Common Lisp the Language
2nd Edition
This example determines a class precedence list for the
class pie. The following classes are defined:
(defclass pie (apple cinnamon) ()) (defclass apple (fruit) ()) (defclass cinnamon (spice) ()) (defclass fruit (food) ()) (defclass spice (food) ()) (defclass food () ())
The set S = {pie apple cinnamon fruit spice food standard-object t}. The set R = {(pie apple) (apple cinnamon) (cinnamon standard-object) (apple fruit) (fruit standard-object) (cinnamon spice) (spice standard-object) (fruit food) (food standard-object) (spice food) (standard-object t)}
[The original CLOS specification [5
6]
contained a minor error in this example: the pairs
(cinnamon
standard-object)
(fruit
standard-object)
and
(spice
standard-object)
were inadvertently omitted from R in the preceding paragraph.
It is important to understand that defclass implicitly appends the
class standard-object to the list of superclasses when the metaclass
is standard-class (the normal situation)
in order to insure that standard-object will be a superclass
of every instance of standard-class except standard-object itself
(see section 28.1.2).
is then generated from this augmented list of superclasses;
this is where the extra pairs come from. I have corrected the example
by adding these pairs as appropriate throughout the example. The final result
the class precedence list for pie
is unchanged.-GLS]
The class pie is not preceded by anything
so it comes first;
the result so far is (pie). Remove pie from S and pairs
mentioning pie from R to get
S = {apple
cinnamon
fruit
spice
food
standard-object
t}
and R = {(apple
cinnamon)
(cinnamon
standard-object)
(apple
fruit)
(fruit
standard-object)
(cinnamon
spice)
(spice
standard-object)
(fruit
food)
(food
standard-object)
(spice
food)
(standard-object
t)}.
The class apple is not preceded by anything
so it is next; the
result is (pie apple). Removing apple and the relevant
pairs results in
S = {cinnamon
fruit
spice
food
standard-object
t}
and R = {(cinnamon
standard-object)
(fruit
standard-object)
(cinnamon
spice)
(spice
standard-object)
(fruit
food)
(food
standard-object)
(spice
food)
(standard-object
t)}.
The classes cinnamon and fruit are not preceded by
anything
so the one with a direct subclass rightmost in the class
precedence list computed so far goes next. The class apple is a
direct subclass of fruit
and the class pie is a direct
subclass of cinnamon. Because apple appears to the right
of pie in the precedence list
fruit goes next
and the
result so far is (pie apple fruit).
S = {cinnamon
spice
food
standard-object
t}
and R = {(cinnamon
standard-object)
(cinnamon
spice)
(spice
standard-object)
(food
standard-object)
(spice
food)
(standard-object
t)}.
The class cinnamon is next
giving the result so far as
(pie apple fruit cinnamon). At this point
S = {spice
food
standard-object
t}
and R = {(spice
standard-object)
(food
standard-object)
(spice
food)
(standard-object
t)}.
The classes spice food standard-object and t are then added in that order and the final class precedence list for pie is
(pie apple fruit cinnamon spice food standard-object t)
It is possible to write a set of class definitions that cannot be ordered. For example:
(defclass new-class (fruit apple) ()) (defclass apple (fruit) ())
The class fruit must precede apple because the local ordering of superclasses must be preserved. The class apple must precede fruit because a class always precedes its own superclasses. When this situation occurs an error is signaled when the system tries to compute the class precedence list.
The following might appear to be a conflicting set of definitions:
(defclass pie (apple cinnamon) ()) (defclass pastry (cinnamon apple) ()) (defclass apple () ()) (defclass cinnamon () ())
The class precedence list for pie is
(pie apple cinnamon standard-object t)
The class precedence list for pastry is
(pastry cinnamon apple standard-object t)
It is not a problem for apple to precede cinnamon in the
ordering of the superclasses of pie but not in the ordering for
pastry. However
it is not possible to build a new class that
has both pie and pastry as superclasses.
