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 @IGindex{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:
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 property 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.
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.
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.
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.
Validity checking of initialization arguments is disabled if the value of the initialization argument :allow-other-keys is true.
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.
@ITindex{order of evaluation}
@ITindex{evaluation order}
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.
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:
[Reviewer Note by 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 Note by 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))
{ 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
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:
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:
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.
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 simply by supplying initial values for slots.
During initialization, initialize-instance is invoked after the following actions have been taken:
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.
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))
The elided code in the definition of make-instance augments the initargs with any defaulted initialization arguments and checks the resulting initialization arguments to determine whether an initialization argument was supplied that neither filled a slot nor supplied an argument to an applicable method.
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)))
These procedures can be customized.
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, allocate-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.
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.
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.
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.
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.
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 Declaring the Validity of Initialization Arguments.) Then it calls the generic function shared-initialize with the following arguments: the new instance, a list of names of the newly added slots, and the initialization arguments it received.
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.
Methods for shared-initialize may be defined to customize class redefinition. For more information, see section Shared-Initialize.
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 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.
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 Shared-Initialize.
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.
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.
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 Note by 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 Note by Barmar: from an object whose metaclass is standard-class?] the generic function slot-unbound is invoked. The system-supplied primary method for slot-unbound 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 Object Creation and Initialization.
A local slot is defined to be a slot that is accessible 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 Inheritance of Slots and Slot Options.
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.
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:
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 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.
A generic function @IGindex{generic function} is a function whose behavior depends on the classes or identities of the arguments supplied to it. A generic function object is associated with a set of methods, a lambda list, a method combination_2, 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.
When a defgeneric form is evaluated, one of three actions is taken (due to ensure-generic-function):
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."
The only standardized operator in this category is defgeneric.
Some operators define methods for a generic function. These operators will be referred to as method-defining operators @IGindex{method-defining operator} ; their associated forms are called method-defining forms. The standardized method-defining operators are listed in Figure 7--2.
defgeneric defmethod defclass define-condition defstruct
Figure 7--2: Standardized Method-Defining Operators
Note that of the standardized method-defining operators 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."
Methods define the class-specific or identity-specific behavior and operations of a generic function.
A method object is associated with 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 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 @IGindex{congruence} , see section 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:
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.
Let < A_1, ..., A_n> be the required arguments to a generic function in order. Let < P_1, ..., 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_i is of the type specified by the type specifier P_i. 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 @IGindex{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 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.
Two methods are said to agree with each other on parameter specializers and qualifiers if the following conditions hold:
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.
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 Error Checking in Function Calls.
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)
When a generic function is called with particular arguments, it must determine the code to execute. This code is called the effective method @IGindex{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 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 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.
The effective method is determined by the following three-step procedure:
This step is described in section Introduction to Methods.
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 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.
In the simple case--if standard method combination is used and all applicable methods are primary methods--the 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 @IGindex{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.
The role of each method in the effective method is determined by its 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 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.
@IRindex{standard}
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 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.
The semantics of standard method combination is as follows:
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.
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 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.
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 Figure 7--3.
@IRindex{+}
@IRindex{and}
@IRindex{append}
@IRindex{list}
@IRindex{max}
@IRindex{min}
@IRindex{nconc}
@IRindex{or}
@IRindex{progn}
@IRindex{standard}
+ append max nconc progn and list min or standard
Figure 7--3: Built-in Method Combination Types
The semantics of the standard built-in method combination type is described in section 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:
The semantics of the simple built-in method combination types is as follows:
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.
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.
The inheritance of methods is described in detail in section Method Selection and Combination.
function-keywords method => keys, allow-other-keys-p
function-keywords (method standard-method)
method---a method.
keys---a list.
allow-other-keys-p---a generalized boolean.
Returns the keyword parameter specifiers for a method.
Two values are returned: a list of the explicitly named keywords and a generalized boolean that states whether &allow-other-keys had been specified in the method definition.
(defmethod gf1 ((a integer) &optional (b 2)
&key (c 3) ((:dee d) 4) e ((eff f)))
(list a b c d e f))
=> #<STANDARD-METHOD GF1 (INTEGER) 36324653>
(find-method #'gf1 '() (list (find-class 'integer)))
=> #<STANDARD-METHOD GF1 (INTEGER) 36324653>
(function-keywords *)
=> (:C :DEE :E EFF), false
(defmethod gf2 ((a integer))
(list a b c d e f))
=> #<STANDARD-METHOD GF2 (INTEGER) 42701775>
(function-keywords (find-method #'gf1 '() (list (find-class 'integer))))
=> (), false
(defmethod gf3 ((a integer) &key b c d &allow-other-keys)
(list a b c d e f))
(function-keywords *)
=> (:B :C :D), true
defmethod
section defmethod [Macro]
ensure-generic-function function-name {&key
argument-precedence-order declare
documentation environment
generic-function-class lambda-list
method-class method-combination}
=> generic-function
function-name---a function name.
The keyword arguments correspond to the option arguments of defgeneric, except that the :method-class and :generic-function-class arguments can be class objects as well as names.
Method-combination -- method combination object.
Environment -- the same as the &environment argument to macro expansion functions and is used to distinguish between compile-time and run-time environments.
[Editorial Note by KMP: What about documentation. Missing from this arguments enumeration, and confusing in description below.]
generic-function---a generic function object.
The function ensure-generic-function is used to define a globally named generic function with no methods or to specify or modify options and declarations that pertain to a globally named generic function as a whole.
If function-name is not fbound in the global environment, a new generic function is created. If
(fdefinition function-name)
is an ordinary function, a macro, or a special operator, an error is signaled.
If function-name is a list, it must be of the form (setf symbol). If function-name specifies a generic function that has a different value for any of the following arguments, the generic function is modified to have the new value: :argument-precedence-order, :declare, :documentation, :method-combination.
If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is congruent with the lambda lists of all existing methods or there are no methods, the value is changed; otherwise an error is signaled.
If function-name specifies a generic function that has a different value for the :generic-function-class argument and if the new generic function class is compatible with the old, change-class is called to change the class of the generic function; otherwise an error is signaled.
If function-name specifies a generic function that has a different value for the :method-class argument, the value is changed, but any existing methods are not changed.
Existing function binding of function-name.
If
(fdefinition function-name)
is an ordinary function, a macro, or a special operator, an error of type error is signaled.
If function-name specifies a generic function that has a different value for the :lambda-list argument, and the new value is not congruent with the lambda list of any existing method, an error of type error is signaled.
If function-name specifies a generic function that has a different value for the :generic-function-class argument and if the new generic function class not is compatible with the old, an error of type error is signaled.
section defgeneric [Macro]
allocate-instance class {&rest initargs {&key} {&allow-other-keys}} => new-instance
allocate-instance (class standard-class) {&rest initargs}
allocate-instance (class structure-class) {&rest initargs}
class---a class.
initargs---a list of keyword/value pairs (initialization argument names and values).
new-instance---an object whose class is class.
The generic function allocate-instance creates and returns a new instance of the class, without initializing it. When the class is a standard class, this means that the slots are unbound; when the class is a structure class, this means the slots' values are unspecified.
The caller of allocate-instance is expected to have already checked the initialization arguments.
The generic function allocate-instance is called by make-instance, as described in section Object Creation and Initialization.
section defclass [Macro] , section make-instance [Standard Generic Function] , section class-of [Function] , section Object Creation and Initialization
The consequences of adding methods to allocate-instance is unspecified. This capability might be added by the Metaobject Protocol.
reinitialize-instance instance {&rest initargs {&key} {&allow-other-keys}} => instance
reinitialize-instance (instance standard-object) {&rest initargs}
instance---an object.
initargs---an initialization argument list.
The generic function reinitialize-instance can be used to change the values of local slots of an instance according to initargs. This generic function can be called by users.
The system-supplied primary method for reinitialize-instance checks the validity of initargs and signals an error if an initarg is supplied that is not declared as valid. The method then calls the generic function shared-initialize with the following arguments: the instance, nil (which means no slots should be initialized according to their initforms), and the initargs it received.
The generic function reinitialize-instance changes the values of local slots.
The system-supplied primary method for reinitialize-instance signals an error if an initarg is supplied that is not declared as valid.
section Initialize-Instance , section Shared-Initialize , section update-instance-for-redefined-class [Standard Generic Function] , section update-instance-for-different-class [Standard Generic Function] , section slot-boundp [Function] , section slot-makunbound [Function] , section Reinitializing an Instance, section Rules for Initialization Arguments, section Declaring the Validity of Initialization Arguments
Initargs are declared as valid by using the :initarg option to defclass, or by defining methods for reinitialize-instance or shared-initialize. The keyword name of each keyword parameter specifier in the lambda list of any method defined on reinitialize-instance or shared-initialize is declared as a valid initialization argument name for all classes for which that method is applicable.
shared-initialize instance slot-names {&rest initargs {&key} {&allow-other-keys}} => instance
shared-initialize (instance standard-object) slot-names {&rest initargs}
instance---an object.
slot-names---a list or t.
initargs---a list of keyword/value pairs (of initialization argument names and values).
