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}~= {pie, apple, cinnamon, fruit, spice, food, standard-object, t }. The set R~= { (pie, apple), (apple, cinnamon), (apple, fruit), (cinnamon, spice), \break (fruit, food), (spice, food), (food, standard-object), (standard-object, t) }.
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), (apple, fruit), (cinnamon, spice),\break (fruit,
food), (spice, food), (food, standard-object),
(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, spice),
(fruit, food), (spice, food), (food, standard-object),\break
(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 class precedence list,
fruit
goes next, and the
result so far is (pie apple fruit)
. S~= { cinnamon,
spice, food, standard-object, t }; R~= {(cinnamon,
spice), (spice, food),\break (food, standard-object),
(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 }; R~= { (spice, food), (food,
standard-object), (standard-object, t) }.
The classes spice
, food
, standard-object, and
t are added in that order, and the class precedence list
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, as happens here
when the system tries to compute the class precedence list
of new-class
.
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.