ANSI Common Lisp   7 Objects   7.5 Slots

7.5.3 Inheritance of Slots and Slot Options

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.