argument satisfies a parameter specializer.

A method all of whose parameter specializers are the class t is called a default method; it is always applicable but may be shadowed by a more specific method.

Methods can have qualifiers, which give the method combination procedure a way to distinguish among methods. A method that has one or more qualifiers is called a qualified method. A method with no qualifiers is called an unqualified method. A qualifier is any non-list. The \term{qualifiers} defined by standard method combination and by the built-in method combination types are \term{symbols}. Simplification per Barrett:The qualifiers defined by the standardized method combination types are symbols.

In this specification, the terms “primary method” and “auxiliary method” are used to partition methods within a method combination type according to their intended use. In standard method combination, primary methods are unqualified methods and auxiliary methods are methods with a single qualifier that is one of :around, :before, or :after. Methods with these qualifiers are called around methods, before methods, and after methods, respectively. When a method combination type is defined using the short form of define-method-combination, primary methods are methods qualified with the name of the type of method combination, and auxiliary methods have the qualifier :around. Thus the terms “primary method” and “auxiliary method” have only a relative definition within a given method combination type.

7.6.3 Agreement on Parameter Specializers and Qualifiers#

Two methods are said to agree with each other on parameter specializers and qualifiers if the following conditions hold:

1. Both methods have the same number of required parameters. Suppose the parameter specializers of the two methods are $P_{1, 1} \dots P_{1, n}$ and $P_{2, 1} \dots P_{2, n}$ .
2. For each $1 \leq i \leq n$ , $P_{1, i}$ agrees with $P_{2, i}$ . The parameter specializer $P_{1, i}$ agrees with $P_{2, i}$ if $P_{1, i}$ and $P_{2, i}$ are the same class or if $P_{1, i} = (eql object1)$ , $P_{2, i} = (eql object2)$ , and (eql ${object}_{1}$ ${object}_{2}$ ). Otherwise $P_{1, i}$ and $P_{2, i}$ do not agree.
3. The two lists of qualifiers are the same under equal.
\itemitem{3.} The lists of \term{qualifiers} of both methods contain the same \term{non-nil} atoms in the same order. That is, the lists are \funref{equal}.

7.6.4 Congruent Lambda-lists for all Methods of a Generic Function#

These rules define the congruence of a set of lambda lists, including the lambda list of each method for a given generic function and the lambda list specified for the generic function itself, if given.

1. Each lambda list must have the same number of required parameters.
2. Each lambda list must have the same number of optional parameters. Each method can supply its own default for an optional parameter.
3. If any lambda list mentions &rest or &key, each lambda list must mention one or both of them.
4. If the generic function lambda list mentions &key, each method must accept all of the keyword names mentioned after &key, either by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own. The checking of the validity of keyword names is done in the generic function, not in each method. !!! "Leftmost". Sigh.A method is invoked as if the keyword argument pair whose name is :allow-other-keys and whose value is true were supplied, though no such argument pair will be passed. !!! KAB: Alternatively, as if the lambda list of the method specified &allow-other-keys.
5. The use of &allow-other-keys need not be consistent across lambda lists. If &allow-other-keys is mentioned in the lambda list of any applicable method or of the generic function, any keyword arguments may be mentioned in the call to the generic function.
6. The use of &aux need not be consistent across methods.
If a method-defining operator that cannot specify generic function options creates a generic function, and if the lambda list for the method mentions keyword arguments, the lambda list of the generic function will mention &key (but no keyword arguments).

\newpage

7.6.5 Keyword Arguments in Generic Functions and Methods#

When a generic function or any of its methods mentions &key in a lambda list, the specific set of keyword arguments accepted by the generic function varies according to the applicable methods. The set of keyword arguments accepted by the generic function for a particular call is the union of the keyword arguments accepted by all applicable methods and the keyword arguments mentioned after &key in the generic function definition, if any. A method that has &rest but not &key does not affect the set of acceptable keyword arguments. If the lambda list of any applicable method or of the generic function definition contains &allow-other-keys, all keyword arguments are accepted by the generic function.

The lambda list congruence rules require that each method accept all of the keyword arguments mentioned after &key in the generic function definition, by accepting them explicitly, by specifying &allow-other-keys, or by specifying &rest but not &key. Each method can accept additional keyword arguments of its own, in addition to the keyword arguments mentioned in the generic function definition.

If a generic function is passed a keyword argument that no applicable method accepts, an error should be signaled; see Section 3.5 (Error Checking in Function Calls).