The generic function shared-initialize is used to fill the slots of an instance using initargs and :initform forms. It is called 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. The generic function shared-initialize is called by the system-supplied primary method for initialize-instance, reinitialize-instance, update-instance-for-redefined-class, and update-instance-for-different-class.
The generic function shared-initialize takes the following arguments: the instance to be initialized, a specification of a set of slot-names accessible in that instance, and any number of initargs. The arguments after the first two must form an initialization argument list. The system-supplied primary method on shared-initialize initializes the slots with values according to the initargs and supplied :initform forms. Slot-names indicates which slots should be initialized according to their :initform forms if no initargs are provided for those slots.
The system-supplied primary method behaves as follows, regardless of whether the slots are local or shared:
The slots-names argument specifies the slots that are to be initialized according to their :initform forms if no initialization arguments apply. It can be a list of slot names, which specifies the set of those slot names; or it can be the symbol t, which specifies the set of all of the slots.
section Initialize-Instance , section reinitialize-instance [Standard Generic Function] , section update-instance-for-redefined-class [Standard Generic Function] , section update-instance-for-different-class [Standard Generic Function] , section slot-boundp [Function] , section slot-makunbound [Function] , section Object Creation and Initialization, section Rules for Initialization Arguments, section Declaring the Validity of Initialization Arguments
Initargs are declared as valid by using the :initarg option to defclass, or by defining methods for shared-initialize. The keyword name of each keyword parameter specifier in the lambda list of any method defined on shared-initialize is declared as a valid initarg name for all classes for which that method is applicable.
Implementations are permitted to optimize :initform forms that neither produce nor depend on side effects, by evaluating these forms and storing them into slots before running any initialize-instance methods, rather than by handling them in the primary initialize-instance method. (This optimization might be implemented by having the allocate-instance method copy a prototype instance.)
Implementations are permitted to optimize default initial value forms for initargs associated with slots by not actually creating the complete initialization argument list when the only method that would receive the complete list is the method on standard-object. In this case default initial value forms can be treated like :initform forms. This optimization has no visible effects other than a performance improvement.
update-instance-for-different-class previous current
{&rest initargs
{&key} {&allow-other-keys}} => implementation-dependent
update-instance-for-different-class (previous standard-object)
(current standard-object)
{&rest initargs}
previous---a copy of the original instance.
current---the original instance (altered).
initargs---an initialization argument list.
The generic function update-instance-for-different-class is not intended to be called by programmers. Programmers may write methods for it. The function update-instance-for-different-class is called only by the function change-class.
The system-supplied primary method on update-instance-for-different-class checks the validity of initargs and signals an error if an initarg is supplied that is not declared as valid. This method then initializes slots with values according to the initargs, and initializes the newly added slots with values according to their :initform forms. It does this by calling the generic function shared-initialize with the following arguments: the instance (current), a list of names of the newly added slots, and the initargs it received. Newly added slots are those local slots for which no slot of the same name exists in the previous class.
Methods for update-instance-for-different-class can 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 therefore will not interfere with the default behavior of update-instance-for-different-class.
Methods on update-instance-for-different-class can be defined to initialize slots differently from change-class. The default behavior of change-class is described in section Changing the Class of an Instance.
The arguments to update-instance-for-different-class are computed by change-class. When change-class is invoked on an instance, a copy of that instance is made; change-class then destructively alters the original instance. The first argument to update-instance-for-different-class, previous, is that copy; it holds the old slot values temporarily. This argument has dynamic extent within change-class; if it is referenced in any way once update-instance-for-different-class returns, the results are undefined. The second argument to update-instance-for-different-class, current, is the altered original instance. The intended use of previous is to extract old slot values by using slot-value or with-slots or by invoking a reader generic function, or to run other methods that were applicable to instances of the original class.
See the example for the function change-class.
The system-supplied primary method on update-instance-for-different-class signals an error if an initialization argument is supplied that is not declared as valid.
section change-class [Standard Generic Function] , section Shared-Initialize , section Changing the Class of an Instance, section Rules for Initialization Arguments, section Declaring the Validity of Initialization Arguments
Initargs are declared as valid by using the :initarg option to defclass, or by defining methods for update-instance-for-different-class or shared-initialize. The keyword name of each keyword parameter specifier in the lambda list of any method defined on update-instance-for-different-class or shared-initialize is declared as a valid initarg name for all classes for which that method is applicable.
The value returned by update-instance-for-different-class is ignored by change-class.
update-instance-for-redefined-class instance
added-slots discarded-slots
property-list
{&rest initargs {&key} {&allow-other-keys}}
=> {result}{*}
update-instance-for-redefined-class (instance standard-object)
added-slots discarded-slots
property-list
{&rest initargs}
instance---an object.
added-slots---a list.
discarded-slots---a list.
property-list---a list.
initargs---an initialization argument list.
result---an object.
The generic function update-instance-for-redefined-class is not intended to be called by programmers. Programmers may write methods for it. The generic function update-instance-for-redefined-class is called by the mechanism activated by make-instances-obsolete.
The system-supplied primary method on update-instance-for-redefined-class checks the validity of initargs and signals an error if an initarg is supplied that is not declared as valid. This method then initializes slots with values according to the initargs, and initializes the newly added-slots with values according to their :initform forms. It does this by calling the generic function shared-initialize with the following arguments: the instance, a list of names of the newly added-slots to instance, and the initargs it received. Newly added-slots are those local slots for which no slot of the same name exists in the old version of the class.
When make-instances-obsolete is invoked or when a class has been redefined and an instance is being updated, a property-list is created that captures the slot names and values of all the discarded-slots with values in the original instance. The structure of the instance is transformed so that it conforms to the current class definition. The arguments to update-instance-for-redefined-class are this transformed instance, a list of added-slots to the instance, a list discarded-slots from the instance, and the property-list containing the slot names and values for slots that were discarded and had values. Included in this list of discarded slots are slots that were local in the old class and are shared in the new class.
The value returned by update-instance-for-redefined-class is ignored.
(defclass position () ())
(defclass x-y-position (position)
((x :initform 0 :accessor position-x)
(y :initform 0 :accessor position-y)))
;;; It turns out polar coordinates are used more than Cartesian
;;; coordinates, so the representation is altered and some new
;;; accessor methods are added.
(defmethod update-instance-for-redefined-class :before
((pos x-y-position) added deleted plist &key)
;; Transform the x-y coordinates to polar coordinates
;; and store into the new slots.
(let ((x (getf plist 'x))
(y (getf plist 'y)))
(setf (position-rho pos) (sqrt (+ (* x x) (* y y)))
(position-theta pos) (atan y x))))
(defclass x-y-position (position)
((rho :initform 0 :accessor position-rho)
(theta :initform 0 :accessor position-theta)))
;;; All instances of the old x-y-position class will be updated
;;; automatically.
;;; The new representation is given the look and feel of the old one.
(defmethod position-x ((pos x-y-position))
(with-slots (rho theta) pos (* rho (cos theta))))
(defmethod (setf position-x) (new-x (pos x-y-position))
(with-slots (rho theta) pos
(let ((y (position-y pos)))
(setq rho (sqrt (+ (* new-x new-x) (* y y)))
theta (atan y new-x))
new-x)))
(defmethod position-y ((pos x-y-position))
(with-slots (rho theta) pos (* rho (sin theta))))
(defmethod (setf position-y) (new-y (pos x-y-position))
(with-slots (rho theta) pos
(let ((x (position-x pos)))
(setq rho (sqrt (+ (* x x) (* new-y new-y)))
theta (atan new-y x))
new-y)))
The system-supplied primary method on update-instance-for-redefined-class signals an error if an initarg is supplied that is not declared as valid.
section make-instances-obsolete [Standard Generic Function] , section Shared-Initialize , section Redefining Classes, section Rules for Initialization Arguments, section Declaring the Validity of Initialization Arguments
Initargs are declared as valid by using the :initarg option to defclass, or by defining methods for update-instance-for-redefined-class or shared-initialize. The keyword name of each keyword parameter specifier in the lambda list of any method defined on update-instance-for-redefined-class or shared-initialize is declared as a valid initarg name for all classes for which that method is applicable.
change-class instance new-class {&key {&allow-other-keys}} => instance
change-class (instance standard-object)
(new-class standard-class)
{&rest initargs}
change-class (instance t)
(new-class symbol)
{&rest initargs}
instance---an object.
new-class---a class designator.
initargs---an initialization argument list.
The generic function change-class changes the class of an instance to new-class. It destructively modifies and returns the instance.
If in the old class there is any slot of the same name as a local slot in the new-class, the value of that slot is retained. This means that if the slot has a value, the value returned by slot-value after change-class is invoked is eql to the value returned by slot-value before change-class is invoked. Similarly, if the slot was unbound, it remains unbound. The other slots are initialized as described in section Changing the Class of an Instance.
