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80 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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<HEAD>
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<TITLE>28.1.9.1. Initialization Arguments</TITLE>
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<meta name="description" value=" Initialization Arguments">
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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=tex2html5323 HREF="node294.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5321 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5315 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5325 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5326 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5324 HREF="node294.html"> Declaring the Validity </A>
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<B>Up:</B> <A NAME=tex2html5322 HREF="node292.html"> Object Creation and </A>
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<B> Previous:</B> <A NAME=tex2html5316 HREF="node292.html"> Object Creation and </A>
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<HR> <P>
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<H3><A NAME=SECTION003219100000000000000>28.1.9.1. Initialization Arguments</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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An initialization argument controls object creation and
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initialization. It is often convenient to use keyword symbols to name
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initialization arguments, but the name of an initialization argument
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can be any symbol, including <tt>nil</tt>. An initialization argument
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can be used in two ways: to fill a slot with a value or to provide an
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argument for an initialization method. A single initialization
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argument can be used for both purposes.
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<P>
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An <i>initialization argument list</i> is a list of alternating
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initialization argument names and values. Its structure is identical
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to a property list and also to the portion of an argument list
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processed for <tt>&key</tt> parameters. As in those lists, if an
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initialization argument name appears more than once in an
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initialization argument list, the leftmost occurrence supplies the
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value and the remaining occurrences are ignored. The arguments to
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<tt>make-instance</tt> (after the first argument) form an initialization
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argument list. Error checking of initialization argument names is
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disabled if the keyword argument pair whose keyword is
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<tt>:allow-other-keys</tt> and whose value is non-<tt>nil</tt> appears in the
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initialization argument list.
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<P>
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An initialization argument can be associated with a slot. If the
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initialization argument has a value in the initialization argument
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list, the value is stored into the slot of the newly created object,
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overriding any <tt>:initform</tt> form associated with the slot. A
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single initialization argument can initialize more than one slot. An
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initialization argument that initializes a shared slot stores its
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value into the shared slot, replacing any previous value.
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<P>
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An initialization argument can be associated with a method. When an
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object is created and a particular initialization argument is
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supplied, the generic functions <tt>initialize-instance</tt>,
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<tt>shared-initialize</tt>, and <tt>allocate-instance</tt> are called with that
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initialization argument's name and value as a keyword argument pair.
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If a value for the initialization argument is not supplied in the
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initialization argument list, the method's lambda-list supplies a
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default value.
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<P>
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Initialization arguments are used in four situations: when making an
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instance, when re-initializing an instance, when updating an instance to
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conform to a redefined class, and when updating an instance to conform
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to the definition of a different class.
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<P>
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Because initialization arguments are used to control the creation and
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initialization of an instance of some particular class, we say that an
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initialization argument is ``an initialization argument for'' that
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class.
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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=tex2html5323 HREF="node294.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5321 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5315 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5325 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5326 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5324 HREF="node294.html"> Declaring the Validity </A>
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<B>Up:</B> <A NAME=tex2html5322 HREF="node292.html"> Object Creation and </A>
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<B> Previous:</B> <A NAME=tex2html5316 HREF="node292.html"> Object Creation and </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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