7.5 Slots

7.5.1 Introduction to Slots

An object of metaclass standard-class has zero or more named slots. The slots of an object are determined by the class of the object. Each slot can hold one value. 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, the generic function slot-unbound is invoked. The system-supplied primary method for slot-unbound on class t signals an error. If slot-unbound returns, its primary value is used that time as the value of the slot.

The default initial value form for a slot is defined by the :initform slot option. When the :initform form is used to supply a value, it is evaluated in the lexical environment in which the defclass form was evaluated. The :initform along with the lexical environment in which the defclass form was evaluated is called a captured initialization form. For more details, see Section 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 Section 7.5.3 (Inheritance of Slots and Slot Options).

7.5.2 Accessing Slots

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

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

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

are implemented using slot-value.

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

the slot is the symbol name, and the name of the generic function for writing the slot is the list (setf name).

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

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

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

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

7.5.3 Inheritance of Slots and Slot Options

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

In the simplest case, only one class among C and its superclasses defines a slot with a given slot name. If a slot is defined by a superclass of C, the slot is said to be inherited. The characteristics of the slot are determined by the slot specifier of the defining class. Consider the defining class for

a slot S. If the value of the :allocation slot option is :instance, then S is a local slot and each instance of C has its own slot named S that stores its own value. If the value of the :allocation slot option is :class, then S is a shared slot, the class that defined S stores the value, and all instances of C can access that single slot. If the :allocation slot option is omitted, :instance is used.

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

• All the slot specifiers for a given slot name are ordered from most specific to least specific, according to the order in C’s class precedence list of the classes that define them. All references to the specificity of slot specifiers immediately below refers to this ordering.

• The allocation of a slot is controlled by the most specific slot specifier . If the most specific slot specifier does not contain an :allocation slot option, :instance is used. Less specific slot specifiers do not affect the allocation.

• The default initial value form for a slot is the value of the :initform slot option in the most specific slot specifier that contains one. If no slot specifier contains an :initform slot option, the slot has no default initial value form.

• The contents of a slot will always be of type (and T₁ . . . T_n) where T₁ . . . T_n are the values of the :type slot options contained in all of the slot specifiers. If no slot specifier contains the :type slot option, the contents of the slot will always be of type t. The consequences of attempting to store in a slot a value that does not satisfy the type of the slot are undefined.

• The set of initialization arguments that initialize a given slot is the union of the initialization arguments declared in the :initarg slot options in all the slot specifiers.

• The documentation string for a slot is the value of the :documentation slot option in the most specific slot specifier that contains one. If no slot specifier contains a :documentation slot option, the slot has no documentation string.

A consequence of the allocation rule is that a shared slot can be shadowed. For example, if a class C₁ defines a slot named S whose value for the :allocation slot option is :class, that slot is accessible in instances of C₁ and all of its subclasses. However, if C₂ is a subclass of C₁ and also defines a slot named S, C₁’s slot is not shared by instances of C₂ and its subclasses. When a class C₁ defines a shared slot, any subclass C₂ of C₁ will share this single slot unless the defclass form for C₂ specifies a slot of the same name or there is a superclass of C₂ that precedes C₁ in the class precedence list of C₂ that defines a slot of the same name.

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

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

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