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85 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>28.1.9.3. Defaulting of Initialization Arguments</TITLE>
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<BODY>
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<meta name="description" value=" Defaulting of Initialization Arguments">
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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=tex2html5347 HREF="node296.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5345 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5339 HREF="node294.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5349 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5350 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5348 HREF="node296.html"> Rules for Initialization </A>
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<B>Up:</B> <A NAME=tex2html5346 HREF="node292.html"> Object Creation and </A>
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<B> Previous:</B> <A NAME=tex2html5340 HREF="node294.html"> Declaring the Validity </A>
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<HR> <P>
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<H3><A NAME=SECTION003219300000000000000>28.1.9.3. Defaulting of 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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A <i>default value form</i> can be supplied for an initialization
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argument by using the <tt>:default-initargs</tt> class option. If an
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initialization argument is declared valid by some particular class,
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its default value form might be specified by a different class.
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In this case <tt>:default-initargs</tt> is used to supply a default value
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for an inherited initialization argument.
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<P>
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The <tt>:default-initargs</tt> option is used only to provide default
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values for initialization arguments; it does not declare a symbol as a
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valid initialization argument name. Furthermore, the
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<tt>:default-initargs</tt> option is used only to provide default values for
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initialization arguments when making an instance.
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<P>
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The argument to the <tt>:default-initargs</tt> class option is a list of
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alternating initialization argument names and forms. Each form is the
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default value form for the corresponding initialization
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argument. The default value form of an initialization
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argument is used and evaluated only if that initialization argument
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does not appear in the arguments to <tt>make-instance</tt> and is not
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defaulted by a more specific class. The default value form is
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evaluated in the lexical environment of the <tt>defclass</tt> form that
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supplied it; the result is used as the initialization
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argument's value.
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<P>
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The initialization arguments supplied to <tt>make-instance</tt> are combined
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with defaulted initialization arguments to produce a <i>defaulted initialization argument list</i>. A defaulted initialization
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argument list is a list of alternating initialization argument names and
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values in which unsupplied initialization arguments are defaulted and in
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which the explicitly supplied initialization arguments appear earlier in
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the list than the defaulted initialization arguments. Defaulted
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initialization arguments are ordered according to the order in the class
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precedence list of the classes that supplied the default values.
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<P>
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There is a distinction between the purposes of the
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<tt>:default-initargs</tt> and the <tt>:initform</tt> options with respect to the
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initialization of slots. The <tt>:default-initargs</tt> class option
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provides a mechanism for the user to give a default value form
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for an initialization argument without knowing whether the
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initialization argument initializes a slot or is passed to a method.
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If that initialization argument is not explicitly supplied in a call
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to <tt>make-instance</tt>, the default value form is used, just
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as if it had been supplied in the call. In contrast, the
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<tt>:initform</tt> slot option provides a mechanism for the user to give a
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default initial value form for a slot. An <tt>:initform</tt> form is
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used to initialize a slot only if no initialization argument
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associated with that slot is given as an argument to
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<tt>make-instance</tt> or is defaulted by <tt>:default-initargs</tt>.
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<P>
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The order of evaluation of default value forms for initialization
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arguments and the order of evaluation of <tt>:initform</tt> forms are
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undefined. If the order of evaluation matters, use
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<tt>initialize-instance</tt> or <tt>shared-initialize</tt> methods.
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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=tex2html5347 HREF="node296.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5345 HREF="node292.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5339 HREF="node294.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5349 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5350 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5348 HREF="node296.html"> Rules for Initialization </A>
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<B>Up:</B> <A NAME=tex2html5346 HREF="node292.html"> Object Creation and </A>
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<B> Previous:</B> <A NAME=tex2html5340 HREF="node294.html"> Declaring the Validity </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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