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122 lines
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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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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<TITLE>28.1.3.2. Inheritance of Slots and Slot Options</TITLE>
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<meta name="description" value=" Inheritance of Slots and Slot Options">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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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=tex2html5036 HREF="node271.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5034 HREF="node268.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5028 HREF="node269.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5038 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5039 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5037 HREF="node271.html"> Inheritance of Class </A>
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<B>Up:</B> <A NAME=tex2html5035 HREF="node268.html"> Inheritance</A>
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<B> Previous:</B> <A NAME=tex2html5029 HREF="node269.html"> Inheritance of Methods</A>
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<HR> <P>
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<H3><A NAME=SECTION003213200000000000000>28.1.3.2. Inheritance of Slots and Slot Options</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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<A NAME=InheritanceofSlotsandSlotOptionsSECTION>The</A>
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set of names of all slots accessible in an instance of a class
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<i>C</i> is the union of the sets of names of slots defined by <i>C</i> and its
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superclasses. The <i>structure</i> of an instance is the set of names
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of local slots in that instance.
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<P>
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In the simplest case, only one class among <i>C</i> and its superclasses
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defines a slot with a given slot name. If a slot is defined by a
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superclass of <i>C</i>, the slot is said to be <i>inherited</i>. The characteristics of the slot are determined by the
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slot specifier of the defining class. Consider the defining class for
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a slot <i>S</i>. If the value of the <tt>:allocation</tt> slot
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option is <tt>:instance</tt>, then <i>S</i> is a local slot and each instance
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of <i>C</i> has its own slot named <i>S</i> that stores its own value. If the
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value of the <tt>:allocation</tt> slot option is <tt>:class</tt>, then <i>S</i>
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is a shared slot, the class that defined <i>S</i> stores the value, and all
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instances of <i>C</i> can access that single slot. If the
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<tt>:allocation</tt> slot option is omitted, <tt>:instance</tt> is used.
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<P>
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In general, more than one class among <i>C</i> and its superclasses can
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define a slot with a given name. In such cases, only one slot with
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the given name is accessible in an instance of <i>C</i>, and
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the characteristics of that slot are a combination of the several slot
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specifiers, computed as follows:
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<P>
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<UL><LI> All the slot specifiers for a given slot name are ordered
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from most specific to least specific, according to the order in <i>C</i>'s
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class precedence list of the classes that define them. All references
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to the specificity of slot specifiers immediately following refer to this
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ordering.
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<P>
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<LI> The allocation of a slot is controlled by the most specific
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slot specifier. If the most specific slot specifier does not contain an
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<tt>:allocation</tt> slot option, <tt>:instance</tt> is used. Less specific
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slot specifiers do not affect the allocation.
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<P>
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<LI> The default initial value form for a
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slot is the value of the <tt>:initform</tt> slot option in the most
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specific slot specifier that contains one. If no slot specifier
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contains an <tt>:initform</tt> slot option, the slot has no default
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initial value form.
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<P>
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<LI> The contents of a slot will always be of type
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<tt>(and <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43475.gif"> <b>...</b> <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43367.gif">)</tt>
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where <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43369.gif"> are
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the values of the <tt>:type</tt> slot options contained in all of the slot
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specifiers. If no slot specifier contains the <tt>:type</tt> slot option, the
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contents of the slot will always be of type <tt>t</tt>. The result
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of attempting to store in a slot
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a value that does not satisfy the type of the slot is undefined.
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<P>
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<LI> The set of initialization arguments that initialize a given
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slot is the union of the initialization arguments declared in the
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<tt>:initarg</tt> slot options in all the slot specifiers.
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<P>
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<LI> The documentation string for a slot is the value of the
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<tt>:documentation</tt> slot option in the most specific slot specifier
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that contains one. If no slot specifier contains a
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<tt>:documentation</tt> slot option, the slot has no documentation string.
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</UL>
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<P>
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A consequence of the allocation rule is that a shared slot can be
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shadowed. For example, if a class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> defines a slot named <i>S</i>
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whose value for the <tt>:allocation</tt> slot option is <tt>:class</tt>,
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that slot is accessible in instances of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> and all of its
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subclasses. However, if <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> is a subclass of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> and also
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defines a slot named <i>S</i>, <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif">'s slot is not shared
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by instances of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> and its subclasses. When a class
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> defines a shared slot, any subclass <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> will share this single slot unless the <tt>defclass</tt> form for
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> specifies a slot of the same name or there is a superclass
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of <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> that precedes <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> in the class precedence list of
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<IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> that defines a slot of the same name.
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<P>
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A consequence of the type rule is that the value of a slot satisfies
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the type constraint of each slot specifier that contributes to that
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slot. Because the result of attempting to store in a slot a value
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that does not satisfy the type constraint for the slot is undefined,
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the value in a slot might fail to satisfy its type constraint.
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<P>
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The <tt>:reader</tt>, <tt>:writer</tt>, and <tt>:accessor</tt> slot options
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create methods rather than define the characteristics of a slot.
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Reader and writer methods are inherited in the sense described in
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section <A HREF="node269.html#InheritanceofMethodsSECTION">28.1.3.1</A>.
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<P>
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Methods that access slots use only the name of the slot and the type
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of the slot's value. Suppose a superclass provides a method that
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expects to access a shared slot of a given name, and a subclass defines
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a local slot with the same name. If the method provided by the
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superclass is used on an instance of the subclass, the method accesses
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the local slot.
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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=tex2html5036 HREF="node271.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5034 HREF="node268.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5028 HREF="node269.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5038 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5039 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5037 HREF="node271.html"> Inheritance of Class </A>
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<B>Up:</B> <A NAME=tex2html5035 HREF="node268.html"> Inheritance</A>
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<B> Previous:</B> <A NAME=tex2html5029 HREF="node269.html"> Inheritance of Methods</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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</ADDRESS>
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