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73 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>19.4. Defstruct Slot-Options</TITLE>
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<meta name="description" value=" Defstruct Slot-Options">
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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=tex2html3678 HREF="node173.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3676 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3670 HREF="node171.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3680 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3681 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3679 HREF="node173.html"> Defstruct Options</A>
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<B>Up:</B> <A NAME=tex2html3677 HREF="node168.html"> Structures</A>
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<B> Previous:</B> <A NAME=tex2html3671 HREF="node171.html"> Using the Automatically </A>
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<HR> <P>
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<H1><A NAME=SECTION002340000000000000000>19.4. Defstruct Slot-Options</A></H1>
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<P>
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<A NAME=DefstructSlotOptions>Each</A>
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<i>slot-description</i> in a <tt>defstruct</tt> form may specify one or more
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slot-options. A slot-option consists of a pair of a keyword and
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a value (which is not a form to be evaluated, but the value itself).
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For example:
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<P><pre>
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(defstruct ship
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(x-position 0.0 <tt>:type</tt> short-float)
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(y-position 0.0 <tt>:type</tt> short-float)
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(x-velocity 0.0 <tt>:type</tt> short-float)
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(y-velocity 0.0 <tt>:type</tt> short-float)
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(mass *default-ship-mass* <tt>:type</tt> short-float <tt>:read-only</tt> t))
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</pre><P>
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This specifies that each slot will always contain a
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short-format floating-point number,
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and that the last slot may not be altered once a ship is constructed.
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<P>
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The available slot-options are as follows.
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<DL COMPACT><DT><tt>:type</tt>
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<DD>
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The option <tt><tt>:type</tt> <i>type</i></tt> specifies that the contents of the
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slot will always be of the specified data type. This is entirely
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analogous to the declaration of a variable or function; indeed, it
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effectively declares the result type of the access function. An
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implementation may or may not choose to check the type of the new object
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when initializing or assigning to a slot.
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Note that the argument form <i>type</i> is not evaluated;
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it must be a valid type specifier.
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<P>
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<DT><tt>:read-only</tt>
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<DD>
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The option <tt><tt>:read-only</tt> <i>x</i></tt>, where <i>x</i> is not <tt>nil</tt>,
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specifies that this slot may not be
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altered; it will always contain the value specified at construction time.
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<tt>setf</tt> will not accept the access function for this slot.
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If <i>x</i> is <tt>nil</tt>, this slot-option has no effect.
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Note that the argument form <i>x</i> is not evaluated.
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<P>
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</DL>
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<P>
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Note that it is impossible to specify a slot-option unless
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a default value is specified first.
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<P>
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<BR> <HR><A NAME=tex2html3678 HREF="node173.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3676 HREF="node168.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3670 HREF="node171.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3680 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3681 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3679 HREF="node173.html"> Defstruct Options</A>
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<B>Up:</B> <A NAME=tex2html3677 HREF="node168.html"> Structures</A>
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<B> Previous:</B> <A NAME=tex2html3671 HREF="node171.html"> Using the Automatically </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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