After completing all other actions, change-class invokes update-instance-for-different-class. The generic function update-instance-for-different-class can be used to assign values to slots in the transformed instance.
See @xref{Initializing Newly Added Local Slots}.
If the second of the above methods is selected, that method invokes change-class on instance, (find-class new-class), and the initargs.
(defclass position () ())
(defclass x-y-position (position)
((x :initform 0 :initarg :x)
(y :initform 0 :initarg :y)))
(defclass rho-theta-position (position)
((rho :initform 0)
(theta :initform 0)))
(defmethod update-instance-for-different-class :before ((old x-y-position)
(new rho-theta-position)
&key)
;; Copy the position information from old to new to make new
;; be a rho-theta-position at the same position as old.
(let ((x (slot-value old 'x))
(y (slot-value old 'y)))
(setf (slot-value new 'rho) (sqrt (+ (* x x) (* y y)))
(slot-value new 'theta) (atan y x))))
;;; At this point an instance of the class x-y-position can be
;;; changed to be an instance of the class rho-theta-position using
;;; change-class:
(setq p1 (make-instance 'x-y-position :x 2 :y 0))
(change-class p1 'rho-theta-position)
;;; The result is that the instance bound to p1 is now an instance of
;;; the class rho-theta-position. The update-instance-for-different-class
;;; method performed the initialization of the rho and theta slots based
;;; on the value of the x and y slots, which were maintained by
;;; the old instance.
section update-instance-for-different-class [Standard Generic Function] , section Changing the Class of an Instance
The generic function change-class has several semantic difficulties. First, it performs a destructive operation that can be invoked within a method on an instance that was used to select that method. When multiple methods are involved because methods are being combined, the methods currently executing or about to be executed may no longer be applicable. Second, some implementations might use compiler optimizations of slot access, and when the class of an instance is changed the assumptions the compiler made might be violated. This implies that a programmer must not use change-class inside a method if any methods for that generic function access any slots, or the results are undefined.
slot-boundp instance slot-name => generalized-boolean
instance---an object.
slot-name---a symbol naming a slot of instance.
generalized-boolean---a generalized boolean.
Returns true if the slot named slot-name in instance is bound; otherwise, returns false.
If no slot of the name slot-name exists in the instance, slot-missing is called as follows:
(slot-missing (class-of instance)
instance
slot-name
'slot-boundp)
(If slot-missing is invoked and returns a value, a boolean equivalent to its primary value is returned by slot-boundp.)
The specific behavior depends on instance's metaclass. An error is never signaled if instance has metaclass standard-class. An error is always signaled if instance has metaclass built-in-class. The consequences are undefined if instance has any other metaclass--an error might or might not be signaled in this situation. Note in particular that the behavior for conditions and structures is not specified.
section slot-makunbound [Function] , section slot-missing [Standard Generic Function]
The function slot-boundp allows for writing after methods on initialize-instance in order to initialize only those slots that have not already been bound.
Although no implementation is required to do so, implementors are strongly encouraged to implement the function slot-boundp using the function slot-boundp-using-class described in the Metaobject Protocol.
slot-exists-p object slot-name => generalized-boolean
object---an object.
slot-name---a symbol.
generalized-boolean---a generalized boolean.
Returns true if the object has a slot named slot-name.
defclass, defstruct
section defclass [Macro] , section slot-missing [Standard Generic Function]
Although no implementation is required to do so, implementors are strongly encouraged to implement the function slot-exists-p using the function slot-exists-p-using-class described in the Metaobject Protocol.
slot-makunbound instance slot-name => instance
instance -- instance.
Slot-name---a symbol.
The function slot-makunbound restores a slot of the name slot-name in an instance to the unbound state.
If no slot of the name slot-name exists in the instance, slot-missing is called as follows:
(slot-missing (class-of instance)
instance
slot-name
'slot-makunbound)
(Any values returned by slot-missing in this case are ignored by slot-makunbound.)
The specific behavior depends on instance's metaclass. An error is never signaled if instance has metaclass standard-class. An error is always signaled if instance has metaclass built-in-class. The consequences are undefined if instance has any other metaclass--an error might or might not be signaled in this situation. Note in particular that the behavior for conditions and structures is not specified.
section slot-boundp [Function] , section slot-missing [Standard Generic Function]
Although no implementation is required to do so, implementors are strongly encouraged to implement the function slot-makunbound using the function slot-makunbound-using-class described in the Metaobject Protocol.
slot-missing class object slot-name operation {&optional new-value} => {result}{*}
slot-missing (class t)
object slot-name
operation {&optional new-value}
class---the class of object.
object---an object.
slot-name---a symbol (the name of a would-be slot).
operation---one of the symbols setf, slot-boundp, slot-makunbound, or slot-value.
new-value---an object.
result---an object.
The generic function slot-missing is invoked when an attempt is made to access a slot in an object whose metaclass is standard-class and the slot of the name slot-name is not a name of a slot in that class. The default method signals an error.
The generic function slot-missing is not intended to be called by programmers. Programmers may write methods for it.
The generic function slot-missing may be called during evaluation of slot-value, (setf slot-value), slot-boundp, and slot-makunbound. For each of these operations the corresponding symbol for the operation argument is slot-value, setf, slot-boundp, and slot-makunbound respectively.
The optional new-value argument to slot-missing is used when the operation is attempting to set the value of the slot.
If slot-missing returns, its values will be treated as follows:
The default method on slot-missing signals an error of type error.
section defclass [Macro] , section slot-exists-p [Function] , section slot-value [Function]
The set of arguments (including the class of the instance) facilitates defining methods on the metaclass for slot-missing.
slot-unbound class instance slot-name => {result}{*}
slot-unbound (class t)
instance slot-name
class---the class of the instance.
instance---the instance in which an attempt was made to read the unbound slot.
slot-name---the name of the unbound slot.
result---an object.
The generic function slot-unbound is called when an unbound slot is read in an instance whose metaclass is standard-class. The default method signals an error
of type unbound-slot. The name slot of the unbound-slot condition is initialized to the name of the offending variable, and the instance slot of the unbound-slot condition is initialized to the offending instance.
The generic function slot-unbound is not intended to be called by programmers. Programmers may write methods for it. The function slot-unbound is called only indirectly by slot-value.
If slot-unbound returns, only the primary value will be used by the caller, and all other values will be ignored.
The default method on slot-unbound signals an error of type unbound-slot.
section slot-makunbound [Function]
An unbound slot may occur if no :initform form was specified for the slot and the slot value has not been set, or if slot-makunbound has been called on the slot.
slot-value object slot-name => value
object---an object.
name---a symbol.
value---an object.
The function slot-value returns the value of the slot named slot-name in the object. If there is no slot named slot-name, slot-missing is called. If the slot is unbound, slot-unbound is called.
The macro setf can be used with slot-value to change the value of a slot.
(defclass foo ()
((a :accessor foo-a :initarg :a :initform 1)
(b :accessor foo-b :initarg :b)
(c :accessor foo-c :initform 3)))
=> #<STANDARD-CLASS FOO 244020371>
(setq foo1 (make-instance 'foo :a 'one :b 'two))
=> #<FOO 36325624>
(slot-value foo1 'a) => ONE
(slot-value foo1 'b) => TWO
(slot-value foo1 'c) => 3
(setf (slot-value foo1 'a) 'uno) => UNO
(slot-value foo1 'a) => UNO
(defmethod foo-method ((x foo))
(slot-value x 'a))
=> #<STANDARD-METHOD FOO-METHOD (FOO) 42720573>
(foo-method foo1) => UNO
If an attempt is made to read a slot and no slot of the name slot-name exists in the object, slot-missing is called as follows:
(slot-missing (class-of instance)
instance
slot-name
'slot-value)
(If slot-missing is invoked, its primary value is returned by slot-value.)
If an attempt is made to write a slot and no slot of the name slot-name exists in the object, slot-missing is called as follows:
(slot-missing (class-of instance)
instance
slot-name
'setf
new-value)
(If slot-missing returns in this case, any values are ignored.)
The specific behavior depends on object's metaclass. An error is never signaled if object has metaclass standard-class. An error is always signaled if object has metaclass built-in-class. The consequences are unspecified if object has any other metaclass--an error might or might not be signaled in this situation. Note in particular that the behavior for conditions and structures is not specified.
section slot-missing [Standard Generic Function] , section slot-unbound [Standard Generic Function] , section with-slots [Macro]
Although no implementation is required to do so, implementors are strongly encouraged to implement the function slot-value using the function slot-value-using-class described in the Metaobject Protocol.
Implementations may optimize slot-value by compiling it inline.
method-qualifiers method => qualifiers
method-qualifiers (method standard-method)
method---a method.
qualifiers---a proper list.
Returns a list of the qualifiers of the method.