7.6.5.1 Examples of Keyword Arguments in Generic Functions and Methods#

For example, suppose there are two methods defined for width as follows:

 (defmethod width ((c character-class) &key font) ...)
 
 (defmethod width ((p picture-class) &key pixel-size) ...)

Assume that there are no other methods and no generic function definition for width. The evaluation of the following form should signal an error because the keyword argument :pixel-size is not accepted by the applicable method.

 (width (make-instance `character-class :char #\Q) 
        :font 'baskerville :pixel-size 10)

The evaluation of the following form should signal an error.

 (width (make-instance `picture-class :glyph (glyph #\Q)) 
        :font 'baskerville :pixel-size 10)

The evaluation of the following form will not signal an error if the class named character-picture-class is a subclass of both picture-class and character-class.

 (width (make-instance `character-picture-class :char #\Q)
        :font 'baskerville :pixel-size 10)

7.6.6 Method Selection and Combination#

When a generic function is called with particular arguments, it must determine the code to execute. This code is called the effective method for those arguments. The effective method is a combination of the applicable methods in the generic function that calls some or all of the methods.

If a \term{generic function} is called and no \term{methods} are \term{applicable}, the \term{generic function} \funref{no-applicable-method} is invoked.If a generic function is called and no methods are applicable, the generic function no-applicable-method is invoked, with the results from that call being used as the results of the call to the original generic function. Calling no-applicable-method takes precedence over checking for acceptable keyword arguments; see Section 7.6.5 (Keyword Arguments in Generic Functions and Methods).

When the effective method has been determined, it is invoked with the same arguments as were passed to the generic function. Whatever values it returns are returned as the values of the generic function.

7.6.6.1 Determining the Effective Method#

The effective method is determined by the following three-step procedure:

1.Select the applicable methods.
2.Sort the applicable methods by precedence order, putting the most specific method first.
3.Apply method combination to the sorted list of applicable methods, producing the effective method.

7.6.6.1.1 Selecting the Applicable Methods#

This step is described in Section 7.6.2 (Introduction to Methods).

7.6.6.1.2 Sorting the Applicable Methods by Precedence Order#

To compare the precedence of two methods, their parameter specializers are examined in order. The default examination order is from left to right, but an alternative order may be specified by the :argument-precedence-order option to defgeneric or to any of the other operators that specify generic function options.

The corresponding parameter specializers from each method are compared. When a pair of parameter specializers agree, the next pair are compared for agreement. If all corresponding parameter specializers agree, the two methods must have different qualifiers; in this case, either method can be selected to precede the other. For information about agreement, see Section 7.6.3 (Agreement on Parameter Specializers and Qualifiers).

If some corresponding parameter specializers do not agree, the first pair of parameter specializers that do not agree determines the precedence. If both parameter specializers are classes, the more specific of the two methods is the method whose parameter specializer appears earlier in the class precedence list of the corresponding argument. Because of the way in which the set of applicable methods is chosen, the parameter specializers are guaranteed to be present in the class precedence list of the class of the argument.

If just one of a pair of corresponding parameter specializers is (eql object), the method with that parameter specializer precedes the other method. If both parameter specializers are eql expressions, the specializers must agree (otherwise the two methods would not both have been applicable to this argument).

The resulting list of applicable methods has the most specific method first and the least specific method last.

7.6.6.1.3 Applying method combination to the sorted list of applicable methods#

In the simple case—if standard method combination is used and all applicable methods are primary methods—the !!! Barrett suggests that this is not the normal meaning of "effective method"effective method is the most specific method. That method can call the next most specific method by using the function call-next-method. The method that call-next-method will call is referred to as the next method. The predicate next-method-p tests whether a next method exists. If call-next-method is called and there is no next most specific method, the generic function no-next-method is invoked.

In general, the effective method is some combination of the applicable methods. It is described by a form that contains calls to some or all of the applicable methods, returns the value or values that will be returned as the value or values of the generic function, and optionally makes some of the methods accessible by means of call-next-method. Moon wanted this removed. Barrett agrees. -kmp 9-Feb-92 This Lisp form is the body of the effective method; it is augmented with an appropriate \term{lambda list} to make it a function.

The role of each method in the effective method is determined by its \term{method}qualifiers and the specificity of the method. A qualifier serves to mark a method, and the meaning of a qualifier is determined by the way that these marks are used by this step of the procedure. If an applicable method has an unrecognized qualifier, this step signals an error and does not include that method in the effective method.

When standard method combination is used together with qualified methods, the effective method is produced as described in Section 7.6.6.2 (Standard Method Combination).

