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142 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>28.1.5.2. Examples</TITLE>
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<meta name="description" value=" Examples">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5106 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5104 HREF="node274.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5100 HREF="node275.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5108 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5109 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5107 HREF="node277.html"> Generic Functions and </A>
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<B>Up:</B> <A NAME=tex2html5105 HREF="node274.html"> Determining the Class </A>
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<B> Previous:</B> <A NAME=tex2html5101 HREF="node275.html"> Topological Sorting</A>
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<HR> <P>
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<H3><A NAME=SECTION003215200000000000000>28.1.5.2. Examples</A></H3>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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This example determines a class precedence list for the
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class <tt>pie</tt>. The following classes are defined:
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<P>
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<P><pre>
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(defclass pie (apple cinnamon) ())
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(defclass apple (fruit) ())
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(defclass cinnamon (spice) ())
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(defclass fruit (food) ())
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(defclass spice (food) ())
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(defclass food () ())
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</pre><P>
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The set <i>S</i> = <tt>{pie, apple, cinnamon, fruit, spice, food, standard-object, t}</tt>.
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The set <i>R</i> = <tt>{(pie, apple), (apple, cinnamon), (cinnamon,
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standard-object), (apple, fruit), (fruit, standard-object), (cinnamon,
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spice), (spice, standard-object), (fruit, food), (food,
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standard-object), (spice, food), (standard-object, t)}</tt>
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<P>
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[The original CLOS specification [<A HREF="node368.html#SIGPLANCLOS">5</A>,<A HREF="node368.html#LASCCLOSPART1">6</A>]
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contained a minor error in this example: the pairs
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<tt>(cinnamon, standard-object)</tt>,
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<tt>(fruit, standard-object)</tt>, and
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<tt>(spice, standard-object)</tt>
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were inadvertently omitted from <i>R</i> in the preceding paragraph.
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It is important to understand that <tt>defclass</tt> implicitly appends the
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class <tt>standard-object</tt> to the list of superclasses when the metaclass
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is <tt>standard-class</tt> (the normal situation),
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in order to insure that <tt>standard-object</tt> will be a superclass
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of every instance of <tt>standard-class</tt> except <tt>standard-object</tt> itself
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(see section <A HREF="node263.html#ClassesSECTION">28.1.2</A>).
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43499.gif"> is then generated from this augmented list of superclasses;
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this is where the extra pairs come from. I have corrected the example
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by adding these pairs as appropriate throughout the example. The final result,
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the class precedence list for <tt>pie</tt>,
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is unchanged.-GLS]
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<P>
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The class <tt>pie</tt> is not preceded by anything, so it comes first;
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the result so far is <tt>(pie)</tt>. Remove <tt>pie</tt> from <i>S</i> and pairs
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mentioning <tt>pie</tt> from <i>R</i> to get
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<br><i>S</i> = <tt>{apple, cinnamon, fruit, spice, food, standard-object, t}</tt>
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and <i>R</i> = <tt>{(apple, cinnamon), (cinnamon, standard-object),
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(apple, fruit), (fruit, standard-object), (cinnamon, spice), (spice,
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standard-object), (fruit, food), (food, standard-object), (spice,
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food), (standard-object, t)}</tt>.
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<P>
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The class <tt>apple</tt> is not preceded by anything, so it is next; the
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result is <tt>(pie apple)</tt>. Removing <tt>apple</tt> and the relevant
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pairs results in
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<br><i>S</i> = <tt>{cinnamon, fruit, spice, food, standard-object, t}</tt>
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and <i>R</i> = <tt>{(cinnamon, standard-object), (fruit,
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standard-object), (cinnamon, spice), (spice, standard-object), (fruit,
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food), (food, standard-object), (spice, food), (standard-object, t)}</tt>.
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<P>
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The classes <tt>cinnamon</tt> and <tt>fruit</tt> are not preceded by
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anything, so the one with a direct subclass rightmost in the class
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precedence list computed so far goes next. The class <tt>apple</tt> is a
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direct subclass of <tt>fruit</tt>, and the class <tt>pie</tt> is a direct
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subclass of <tt>cinnamon</tt>. Because <tt>apple</tt> appears to the right
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of <tt>pie</tt> in the precedence list, <tt>fruit</tt> goes next, and the
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result so far is <tt>(pie apple fruit)</tt>.
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<br><i>S</i> = <tt>{cinnamon, spice, food, standard-object, t}</tt>
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and <i>R</i> = <tt>{(cinnamon, standard-object), (cinnamon, spice), (spice, standard-object), (food, standard-object), (spice, food), (standard-object, t)}</tt>.
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<P>
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The class <tt>cinnamon</tt> is next, giving the result so far as
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<tt>(pie apple fruit cinnamon)</tt>. At this point
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<br><i>S</i> = <tt>{spice, food, standard-object, t}</tt>
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and <i>R</i> = <tt>{(spice, standard-object), (food, standard-object), (spice, food), (standard-object, t)}</tt>.
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<P>
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The classes <tt>spice</tt>, <tt>food</tt>, <tt>standard-object</tt>, and
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<tt>t</tt> are then added in that order, and the final class precedence list for <tt>pie</tt> is
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<P><pre>
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(pie apple fruit cinnamon spice food standard-object t)
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</pre><P>
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<P>
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It is possible to write a set of class definitions that cannot be
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ordered. For example:
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<P>
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<P><pre>
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(defclass new-class (fruit apple) ())
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(defclass apple (fruit) ())
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</pre><P>
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<P>
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The class <tt>fruit</tt> must precede <tt>apple</tt> because the local
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ordering of superclasses must be preserved. The class <tt>apple</tt> must
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precede <tt>fruit</tt> because a class always precedes its own
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superclasses. When this situation occurs, an error is signaled when
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the system tries to compute the class precedence list.
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<P>
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The following might appear to be a conflicting set of definitions:
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<P>
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<P><pre>
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(defclass pie (apple cinnamon) ())
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(defclass pastry (cinnamon apple) ())
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(defclass apple () ())
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(defclass cinnamon () ())
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</pre><P>
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<P>
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The class precedence list for <tt>pie</tt> is
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<P><pre>
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(pie apple cinnamon standard-object t)
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</pre><P>
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<P>
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The class precedence list for <tt>pastry</tt> is
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<P><pre>
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(pastry cinnamon apple standard-object t)
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</pre><P>
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<P>
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It is not a problem for <tt>apple</tt> to precede <tt>cinnamon</tt> in the
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ordering of the superclasses of <tt>pie</tt> but not in the ordering for
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<tt>pastry</tt>. However, it is not possible to build a new class that
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has both <tt>pie</tt> and <tt>pastry</tt> as superclasses.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html5106 HREF="node277.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5104 HREF="node274.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5100 HREF="node275.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5108 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5109 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5107 HREF="node277.html"> Generic Functions and </A>
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<B>Up:</B> <A NAME=tex2html5105 HREF="node274.html"> Determining the Class </A>
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<B> Previous:</B> <A NAME=tex2html5101 HREF="node275.html"> Topological Sorting</A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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