(defmethod some-gf :before ((a integer)) a) => #<STANDARD-METHOD SOME-GF (:BEFORE) (INTEGER) 42736540> (method-qualifiers *) => (:BEFORE)
section define-method-combination [Macro]
no-applicable-method generic-function {&rest function-arguments} => {result}{*}
no-applicable-method (generic-function t)
{&rest function-arguments}
generic-function---a generic function on which no applicable method was found.
function-arguments---arguments to the generic-function.
result---an object.
The generic function no-applicable-method is called when a generic function is invoked and no method on that generic function is applicable. The default method signals an error.
The generic function no-applicable-method is not intended to be called by programmers. Programmers may write methods for it.
The default method signals an error of type error.
no-next-method generic-function method {&rest args} => {result}{*}
no-next-method (generic-function standard-generic-function)
(method standard-method)
{&rest args}
generic-function -- generic function to which method belongs.
method -- method that contained the call to call-next-method for which there is no next method.
args -- arguments to call-next-method.
result---an object.
The generic function no-next-method is called by call-next-method when there is no next method.
The generic function no-next-method is not intended to be called by programmers. Programmers may write methods for it.
The system-supplied method on no-next-method signals an error of type error. [Editorial Note by KMP: perhaps control-error??]
section call-next-method [Local Function]
remove-method generic-function method => generic-function
remove-method (generic-function standard-generic-function)
method
generic-function---a generic function.
method---a method.
The generic function remove-method removes a method from generic-function by modifying the generic-function (if necessary).
remove-method must not signal an error if the method is not one of the methods on the generic-function.
section find-method [Standard Generic Function]
make-instance class {&rest initargs {&key} {&allow-other-keys}} => instance
make-instance (class standard-class) {&rest initargs}
make-instance (class symbol) {&rest initargs}
class---a class, or a symbol that names a class.
initargs---an initialization argument list.
instance---a fresh instance of class class.
The generic function make-instance creates and returns a new instance of the given class.
If the second of the above methods is selected, that method invokes make-instance on the arguments (find-class class) and initargs.
The initialization arguments are checked within make-instance.
The generic function make-instance may be used as described in section Object Creation and Initialization.
If any of the initialization arguments has not been declared as valid, an error of type error is signaled.
section defclass [Macro] , section class-of [Function] , section allocate-instance [Standard Generic Function] , section Initialize-Instance , section Object Creation and Initialization
make-instances-obsolete class => class
make-instances-obsolete (class standard-class)
make-instances-obsolete (class symbol)
class---a class designator.
The function make-instances-obsolete has the effect of initiating the process of updating the instances of the class. During updating, the generic function update-instance-for-redefined-class will be invoked.
The generic function make-instances-obsolete is invoked automatically by the system when defclass has been used to redefine an existing standard class and the set of local slots accessible in an instance is changed or the order of slots in storage is changed. It can also be explicitly invoked by the user.
If the second of the above methods is selected, that method invokes make-instances-obsolete on (find-class class).
section update-instance-for-redefined-class [Standard Generic Function] , section Redefining Classes
make-load-form object {&optional environment} => creation-form[, initialization-form]
make-load-form (object standard-object) {&optional environment}
make-load-form (object structure-object) {&optional environment}
make-load-form (object condition) {&optional environment}
make-load-form (object class) {&optional environment}
object---an object.
environment---an environment object.
creation-form---a form.
initialization-form---a form.
The generic function make-load-form creates and returns one or two forms, a creation-form and an initialization-form, that enable load to construct an object equivalent to object. Environment is an environment object corresponding to the lexical environment in which the forms will be processed.
The file compiler calls make-load-form to process certain classes of literal objects; see section Additional Constraints on Externalizable Objects.
Conforming programs may call make-load-form directly, providing object is a generalized instance of standard-object, structure-object, or condition.
The creation form is a form that, when evaluated at load time, should return an object that is equivalent to object. The exact meaning of equivalent depends on the type of object and is up to the programmer who defines a method for make-load-form; see section Literal Objects in Compiled Files.
The initialization form is a form that, when evaluated at load time, should perform further initialization of the object. The value returned by the initialization form is ignored. If make-load-form returns only one value, the initialization form is nil, which has no effect. If object appears as a constant in the initialization form, at load time it will be replaced by the equivalent object constructed by the creation form; this is how the further initialization gains access to the object.
Both the creation-form and the initialization-form may contain references to any externalizable object. However, there must not be any circular dependencies in creation forms. An example of a circular dependency is when the creation form for the object X contains a reference to the object Y, and the creation form for the object Y contains a reference to the object X. Initialization forms are not subject to any restriction against circular dependencies, which is the reason that initialization forms exist; see the example of circular data structures below.
The creation form for an object is always evaluated before the initialization form for that object. When either the creation form or the initialization form references other objects that have not been referenced earlier in the file being compiled, the compiler ensures that all of the referenced objects have been created before evaluating the referencing form. When the referenced object is of a type which the file compiler processes using make-load-form, this involves evaluating the creation form returned for it. (This is the reason for the prohibition against circular references among creation forms).
Each initialization form is evaluated as soon as possible after its associated creation form, as determined by data flow. If the initialization form for an object does not reference any other objects not referenced earlier in the file and processed by the file compiler using make-load-form, the initialization form is evaluated immediately after the creation form. If a creation or initialization form F does contain references to such objects, the creation forms for those other objects are evaluated before F, and the initialization forms for those other objects are also evaluated before F whenever they do not depend on the object created or initialized by F. Where these rules do not uniquely determine an order of evaluation between two creation/initialization forms, the order of evaluation is unspecified.
While these creation and initialization forms are being evaluated, the objects are possibly in an uninitialized state, analogous to the state of an object between the time it has been created by allocate-instance and it has been processed fully by initialize-instance. Programmers writing methods for make-load-form must take care in manipulating objects not to depend on slots that have not yet been initialized.
It is implementation-dependent whether load calls eval on the forms or does some other operation that has an equivalent effect. For example, the forms might be translated into different but equivalent forms and then evaluated, they might be compiled and the resulting functions called by load, or they might be interpreted by a special-purpose function different from eval. All that is required is that the effect be equivalent to evaluating the forms.
The method specialized on class returns a creation form using the name of the class if the class has a proper name in environment, signaling an error of type error if it does not have a proper name. Evaluation of the creation form uses the name to find the class with that name, as if by calling find-class. If a class with that name has not been defined, then a class may be computed in an implementation-defined manner. If a class cannot be returned as the result of evaluating the creation form, then an error of type error is signaled.
Both conforming implementations and conforming programs may further specialize make-load-form.
(defclass obj ()
((x :initarg :x :reader obj-x)
(y :initarg :y :reader obj-y)
(dist :accessor obj-dist)))
=> #<STANDARD-CLASS OBJ 250020030>
(defmethod shared-initialize :after ((self obj) slot-names &rest keys)
(declare (ignore slot-names keys))
(unless (slot-boundp self 'dist)
(setf (obj-dist self)
(sqrt (+ (expt (obj-x self) 2) (expt (obj-y self) 2))))))
=> #<STANDARD-METHOD SHARED-INITIALIZE (:AFTER) (OBJ T) 26266714>
(defmethod make-load-form ((self obj) &optional environment)
(declare (ignore environment))
;; Note that this definition only works because X and Y do not
;; contain information which refers back to the object itself.
;; For a more general solution to this problem, see revised example below.
`(make-instance ',(class-of self)
:x ',(obj-x self) :y ',(obj-y self)))
=> #<STANDARD-METHOD MAKE-LOAD-FORM (OBJ) 26267532>
(setq obj1 (make-instance 'obj :x 3.0 :y 4.0)) => #<OBJ 26274136>
(obj-dist obj1) => 5.0
(make-load-form obj1) => (MAKE-INSTANCE 'OBJ :X '3.0 :Y '4.0)
In the above example, an equivalent instance of obj is reconstructed by using the values of two of its slots. The value of the third slot is derived from those two values.
Another way to write the make-load-form method in that example is to use make-load-form-saving-slots. The code it generates might yield a slightly different result from the make-load-form method shown above, but the operational effect will be the same. For example:
;; Redefine method defined above.
(defmethod make-load-form ((self obj) &optional environment)
(make-load-form-saving-slots self
:slot-names '(x y)
:environment environment))
=> #<STANDARD-METHOD MAKE-LOAD-FORM (OBJ) 42755655>
;; Try MAKE-LOAD-FORM on object created above.
(make-load-form obj1)
=> (ALLOCATE-INSTANCE '#<STANDARD-CLASS OBJ 250020030>),
(PROGN
(SETF (SLOT-VALUE '#<OBJ 26274136> 'X) '3.0)
(SETF (SLOT-VALUE '#<OBJ 26274136> 'Y) '4.0)
(INITIALIZE-INSTANCE '#<OBJ 26274136>))
In the following example, instances of my-frob are "interned" in some way. An equivalent instance is reconstructed by using the value of the name slot as a key for searching existing objects. In this case the programmer has chosen to create a new object if no existing object is found; alternatively an error could have been signaled in that case.