Another type of method combination can be specified by using the :method-combination option of defgeneric or of any of the other operators that specify generic function options. In this way this step of the procedure can be customized.

New types of method combination can be defined by using the define-method-combination macro.

7.6.6.2 Standard Method Combination#

!!! Barrett: "supported" ?Standard method combination is supported by the class standard-generic-function. It is used if no other type of method combination is specified or if the built-in method combination type standard is specified.

Primary methods define the main action of the effective method, while auxiliary methods modify that action in one of three ways. A primary method has no method qualifiers.

An auxiliary method is a method whose \term{method}qualifier is :before, :after, or :around. Standard method combination allows no more than one qualifier per method; if a method definition specifies more than one qualifier per method, an error is signaled.

A before method has the keyword :before as its only qualifier. A before method specifies code that is to be run before any primary methods.
An after method has the keyword :after as its only qualifier. An after method specifies code that is to be run after primary methods.
An around method has the keyword :around as its only qualifier. An around method specifies code that is to be run instead of other applicable methods, I found this to be too vague. -kmp 9-Jan-91 but which is able to cause some of them to be run.but which might contain explicit code which calls some of those shadowed methods (via call-next-method).

The semantics of standard method combination is as follows:

If there are any around methods, the most specific around method is called. It supplies the value or values of the generic function.
Inside the body of an around method, call-next-method can be used to call the next method. When the next method returns, the around method can execute more code, perhaps based on the returned value or values. !!! Moon: Can't happen, `next page' says signals an error if there are no primaries. Barrett: This is a bone of contention. (e.g., no-next-method might -do- the signaling)The generic function no-next-method is invoked if call-next-method is used and there is no applicable method to call. The function next-method-p may be used to determine whether a next method exists.
If an around method invokes call-next-method, the next most specific around method is called, if one is applicable. If there are no around methods or if call-next-method is called by the least specific around method, the other methods are called as follows:
- – All the before methods are called, in most-specific-first order. Their values are ignored. An error is signaled if call-next-method is used in a before method.
- – The most specific primary method is called. Inside the body of a primary method, call-next-method may be used to call the next most specific primary method. When that method returns, the previous primary method can execute more code, perhaps based on the returned value or values. The generic function no-next-method is invoked if call-next-method is used and there are no more applicable primary methods. The function next-method-p may be used to determine whether a next method exists. If call-next-method is not used, only the most specific primary method is called.
- – All the after methods are called in most-specific-last order. Their values are ignored. An error is signaled if call-next-method is used in an after method.
If no around methods were invoked, the most specific primary method supplies the value or values returned by the generic function. The value or values returned by the invocation of call-next-method in the least specific around method are those returned by the most specific primary method.

In standard method combination, if there is an applicable method but no applicable primary method, an error is signaled.

The before methods are run in most-specific-first order while the after methods are run in least-specific-first order. The design rationale for this difference can be illustrated with an example. Suppose class $C_{1}$ modifies the behavior of its superclass, $C_{2}$ , by adding before methods and after methods. Whether the behavior of the class $C_{2}$ is defined directly by methods on $C_{2}$ or is inherited from its superclasses does not affect the relative order of invocation of methods on instances of the class $C_{1}$ . Class $C_{1}$ 's before method runs before all of class $C_{2}$ 's methods. Class $C_{1}$ 's after method runs after all of class $C_{2}$ 's methods.

By contrast, all around methods run before any other methods run. Thus a less specific around method runs before a more specific primary method.

If only primary methods are used and if call-next-method is not used, only the most specific method is invoked; that is, more specific methods shadow more general ones.

7.6.6.3 Declarative Method Combination#

The macro define-method-combination defines new forms of method combination. It provides a mechanism for customizing the production of the effective method. The default procedure for producing an effective method is described in Section 7.6.6.1 (Determining the Effective Method). There are two forms of define-method-combination. The short form is a simple facility while the long form is more powerful and more verbose. The long form resembles defmacro in that the body is an expression that computes a Lisp form; it provides mechanisms for implementing arbitrary control structures within method combination and for arbitrary processing of method qualifiers.

7.6.6.4 Built-in Method Combination Types#

The object system provides a set of built-in method combination types. To specify that a generic function is to use one of these method combination types, the name of the method combination type is given as the argument to the :method-combination option to defgeneric or to the :method-combination option to any of the other operators that specify generic function options.

The names of the built-in method combination types are listed in the next figure.

