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).