(defclass my-frob ()
((name :initarg :name :reader my-name)))
(defmethod make-load-form ((self my-frob) &optional environment)
(declare (ignore environment))
`(find-my-frob ',(my-name self) :if-does-not-exist :create))
In the following example, the data structure to be dumped is circular, because each parent has a list of its children and each child has a reference back to its parent. If make-load-form is called on one object in such a structure, the creation form creates an equivalent object and fills in the children slot, which forces creation of equivalent objects for all of its children, grandchildren, etc. At this point none of the parent slots have been filled in. The initialization form fills in the parent slot, which forces creation of an equivalent object for the parent if it was not already created. Thus the entire tree is recreated at load time. At compile time, make-load-form is called once for each object in the tree. All of the creation forms are evaluated, in implementation-dependent order, and then all of the initialization forms are evaluated, also in implementation-dependent order.
(defclass tree-with-parent () ((parent :accessor tree-parent)
(children :initarg :children)))
(defmethod make-load-form ((x tree-with-parent) &optional environment)
(declare (ignore environment))
(values
;; creation form
`(make-instance ',(class-of x) :children ',(slot-value x 'children))
;; initialization form
`(setf (tree-parent ',x) ',(slot-value x 'parent))))
In the following example, the data structure to be dumped has no special properties and an equivalent structure can be reconstructed simply by reconstructing the slots' contents.
(defstruct my-struct a b c)
(defmethod make-load-form ((s my-struct) &optional environment)
(make-load-form-saving-slots s :environment environment))
The methods specialized on standard-object, structure-object, and condition all signal an error of type error.
It is implementation-dependent whether calling make-load-form on a generalized instance of a system class signals an error or returns creation and initialization forms.
section compile-file [Function] , section make-load-form-saving-slots [Function] , section Additional Constraints on Externalizable Objects section Evaluation, section Compilation
The file compiler calls make-load-form in specific circumstances detailed in section Additional Constraints on Externalizable Objects.
Some implementations may provide facilities for defining new subclasses of classes which are specified as system classes. (Some likely candidates include generic-function, method, and stream). Such implementations should document how the file compiler processes instances of such classes when encountered as literal objects, and should document any relevant methods for make-load-form.
make-load-form-saving-slots object {&key slot-names environment}
=> creation-form, initialization-form
object---an object.
slot-names---a list.
environment---an environment object.
creation-form---a form.
initialization-form---a form.
Returns forms that, when evaluated, will construct an object equivalent to object, without executing initialization forms. The slots in the new object that correspond to initialized slots in object are initialized using the values from object. Uninitialized slots in object are not initialized in the new object. make-load-form-saving-slots works for any instance of standard-object or structure-object.
Slot-names is a list of the names of the slots to preserve. If slot-names is not supplied, its value is all of the local slots.
make-load-form-saving-slots returns two values, thus it can deal with circular structures. Whether the result is useful in an application depends on whether the object's type and slot contents fully capture the application's idea of the object's state.
Environment is the environment in which the forms will be processed.
section make-load-form [Standard Generic Function] , section make-instance [Standard Generic Function] , @xref{setf; psetf} , section slot-value [Function] , section slot-makunbound [Function]
make-load-form-saving-slots can be useful in user-written make-load-form methods.
When the object is an instance of standard-object, make-load-form-saving-slots could return a creation form that calls allocate-instance and an initialization form that contains calls to setf of slot-value and slot-makunbound, though other functions of similar effect might actually be used.
with-accessors {({slot-entry}{*)}
instance-form
{declaration}{*} {form}{*}}
=> {result}{*}
slot-entry ::=(variable-name accessor-name)
variable-name---a variable name; not evaluated.
accessor-name---a function name; not evaluated.
instance-form---a form; evaluated.
declaration---a declare expression; not evaluated.
forms---an implicit progn.
results---the values returned by the forms.
Creates a lexical environment in which the slots specified by slot-entry are lexically available through their accessors as if they were variables. The macro with-accessors invokes the appropriate accessors to access the slots specified by slot-entry. Both setf and setq can be used to set the value of the slot.
(defclass thing ()
((x :initarg :x :accessor thing-x)
(y :initarg :y :accessor thing-y)))
=> #<STANDARD-CLASS THING 250020173>
(defmethod (setf thing-x) :before (new-x (thing thing))
(format t "~&Changing X from ~D to ~D in ~S.~
(thing-x thing) new-x thing))
(setq thing1 (make-instance 'thing :x 1 :y 2)) => #<THING 43135676>
(setq thing2 (make-instance 'thing :x 7 :y 8)) => #<THING 43147374>
(with-accessors ((x1 thing-x) (y1 thing-y))
thing1
(with-accessors ((x2 thing-x) (y2 thing-y))
thing2
(list (list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2))
(setq x1 (+ y1 x2))
(list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2))
(setf (thing-x thing2) (list x1))
(list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2)))))
|> Changing X from 1 to 9 in #<THING 43135676>.
|> Changing X from 7 to (9) in #<THING 43147374>.
=> ((1 1 2 2 7 7 8 8)
9
(9 9 2 2 7 7 8 8)
(9)
(9 9 2 2 (9) (9) 8 8))
defclass
The consequences are undefined if any accessor-name is not the name of an accessor for the instance.
section with-slots [Macro] , section symbol-macrolet [Special Operator]
A with-accessors expression of the form:
(with-accessors ({slot-entry_1 ...{slot-entry}_n) instance-form {form}_1 ...{form}_k)}
expands into the equivalent of
(let ((in instance-form))
(symbol-macrolet ({Q_1... {Q}_n) {form}_1 ...{form}_k))}
where {Q}_i is
{
({variable-name}_i ()
({accessor-name_i in))}
}
with-slots ({slot-entry}{*)
instance-form
{declaration}{*} {form}{*}}
=> {result}{*}
slot-entry ::=slot-name | (variable-name slot-name)
slot-name---a slot name; not evaluated.
variable-name---a variable name; not evaluated.
instance-form---a form; evaluted to produce instance.
instance---an object.
declaration---a declare expression; not evaluated.
forms---an implicit progn.
results---the values returned by the forms.
The macro with-slots establishes a lexical environment for referring to the slots in the instance named by the given slot-names as though they were variables. Within such a context the value of the slot can be specified by using its slot name, as if it were a lexically bound variable. Both setf and setq can be used to set the value of the slot.
The macro with-slots translates an appearance of the slot name as a variable into a call to slot-value.
(defclass thing ()
((x :initarg :x :accessor thing-x)
(y :initarg :y :accessor thing-y)))
=> #<STANDARD-CLASS THING 250020173>
(defmethod (setf thing-x) :before (new-x (thing thing))
(format t "~&Changing X from ~D to ~D in ~S.~
(thing-x thing) new-x thing))
(setq thing (make-instance 'thing :x 0 :y 1)) => #<THING 62310540>
(with-slots (x y) thing (incf x) (incf y)) => 2
(values (thing-x thing) (thing-y thing)) => 1, 2
(setq thing1 (make-instance 'thing :x 1 :y 2)) => #<THING 43135676>
(setq thing2 (make-instance 'thing :x 7 :y 8)) => #<THING 43147374>
(with-slots ((x1 x) (y1 y))
thing1
(with-slots ((x2 x) (y2 y))
thing2
(list (list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2))
(setq x1 (+ y1 x2))
(list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2))
(setf (thing-x thing2) (list x1))
(list x1 (thing-x thing1) y1 (thing-y thing1)
x2 (thing-x thing2) y2 (thing-y thing2)))))
|> Changing X from 7 to (9) in #<THING 43147374>.
=> ((1 1 2 2 7 7 8 8)
9
(9 9 2 2 7 7 8 8)
(9)
(9 9 2 2 (9) (9) 8 8))
defclass
The consequences are undefined if any slot-name is not the name of a slot in the instance.
section with-accessors [Macro] , section slot-value [Function] , section symbol-macrolet [Special Operator]
A with-slots expression of the form:
(with-slots ({slot-entry_1 ...{slot-entry}_n) instance-form {form}_1 ...{form}_k)}
expands into the equivalent of
(let ((in instance-form))
(symbol-macrolet ({Q_1... {Q}_n) {form}_1 ...{form}_k))}
where {Q}_i is
({slot-entry}_i ()
(slot-value in '{slot-entry}_i))
if {slot-entry}_i is a symbol and is
{
({variable-name}_i ()
(slot-value in '{slot-name}_i))
}
if {slot-entry}_i is of the form
({variable-name}_i
{slot-name}_i)
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)
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.
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 section 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 section Inheritance.
The following slot options are available:
Each class option is an option that refers to the class as a whole. The following class options are available:
Note the following rules of defclass for standard classes:
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 section 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.
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.
