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 T_1 ... T_n) where T_1 ... 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 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
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.