Introduction to Slots (ANSI and GNU Common Lisp Document)

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#### 7.5.1 Introduction to Slots

An object of metaclass standard-class has zero or more named slots. The slots of an object are determined by the class of the object. Each slot can hold one value.

[Reviewer 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 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 Inheritance of Slots and Slot Options.

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