@xref{documentation; (setf documentation)} , section Initialize-Instance , section make-instance [Standard Generic Function] , section slot-value [Function] , section Classes, section Inheritance, section Redefining Classes, section Determining the Class Precedence List, section Object Creation and Initialization
defgeneric function-name gf-lambda-list
[[!option | {!method-description}{*]]}
=> new-generic
option ::=(:argument-precedence-order {parameter-name}^+) | (declare {gf-declaration}^+) | (:documentation gf-documentation) | (:method-combination method-combination {method-combination-argument}{*) |} (:generic-function-class generic-function-class) | (:method-class method-class)
method-description ::=(:method {method-qualifier}{* specialized-lambda-list {[[{declaration}{*} | documentation]]} {form}{*})}
function-name---a function name.
generic-function-class---a non-nil symbol naming a class.
gf-declaration---an optimize declaration specifier; other declaration specifiers are not permitted.
gf-documentation---a string; not evaluated.
gf-lambda-list---a generic function lambda list.
method-class---a non-nil symbol naming a class.
method-combination-argument---an object.
method-combination-name---a symbol naming a method combination type.
method-qualifiers, specialized-lambda-list, declarations, documentation, forms---as per defmethod.
new-generic---the generic function object.
parameter-name---a symbol that names a required parameter in the lambda-list. (If the :argument-precedence-order option is specified, each required parameter in the lambda-list must be used exactly once as a parameter-name.)
The macro defgeneric is used to define a generic function or to specify options and declarations that pertain to a generic function as a whole.
If function-name is a list it must be of the form (setf symbol). If (fboundp function-name) is false, a new generic function is created.
If (fdefinition function-name) is a generic function, that
generic function is modified. If function-name names an ordinary function, a macro, or a special operator, an error is signaled.
The effect of the defgeneric macro is as if the following three steps were performed: first, methods defined by previous defgeneric forms are removed;
[Reviewer Note by Barmar: Shouldn't this (second) be first?] second, ensure-generic-function is called; and finally, methods specified by the current defgeneric form are added to the generic function.
Each method-description defines a method on the generic function. The lambda list of each method must be congruent with the lambda list specified by the gf-lambda-list option. If no method descriptions are specified and a generic function of the same name does not already exist, a generic function with no methods is created.
The gf-lambda-list argument of defgeneric specifies the shape of lambda lists for the methods on this generic function. All methods on the resulting generic function must have lambda lists that are congruent with this shape. If a defgeneric form is evaluated and some methods for that generic function have lambda lists that are not congruent with that given in the defgeneric form, an error is signaled. For further details on method congruence, see section Congruent Lambda-lists for all Methods of a Generic Function.
The generic function passes to the method all the argument values passed to it, and only those; default values are not supported. Note that optional and keyword arguments in method definitions, however, can have default initial value forms and can use supplied-p parameters.
The following options are provided.
Except as otherwise noted,
a given option may occur only once.
The method-description arguments define methods that will be associated with the generic function. The method-qualifier and specialized-lambda-list arguments in a method description are the same as for defmethod.
The form arguments specify the method body. The body of the method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol.
Implementations can extend defgeneric to include other options. It is required that an implementation signal an error if it observes an option that is not implemented locally.
defgeneric is not required to perform any compile-time side effects. In particular, the methods are not installed for invocation during compilation. An implementation may choose to store information about the generic function for the purposes of compile-time error-checking (such as checking the number of arguments on calls, or noting that a definition for the function name has been seen).
If function-name names an ordinary function, a macro, or a special operator, an error of type program-error is signaled.
Each required argument, as specified in the gf-lambda-list argument, must be included exactly once as a parameter-name, or an error of type program-error is signaled.
The lambda list of each method specified by a method-description must be congruent with the lambda list specified by the gf-lambda-list option, or an error of type error is signaled.
If a defgeneric form is evaluated and some methods for that generic function have lambda lists that are not congruent with that given in the defgeneric form, an error of type error is signaled.
A given option may occur only once, or an error of type program-error is signaled.
[Reviewer Note by Barmar: This says that an error is signaled if you specify the same generic function class as it already has!] If function-name specifies an existing generic function that has a different value for the :generic-function-class argument and the new generic function class is compatible with the old, change-class is called to change the class of the generic function; otherwise an error of type error is signaled.
Implementations can extend defgeneric to include other options. It is required that an implementation signal an error of type program-error if it observes an option that is not implemented locally.
section defmethod [Macro] , @xref{documentation; (setf documentation)} , section ensure-generic-function [Function] ,
generic-function,
section Congruent Lambda-lists for all Methods of a Generic Function
defmethod function-name
{{method-qualifier}{*}
specialized-lambda-list
{[[{declaration}{*} | documentation]]} {form}{*}}
=> new-method
function-name::= {symbol | (setf symbol)}
method-qualifier::= non-list
specialized-lambda-list::= ({var | ({var parameter-specializer-name)}{*}}
[{&optional {var | (var [initform {[supplied-p-parameter]} ])}{*}]}
[&rest var]
{[{{&key}{}}{var | ({var | (keywordvar)} [initform [supplied-p-parameter] ])}{*}}
[&allow-other-keys] {]}
[&aux {var | (var [initform] )}{*] )}
parameter-specializer-name::= symbol | (eql eql-specializer-form)
declaration---a declare expression; not evaluated.
documentation---a string; not evaluated.
var---a variable name.
eql-specializer-form---a form.
Form---a form.
Initform---a form.
Supplied-p-parameter---variable name.
new-method---the new method object.
The macro defmethod defines a method on a generic function.
If (fboundp function-name) is nil, a generic function is created with default values for the argument precedence order (each argument is more specific than the arguments to its right in the argument list), for the generic function class (the class standard-generic-function), for the method class (the class standard-method), and for the method combination type (the standard method combination type). The lambda list of the generic function is congruent with the lambda list of the method being defined; if the defmethod form mentions keyword arguments, the lambda list of the generic function will mention &key (but no keyword arguments). If function-name names an ordinary function, a macro, or a special operator, an error is signaled.
If a generic function is currently named by function-name, the lambda list of the method must be congruent with the lambda list of the generic function. If this condition does not hold, an error is signaled. For a definition of congruence in this context, see section Congruent Lambda-lists for all Methods of a Generic Function.
Each method-qualifier argument is an object that is used by method combination to identify the given method. The method combination type might further restrict what a method qualifier can be. The standard method combination type allows for unqualified methods and methods whose sole qualifier is one of the keywords :before, :after, or :around.
The specialized-lambda-list argument is like an ordinary lambda list except that the names of required parameters can be replaced by specialized parameters. A specialized parameter is a list of the form (var parameter-specializer-name). Only required parameters can be specialized. If parameter-specializer-name is a symbol it names a class; if it is a list, it is of the form (eql eql-specializer-form). The parameter specializer name (eql eql-specializer-form) indicates that the corresponding argument must be eql to the object that is the value of eql-specializer-form for the method to be applicable. The eql-specializer-form is evaluated at the time that the expansion of the defmethod macro is evaluated. If no parameter specializer name is specified for a given required parameter, the parameter specializer defaults to the class t. For further discussion, see section Introduction to Methods.
The form arguments specify the method body. The body of the method is enclosed in an implicit block. If function-name is a symbol, this block bears the same name as the generic function. If function-name is a list of the form (setf symbol), the name of the block is symbol.
The class of the method object that is created is that given by the method class option of the generic function on which the method is defined.
If the generic function already has a method that agrees with the method being defined on parameter specializers and qualifiers, defmethod replaces the existing method with the one now being defined. For a definition of agreement in this context. see section Agreement on Parameter Specializers and Qualifiers.
The parameter specializers are derived from the parameter specializer names as described in section Introduction to Methods.
The expansion of the defmethod macro "refers to" each specialized parameter (see the description of ignore within the description of declare). This includes parameters that have an explicit parameter specializer name of t. This means that a compiler warning does not occur if the body of the method does not refer to a specialized parameter, while a warning might occur if the body of the method does not refer to an unspecialized parameter. For this reason, a parameter that specializes on t is not quite synonymous with an unspecialized parameter in this context.
Declarations at the head of the method body that apply to the method's lambda variables are treated as bound declarations whose scope is the same as the corresponding bindings.
Declarations at the head of the method body that apply to the functional bindings of call-next-method or next-method-p apply to references to those functions within the method body forms. Any outer bindings of the function names call-next-method and next-method-p, and declarations associated with such bindings are shadowed_2 within the method body forms.
The scope of free declarations at the head of the method body is the entire method body, which includes any implicit local function definitions but excludes initialization forms for the lambda variables.
defmethod is not required to perform any compile-time side effects. In particular, the methods are not installed for invocation during compilation. An implementation may choose to store information about the generic function for the purposes of compile-time error-checking (such as checking the number of arguments on calls, or noting that a definition for the function name has been seen).
Documentation is attached as a documentation string to the method object.
The definition of the referenced generic function.
If function-name names an ordinary function, a macro, or a special operator, an error of type error is signaled.
