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4.8 KiB
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83 lines
4.8 KiB
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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>28.1.2.4. Accessing Slots</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Accessing Slots">
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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=tex2html4996 HREF="node268.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4994 HREF="node263.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4990 HREF="node266.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4998 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4999 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4997 HREF="node268.html"> Inheritance</A>
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<B>Up:</B> <A NAME=tex2html4995 HREF="node263.html"> Classes</A>
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<B> Previous:</B> <A NAME=tex2html4991 HREF="node266.html"> Slots</A>
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<HR> <P>
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<H3><A NAME=SECTION003212400000000000000>28.1.2.4. Accessing Slots</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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Slots can be accessed in two ways: by use of the primitive function
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<tt>slot-value</tt> and by use of generic functions generated by the
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<tt>defclass</tt> form.
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<P>
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The function <tt>slot-value</tt> can be used with any slot name
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specified in the <tt>defclass</tt> form to access a specific slot
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accessible in an instance of the given class.
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<P>
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The macro <tt>defclass</tt> provides syntax for generating methods to
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read and write slots. If a <i>reader</i> is requested, a method is
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automatically generated for reading the value of the slot, but no
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method for storing a value into it is generated. If a <i>writer</i>
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is requested, a method is automatically generated for storing a value
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into the slot, but no method for reading its value is generated. If
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an <i>accessor</i> is requested, a method for reading the value of
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the slot and a method for storing a value into the slot are
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automatically generated. Reader and writer methods are implemented
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using <tt>slot-value</tt>.
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<P>
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When a reader or writer is specified for a slot, the name of the
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generic function to which the generated method belongs is directly
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specified. If the name specified for the writer option is the symbol
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<i>name</i>, the name of the generic function for writing the slot
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is the symbol <i>name</i>, and the generic function takes two
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arguments: the new value and the instance, in that order. If the name
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specified for the accessor option is the symbol <i>name</i>, the
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name of the generic function for reading the slot is the symbol <i>name</i>, and the name of the generic function for writing the slot is
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the list <tt>(setf <i>name</i>)</tt>.
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<P>
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A generic function created or modified by supplying reader, writer, or
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accessor slot options can be treated exactly as an ordinary generic
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function.
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<P>
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Note that <tt>slot-value</tt> can be used to read or write the value of a
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slot whether or not reader or writer methods exist for that slot.
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When <tt>slot-value</tt> is used, no reader or writer methods are
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invoked.
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<P>
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The macro <tt>with-slots</tt> can be used to establish a lexical
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environment in which specified slots are lexically available as if they
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were variables. The macro <tt>with-slots</tt> invokes the function
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<tt>slot-value</tt> to access the specified slots.
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<P>
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The macro <tt>with-accessors</tt> can be used to establish a lexical
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environment in which specified slots are lexically available through
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their accessors as if they were variables. The macro
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<tt>with-accessors</tt> invokes the appropriate accessors to access the
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specified slots. Any accessors specified by <tt>with-accessors</tt> must
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already have been defined before they are used.
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<P>
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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=tex2html4996 HREF="node268.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4994 HREF="node263.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4990 HREF="node266.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4998 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4999 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4997 HREF="node268.html"> Inheritance</A>
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<B>Up:</B> <A NAME=tex2html4995 HREF="node263.html"> Classes</A>
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<B> Previous:</B> <A NAME=tex2html4991 HREF="node266.html"> Slots</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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</BODY>
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