`+`	`append`	`max`	`nconc`	`progn`
`and`	`list`	`min`	`or`	`standard`

Figure 7–2. Built-in Method Combination Types

The semantics of the standard built-in method combination type is described in Section 7.6.6.2 (Standard Method Combination). The other built-in method combination types are called simple built-in method combination types.

The simple built-in method combination types act as though they were defined by the short form of define-method-combination. They recognize two roles for methods:

An around method has the keyword symbol :around as its sole qualifier. The meaning of :around methods is the same as in standard method combination. Use of the functions call-next-method and next-method-p is supported in around methods.
A primary method has the name of the method combination type as its sole qualifier. For example, the built-in method combination type and recognizes methods whose sole qualifier is and; these are primary methods. Use of the functions call-next-method and next-method-p is not supported in primary methods.

The semantics of the simple built-in method combination types is as follows:

If there are any around methods, the most specific around method is called. It supplies the value or values of the generic function.
Inside the body of an around method, the function call-next-method can be used to call the next method. !!! Moon: Can't happen, `next page' says signals an error if there are no primaries. Barrett: This is a bone of contention. (e.g., no-next-method might -do- the signaling)The generic function no-next-method is invoked if call-next-method is used and there is no applicable method to call. The function next-method-p may be used to determine whether a next method exists. When the next method returns, the around method can execute more code, perhaps based on the returned value or values.
If an around method invokes call-next-method, the next most specific around method is called, if one is applicable. If there are no around methods or if call-next-method is called by the least specific around method, a Lisp form derived from the name of the built-in method combination type and from the list of applicable primary methods is evaluated to produce the value of the generic function. Suppose the name of the method combination type is operator and the call to the generic function is of the form

$(generic-function a_{1} \dots a_{n})$
Let $M_{1}, \dots, M_{k}$ be the applicable primary methods in order; then the derived Lisp form is

$(operator ⟨ M_{1} a_{1} \dots a_{n} ⟩ \dots ⟨ M_{k} a_{1} \dots a_{n} ⟩)$
If the expression $⟨ M_{i} a_{1} \dots a_{n} ⟩$ is evaluated, the method $M_{i}$ will be applied to the arguments $a_{1} \dots a_{n}$ . For example, if operator is or, the expression $⟨ M_{i} a_{1} \dots a_{n} ⟩$ is evaluated only if $⟨ M_{j} a_{1} \dots a_{n} ⟩$ , $1 \leq j < i$ , returned nil.
The default order for the primary methods is :most-specific-first. However, the order can be reversed by supplying :most-specific-last as the second argument to the :method-combination option.

The simple built-in method combination types require exactly one qualifier per method. An error is signaled if there are applicable methods with no qualifiers or with qualifiers that are not supported by the method combination type. An error is signaled if there are applicable around methods and no applicable primary methods.

7.6.7 Inheritance of Methods#

A subclass inherits methods in the sense that any method applicable to all instances of a class is also applicable to all instances of any subclass of that class.

The inheritance of methods acts the same way regardless of which of the method-defining operators created the methods. whether the method was created by using one of the method-defining operators or by using one of the \macref{defclass} options that causes methods to be generated automatically.

The inheritance of methods is described in detail in Section 7.6.6 (Method Selection and Combination).

Dictionary

function-keywords Standard Generic Function
ensure-generic-function Function
allocate-instance Standard Generic Function
reinitialize-instance Standard Generic Function
shared-initialize Standard Generic Function
update-instance-for-different-class Standard Generic Function
update-instance-for-redefined-class Standard Generic Function
change-class Standard Generic Function
slot-boundp Function
slot-exists-p Function
slot-makunbound Function
slot-missing Standard Generic Function
slot-unbound Standard Generic Function
slot-value Function
method-qualifiers Standard Generic Function
no-applicable-method Standard Generic Function
no-next-method Standard Generic Function
remove-method Standard Generic Function
make-instance Standard Generic Function
make-instances-obsolete Standard Generic Function
make-load-form Standard Generic Function
make-load-form-saving-slots Function
with-accessors Macro
with-slots Macro
defclass Macro
defgeneric Macro
defmethod Macro
find-class Accessor
next-method-p Local Function
call-method, make-method Local Macro
call-next-method Local Function
compute-applicable-methods Standard Generic Function
define-method-combination Macro
find-method Standard Generic Function
add-method Standard Generic Function
initialize-instance Standard Generic Function
class-name Standard Generic Function
(setf class-name) Standard Generic Function
class-of Function
unbound-slot Condition Type
unbound-slot-instance Function

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