If a generic function is currently named by function-name, the lambda list of the method must be congruent with the lambda list of the generic function, or an error of type error is signaled.
section defgeneric [Macro] , @xref{documentation; (setf documentation)} , section Introduction to Methods, section Congruent Lambda-lists for all Methods of a Generic Function, section Agreement on Parameter Specializers and Qualifiers, section Syntactic Interaction of Documentation Strings and Declarations
find-class symbol {&optional errorp environment} => class
(setf ( find-class symbol {&optional errorp environment}) new-class)
symbol---a symbol.
errorp---a generalized boolean. The default is true.
environment -- same as the &environment argument to macro expansion functions and is used to distinguish between compile-time and run-time environments.
The &environment argument has dynamic extent; the consequences are undefined if the &environment argument is referred to outside the dynamic extent of the macro expansion function.
class---a class object, or nil.
Returns the class object named by the symbol in the environment. If there is no such class, nil is returned if errorp is false; otherwise, if errorp is true, an error is signaled.
The class associated with a particular symbol can be changed by using setf with find-class;
or, if the new class given to setf is nil, the class association is removed (but the class object itself is not affected).
The results are undefined if the user attempts to change
or remove
the class associated with a symbol that is defined as a type specifier in this standard. See section Integrating Types and Classes.
When using setf of find-class, any errorp argument is evaluated for effect, but any values it returns are ignored; the errorp parameter is permitted primarily so that the environment parameter can be used.
The environment might be used to distinguish between a compile-time and a run-time environment.
If there is no such class and errorp is true, find-class signals an error of type error.
section defmacro [Macro] , section Integrating Types and Classes
next-method-p <no arguments> => generalized-boolean
generalized-boolean---a generalized boolean.
The locally defined function next-method-p can be used
within the body forms (but not the lambda list)
defined by a method-defining form to determine whether a next method exists.
The function next-method-p has lexical scope and indefinite extent.
Whether or not next-method-p is fbound in the global environment is implementation-dependent; however, the restrictions on redefinition and shadowing of next-method-p are the same as for symbols in the COMMON-LISP package which are fbound in the global environment. The consequences of attempting to use next-method-p outside of a method-defining form are undefined.
section call-next-method [Local Function] , section defmethod [Macro] , @xref{call-method; make-method}
call-method method {&optional next-method-list} => {result}{*}
make-method form => method-object
method---a method object, or a list (see below); not evaluated.
method-object---a method object.
next-method-list---a list of method objects; not evaluated.
results---the values returned by the method invocation.
The macro call-method is used in method combination. It hides the implementation-dependent details of how methods are called. The macro call-method has lexical scope and can only be used within an effective method form.
[Editorial Note by KMP: This next paragraph still needs some work.]
Whether or not call-method is fbound in the global environment is implementation-dependent; however, the restrictions on redefinition and shadowing of call-method are the same as for symbols in the COMMON-LISP package which are fbound in the global environment. The consequences of attempting to use call-method outside of an effective method form are undefined.
The macro call-method invokes the specified method, supplying it with arguments and with definitions for call-next-method and for next-method-p. If the invocation of call-method is lexically inside of a make-method, the arguments are those that were supplied to that method. Otherwise the arguments are those that were supplied to the generic function. The definitions of call-next-method and next-method-p rely on the specified next-method-list.
If method is a list, the first element of the list must be the symbol make-method and the second element must be a form. Such a list specifies a method object whose method function has a body that is the given form.
Next-method-list can contain method objects or lists, the first element of which must be the symbol make-method and the second element of which must be a form.
Those are the only two places where make-method can be used. The form used with make-method is evaluated in the null lexical environment augmented with a local macro definition for call-method and with bindings named by symbols not accessible from the COMMON-LISP-USER package.
The call-next-method function available to method will call the first method in next-method-list. The call-next-method function available in that method, in turn, will call the second method in next-method-list, and so on, until the list of next methods is exhausted.
If next-method-list is not supplied, the call-next-method function available to method signals an error of type control-error and the next-method-p function available to method returns {nil}.
section call-next-method [Local Function] , section define-method-combination [Macro] , section next-method-p [Local Function]
call-next-method {&rest args} => {result}{*}
arg---an object.
results---the values returned by the method it calls.
The function call-next-method can be used
within the body forms (but not the lambda list)
of a method defined by a method-defining form to call the next method.
If there is no next method, the generic function no-next-method is called.
The type of method combination used determines which methods can invoke call-next-method. The standard method combination type allows call-next-method to be used within primary methods and around methods. For generic functions using a type of method combination defined by the short form of define-method-combination, call-next-method can be used in around methods only.
When call-next-method is called with no arguments, it passes the current method's original arguments to the next method. Neither argument defaulting, nor using setq, nor rebinding variables with the same names as parameters of the method affects the values call-next-method passes to the method it calls.
When call-next-method is called with arguments, the next method is called with those arguments.
If call-next-method is called with arguments but omits optional arguments, the next method called defaults those arguments.
The function call-next-method returns any values that are returned by the next method.
The function call-next-method has lexical scope and indefinite extent and can only be used within the body of a method defined by a method-defining form.
Whether or not call-next-method is fbound in the global environment is implementation-dependent; however, the restrictions on redefinition and shadowing of call-next-method are the same as for symbols in the COMMON-LISP package which are fbound in the global environment. The consequences of attempting to use call-next-method outside of a method-defining form are undefined.
defmethod, call-method, define-method-combination.
When providing arguments to call-next-method, the following rule must be satisfied or an error of type error should be signaled: the ordered set of applicable methods for a changed set of arguments for call-next-method must be the same as the ordered set of applicable methods for the original arguments to the generic function. Optimizations of the error checking are possible, but they must not change the semantics of call-next-method.
section define-method-combination [Macro] , section defmethod [Macro] , section next-method-p [Local Function] , section no-next-method [Standard Generic Function] , @xref{call-method; make-method} , section Method Selection and Combination, section Standard Method Combination, section Built-in Method Combination Types
compute-applicable-methods generic-function function-arguments => methods
compute-applicable-methods (generic-function standard-generic-function)
generic-function---a generic function.
function-arguments---a list of arguments for the generic-function.
methods---a list of method objects.
Given a generic-function and a set of function-arguments, the function compute-applicable-methods returns the set of methods that are applicable for those arguments sorted according to precedence order. See section Method Selection and Combination.
defmethod
section Method Selection and Combination
define-method-combination name [[!short-form-option]]
=> name
define-method-combination name lambda-list
({method-group-specifier}{*)
[(:arguments . args-lambda-list)]
[(:generic-function
generic-function-symbol)]
[[{declaration}{*} | documentation]]
{form}{*}}
=> name
short-form-option ::=:documentation documentation | :identity-with-one-argument identity-with-one-argument | :operator operator
method-group-specifier ::=(name {{qualifier-pattern}^+ | predicate} [[!long-form-option]])
long-form-option ::=:description description | :order order | :required required-p
args-lambda-list--- a define-method-combination arguments lambda list.
declaration---a declare expression; not evaluated.
description---a format control.
documentation---a string; not evaluated.
forms---an implicit progn that must compute and return the form that specifies how the methods are combined, that is, the effective method.
generic-function-symbol---a symbol.
identity-with-one-argument---a generalized boolean.
lambda-list---ordinary lambda list.
name---a symbol. Non-keyword, non-nil symbols are usually used.
operator---an operator. Name and operator are often the same symbol. This is the default, but it is not required.
order---:most-specific-first or :most-specific-last; evaluated.
predicate---a symbol that names a function of one argument that returns a generalized boolean.
qualifier-pattern---a list, or the symbol *.
required-p---a generalized boolean.
The macro define-method-combination is used to define new types of method combination.
There are two forms of define-method-combination. The short form is a simple facility for the cases that are expected to be most commonly needed. The long form is more powerful but more verbose. It resembles defmacro in that the body is an expression, usually using backquote, that computes a form. Thus arbitrary control structures can be implemented. The long form also allows arbitrary processing of method qualifiers.
If a define-method-combination form appears as a top level form, the compiler must make the method combination name be recognized as a valid method combination name in subsequent defgeneric forms. However, the method combination is executed no earlier than when the define-method-combination form is executed, and possibly as late as the time that generic functions that use the method combination are executed.
Most examples of the long form of define-method-combination also illustrate the use of the related functions that are provided as part of the declarative method combination facility.
;;; Examples of the short form of define-method-combination
(define-method-combination and :identity-with-one-argument t)
(defmethod func and ((x class1) y) ...)
