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7.1 Object Creation and Initialization 7.2 Changing the Class of an Instance 7.3 Reinitializing an Instance 7.4 Meta-Objects 7.5 Slots 7.6 Generic Functions and Methods 7.7 Objects Dictionary
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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:
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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.
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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 presence of {&allow-other-keys} in the lambda list of an applicable method disables validity checking of initialization arguments.
Thus method inheritance controls the set of valid initialization arguments that supply arguments to methods. The generic functions for which method definitions serve to declare initialization arguments valid are as follows:
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.
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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.
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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
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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.
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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.
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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.
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The function change-class can be used to change the class of an instance from its current class, C_{{from}}, to a different class, C_{{to}}; it changes the structure of the instance to conform to the definition of the class C_{{to}}.
Note that changing the class of an instance may cause slots to be added or deleted. Changing the class of an instance does not change its identity as defined by the eq function.
When change-class is invoked on an instance, a two-step updating process takes place. The first step modifies the structure of the instance by adding new local slots and discarding local slots that are not specified in the new version of the instance. The second step initializes the newly added local slots and performs any other user-defined actions. These two steps are further described in the two following sections.
7.2.1 Modifying the Structure of the Instance 7.2.2 Initializing Newly Added Local Slots 7.2.3 Customizing the Change of Class of an Instance
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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.
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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 7.1.2 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.
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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 7.1.5 Shared-Initialize.
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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 7.1.2 Declaring the Validity of Initialization Arguments.) Then it calls the generic function shared-initialize with the following arguments: the instance, nil, and the initialization arguments it received.
7.3.1 Customizing Reinitialization
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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 7.1.5 Shared-Initialize.
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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.
7.4.1 Standard Meta-objects
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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.
and defclass.
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7.5.1 Introduction to Slots 7.5.2 Accessing Slots 7.5.3 Inheritance of Slots and Slot Options
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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 7.1 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 7.5.3 Inheritance of Slots and Slot Options.
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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.
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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 7.6.7 Inheritance of Methods.
Methods that access slots use only the name of the slot and the type of the slot's value. Suppose a superclass provides a method that expects to access a shared slot of a given name, and a subclass defines a local slot with the same name. If the method provided by the superclass is used on an instance of the subclass, the method accesses the local slot.
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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."
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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 7.6.4 Congruent Lambda-lists for all Methods of a Generic Function.
Each method has a specialized lambda list, which determines when that method can be applied. A specialized lambda list is like an ordinary lambda list except that a specialized parameter may occur instead of the name of a required parameter. A specialized parameter is a list (variable-name parameter-specializer-name), where parameter-specializer-name is one of the following:
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.
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Two methods are said to agree with each other on parameter specializers and qualifiers if the following conditions hold:
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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.
If a method-defining operator that cannot specify generic function options creates a generic function, and if the lambda list for the method mentions keyword arguments, the lambda list of the generic function will mention &key (but no keyword arguments).
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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 3.5 Error Checking in Function Calls.
7.6.5.1 Examples of Keyword Arguments in Generic Functions and Methods
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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)
|
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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 7.6.5 Keyword Arguments in Generic Functions and Methods.
When the effective method has been determined, it is invoked with the same arguments as were passed to the generic function. Whatever values it returns are returned as the values of the generic function.
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The effective method is determined by the following three-step procedure:
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This step is described in 7.6.2 Introduction to Methods.
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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 7.6.3 Agreement on Parameter Specializers and Qualifiers.
If some corresponding parameter specializers do not agree, the first pair of parameter specializers that do not agree determines the precedence. If both parameter specializers are classes, the more specific of the two methods is the method whose parameter specializer appears earlier in the class precedence list of the corresponding argument. Because of the way in which the set of applicable methods is chosen, the parameter specializers are guaranteed to be present in the class precedence list of the class of the argument.
If just one of a pair of corresponding parameter specializers is (eql object), the method with that parameter specializer precedes the other method. If both parameter specializers are eql expressions, the specializers must agree (otherwise the two methods would not both have been applicable to this argument).
The resulting list of applicable methods has the most specific method first and the least specific method last.
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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 7.6.6.5 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.
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@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.
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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 7.6.6.1 Determining the Effective Method. There are two forms of define-method-combination. The short form is a simple facility while the long form is more powerful and more verbose. The long form resembles defmacro in that the body is an expression that computes a Lisp form; it provides mechanisms for implementing arbitrary control structures within method combination and for arbitrary processing of method qualifiers.
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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 7.6.6.5 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.
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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 7.6.6 Method Selection and Combination.
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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
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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.
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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 7.1 Object Creation and Initialization.
7.7.25 defclass [Macro] , 7.7.19 make-instance [Standard Generic Function] , 7.7.39 class-of [Function] , 7.1 Object Creation and Initialization
The consequences of adding methods to allocate-instance is unspecified. This capability might be added by the Metaobject Protocol.
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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.
7.1.6 Initialize-Instance , 7.1.5 Shared-Initialize , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.3 Reinitializing an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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)
ecifier 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.
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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.
7.1.6 Initialize-Instance , 7.7.4 reinitialize-instance [Standard Generic Function] , 7.7.7 update-instance-for-redefined-class [Standard Generic Function] , 7.7.6 update-instance-for-different-class [Standard Generic Function] , 7.7.9 slot-boundp [Function] , 7.7.11 slot-makunbound [Function] , 7.1 Object Creation and Initialization, 7.1.4 Rules for Initialization Arguments, 7.1.2 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.
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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 7.2 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.
7.7.8 change-class [Standard Generic Function] , 7.1.5 Shared-Initialize , 7.2 Changing the Class of an Instance, 7.1.4 Rules for Initialization Arguments, 7.1.2 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 b