;;; The equivalent of this example in the long form is:
(define-method-combination and
(&optional (order :most-specific-first))
((around (:around))
(primary (and) :order order :required t))
(let ((form (if (rest primary)
`(and ,@(mapcar #'(lambda (method)
`(call-method ,method))
primary))
`(call-method ,(first primary)))))
(if around
`(call-method ,(first around)
(,@(rest around)
(make-method ,form)))
form)))
;;; Examples of the long form of define-method-combination
;The default method-combination technique
(define-method-combination standard ()
((around (:around))
(before (:before))
(primary () :required t)
(after (:after)))
(flet ((call-methods (methods)
(mapcar #'(lambda (method)
`(call-method ,method))
methods)))
(let ((form (if (or before after (rest primary))
`(multiple-value-prog1
(progn ,@(call-methods before)
(call-method ,(first primary)
,(rest primary)))
,@(call-methods (reverse after)))
`(call-method ,(first primary)))))
(if around
`(call-method ,(first around)
(,@(rest around)
(make-method ,form)))
form))))
;A simple way to try several methods until one returns non-nil
(define-method-combination or ()
((methods (or)))
`(or ,@(mapcar #'(lambda (method)
`(call-method ,method))
methods)))
;A more complete version of the preceding
(define-method-combination or
(&optional (order ':most-specific-first))
((around (:around))
(primary (or)))
;; Process the order argument
(case order
(:most-specific-first)
(:most-specific-last (setq primary (reverse primary)))
(otherwise (method-combination-error "~S is an invalid order.~@
:most-specific-first and :most-specific-last are the possible values."
order)))
;; Must have a primary method
(unless primary
(method-combination-error "A primary method is required."))
;; Construct the form that calls the primary methods
(let ((form (if (rest primary)
`(or ,@(mapcar #'(lambda (method)
`(call-method ,method))
primary))
`(call-method ,(first primary)))))
;; Wrap the around methods around that form
(if around
`(call-method ,(first around)
(,@(rest around)
(make-method ,form)))
form)))
;The same thing, using the :order and :required keyword options
(define-method-combination or
(&optional (order ':most-specific-first))
((around (:around))
(primary (or) :order order :required t))
(let ((form (if (rest primary)
`(or ,@(mapcar #'(lambda (method)
`(call-method ,method))
primary))
`(call-method ,(first primary)))))
(if around
`(call-method ,(first around)
(,@(rest around)
(make-method ,form)))
form)))
;This short-form call is behaviorally identical to the preceding
(define-method-combination or :identity-with-one-argument t)
;Order methods by positive integer qualifiers
;:around methods are disallowed to keep the example small
(define-method-combination example-method-combination ()
((methods positive-integer-qualifier-p))
`(progn ,@(mapcar #'(lambda (method)
`(call-method ,method))
(stable-sort methods #'<
:key #'(lambda (method)
(first (method-qualifiers method)))))))
(defun positive-integer-qualifier-p (method-qualifiers)
(and (= (length method-qualifiers) 1)
(typep (first method-qualifiers) '(integer 0 *))))
;;; Example of the use of :arguments
(define-method-combination progn-with-lock ()
((methods ()))
(:arguments object)
`(unwind-protect
(progn (lock (object-lock ,object))
,@(mapcar #'(lambda (method)
`(call-method ,method))
methods))
(unlock (object-lock ,object))))
The compiler is not required to perform any compile-time side-effects.
Method combination types defined with the short form require exactly one qualifier per method. An error of type error is signaled if there are applicable methods with no qualifiers or with qualifiers that are not supported by the method combination type. At least one primary method must be applicable or an error of type error is signaled.
If an applicable method does not fall into any method group, the system signals an error of type error indicating that the method is invalid for the kind of method combination in use.
If the value of the :required option is true and the method group is empty (that is, no applicable methods match the qualifier patterns or satisfy the predicate), an error of type error is signaled.
If the :order option evaluates to a value other than :most-specific-first or :most-specific-last, an error of type error is signaled.
@xref{call-method; make-method} , section call-next-method [Local Function] , @xref{documentation; (setf documentation)} , section method-qualifiers [Standard Generic Function] , section method-combination-error [Function] , section invalid-method-error [Function] , section defgeneric [Macro] , section Method Selection and Combination, section Built-in Method Combination Types, section Syntactic Interaction of Documentation Strings and Declarations
The :method-combination option of defgeneric is used to specify that a generic function should use a particular method combination type. The first argument to the :method-combination option is the name of a method combination type and the remaining arguments are options for that type.
find-method generic-function method-qualifiers specializers {&optional errorp}
=> method
find-method (generic-function standard-generic-function)
method-qualifiers specializers {&optional errorp}
generic-function---a generic function.
method-qualifiers---a list.
specializers---a list.
errorp---a generalized boolean. The default is true.
method---a method object, or nil.
The generic function find-method takes a generic function and returns the method object that agrees on qualifiers and parameter specializers with the method-qualifiers and specializers arguments of find-method. Method-qualifiers contains the method qualifiers for the method. The order of the method qualifiers is significant. For a definition of agreement in this context, see section Agreement on Parameter Specializers and Qualifiers.
The specializers argument contains the parameter specializers for the method. It must correspond in length to the number of required arguments of the generic function, or an error is signaled. This means that to obtain the default method on a given generic-function, a list whose elements are the class t must be given.
If there is no such method and errorp is true, find-method signals an error. If there is no such method and errorp is false, find-method returns nil.
(defmethod some-operation ((a integer) (b float)) (list a b)) => #<STANDARD-METHOD SOME-OPERATION (INTEGER FLOAT) 26723357> (find-method #'some-operation '() (mapcar #'find-class '(integer float))) => #<STANDARD-METHOD SOME-OPERATION (INTEGER FLOAT) 26723357> (find-method #'some-operation '() (mapcar #'find-class '(integer integer))) |> Error: No matching method (find-method #'some-operation '() (mapcar #'find-class '(integer integer)) nil) => NIL
add-method, defclass, defgeneric, defmethod
If the specializers argument does not correspond in length to the number of required arguments of the generic-function, an an error of type error is signaled.
If there is no such method and errorp is true, find-method signals an error of type error.
section Agreement on Parameter Specializers and Qualifiers
add-method generic-function method => generic-function
add-method (generic-function standard-generic-function)
(method method)
generic-function---a generic function object.
method---a method object.
The generic function add-method adds a method to a generic function.
If method agrees with an existing method of generic-function on parameter specializers and qualifiers, the existing method is replaced.
The lambda list of the method function of method must be congruent with the lambda list of generic-function, or an error of type error is signaled.
If method is a method object of another generic function, an error of type error is signaled.
section defmethod [Macro] , section defgeneric [Macro] , section find-method [Standard Generic Function] , section remove-method [Standard Generic Function] , section Agreement on Parameter Specializers and Qualifiers
initialize-instance instance {&rest initargs {&key} {&allow-other-keys}} => instance
initialize-instance (instance standard-object) {&rest initargs}
instance---an object.
initargs---a defaulted initialization argument list.
Called by make-instance to initialize a newly created instance. The generic function is called with the new instance and the defaulted initialization argument list.
The system-supplied primary method on initialize-instance initializes the slots of the instance with values according to the initargs and the :initform forms of the slots. It does this by calling the generic function shared-initialize with the following arguments: the instance, t (this indicates that all slots for which no initialization arguments are provided should be initialized according to their :initform forms), and the initargs.
Programmers can define methods for initialize-instance to specify actions to be taken when an instance is initialized. If only after methods 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.
section Shared-Initialize , section make-instance [Standard Generic Function] , section slot-boundp [Function] , section slot-makunbound [Function] , section Object Creation and Initialization, section Rules for Initialization Arguments, section Declaring the Validity of Initialization Arguments
class-name class => name
class-name (class class)
class---a class object.
name---a symbol.
Returns the name of the given class.
section find-class [Accessor] , section Classes
If S is a symbol such that S =(class-name C) and C =(find-class S), then S is the proper name of C. For further discussion, see section Classes.
The name of an anonymous class is nil.
(setf class-name) new-value class => new-value
(setf class-name) new-value (class class)
new-value---a symbol.
class---a class.
The generic function (setf class-name) sets the name of a class object.
section find-class [Accessor] , proper name, section Classes
class-of object => class
object---an object.
class---a class object.
Returns the class of which the object is a direct instance.
(class-of 'fred) => #<BUILT-IN-CLASS SYMBOL 610327300> (class-of 2/3) => #<BUILT-IN-CLASS RATIO 610326642> (defclass book () ()) => #<STANDARD-CLASS BOOK 33424745> (class-of (make-instance 'book)) => #<STANDARD-CLASS BOOK 33424745> (defclass novel (book) ()) => #<STANDARD-CLASS NOVEL 33424764> (class-of (make-instance 'novel)) => #<STANDARD-CLASS NOVEL 33424764> (defstruct kons kar kdr) => KONS (class-of (make-kons :kar 3 :kdr 4)) => #<STRUCTURE-CLASS KONS 250020317>
section make-instance [Standard Generic Function] , section type-of [Function]
unbound-slot, cell-error, error, serious-condition, condition, t
The object having the unbound slot is initialized by the :instance initialization argument to make-condition, and is accessed by the function unbound-slot-instance.
The name of the cell (see cell-error) is the name of the slot.
section cell-error-name [Function] , unbound-slot-object, section Condition System Concepts
unbound-slot-instance condition => instance
condition---a condition of type unbound-slot.
instance---an object.
Returns the instance which had the unbound slot in the situation represented by the condition.
section cell-error-name [Function] , unbound-slot, section Condition System Concepts
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