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68 lines
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<!-- Common Lisp HyperSpec (TM), version 7.0 generated by Kent M. Pitman on Mon, 11-Apr-2005 2:31am EDT -->
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<TITLE>CLHS: Standard Generic Function SLOT-MISSING</TITLE>
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<H1><A REV=MADE HREF="http://www.lispworks.com/"><IMG WIDTH=80 HEIGHT=65 ALT="[LISPWORKS]" SRC="../Graphics/LWSmall.gif" ALIGN=Bottom></A><A REL=TOP HREF="../Front/index.htm"><IMG WIDTH=237 HEIGHT=65 ALT="[Common Lisp HyperSpec (TM)]" SRC="../Graphics/CLHS_Sm.gif" ALIGN=Bottom></A> <A REL=PREV HREF="f_slt_ma.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="c_object.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="f_slt_un.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<A NAME="slot-missing"><I>Standard Generic Function</I> <B>SLOT-MISSING</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>slot-missing</B> <I>class object slot-name operation <TT>&optional</TT> new-value</I> => <I><I>result</I><B>*</B></I><P>
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<P>
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<P><B>Method Signatures:</B><P>
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<P>
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<B>slot-missing</B> <I>(<I>class</I><B> <A REL=DEFINITION HREF="t_t.htm#t">t</A></B>) object slot-name operation <TT>&optional</TT> new-value</I><P>
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<P>
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<P><B>Arguments and Values:</B><P>
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<P>
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<I>class</I>---the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of <I>object</I>. <P>
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<I>object</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>slot-name</I>---a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> (the <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> of a would-be <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>). <P>
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<I>operation</I>---one of the <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbols</I></A> <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A>, <A REL=DEFINITION HREF="f_slt_bo.htm#slot-boundp"><B>slot-boundp</B></A>, <A REL=DEFINITION HREF="f_slt_ma.htm#slot-makunbound"><B>slot-makunbound</B></A>, or <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A>. <P>
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<I>new-value</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>result</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<P><B>Description:</B><P>
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<P>
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The generic function <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> is invoked when an attempt is made to <A REL=DEFINITION HREF="26_glo_a.htm#access"><I>access</I></A> a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> in an <I>object</I> whose <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> is <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A> and the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of the name <I>slot-name</I> is not a <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> of a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> in that <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. The default <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> signals an error. <P>
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The generic function <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> is not intended to be called by programmers. Programmers may write <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> for it. <P>
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The generic function <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> may be called during evaluation of <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A>, <TT>(setf slot-value)</TT>, <A REL=DEFINITION HREF="f_slt_bo.htm#slot-boundp"><B>slot-boundp</B></A>, and <A REL=DEFINITION HREF="f_slt_ma.htm#slot-makunbound"><B>slot-makunbound</B></A>. For each of these operations the corresponding <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> for the <I>operation</I> argument is <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A>, <A REL=DEFINITION HREF="a_setf.htm#setf"><B>setf</B></A>, <A REL=DEFINITION HREF="f_slt_bo.htm#slot-boundp"><B>slot-boundp</B></A>, and <A REL=DEFINITION HREF="f_slt_ma.htm#slot-makunbound"><B>slot-makunbound</B></A> respectively. <P>
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The optional <I>new-value</I> argument to <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> is used when the operation is attempting to set the value of the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>. <P>
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If <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> returns, its values will be treated as follows: <P>
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<P><UL><LI> If the <I>operation</I> is <A REL=DEFINITION HREF="a_setf.htm#setf"><B>setf</B></A> or <A REL=DEFINITION HREF="f_slt_ma.htm#slot-makunbound"><B>slot-makunbound</B></A>, any <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> will be ignored by the caller. <P>
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<LI> If the <I>operation</I> is <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A>, only the <A REL=DEFINITION HREF="26_glo_p.htm#primary_value"><I>primary value</I></A> will be used by the caller, and all other values will be ignored. <P>
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<LI> If the <I>operation</I> is <A REL=DEFINITION HREF="f_slt_bo.htm#slot-boundp"><B>slot-boundp</B></A>, any <A REL=DEFINITION HREF="26_glo_b.htm#boolean_equivalent"><I>boolean equivalent</I></A> of the <A REL=DEFINITION HREF="26_glo_p.htm#primary_value"><I>primary value</I></A> of the <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> might be is used, and all other values will be ignored. <P></UL> <P>
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<P><B>Examples:</B> None.
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<P>
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<P><B>Affected By:</B> None.
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<P>
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<P><B>Exceptional Situations:</B><P>
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<P>
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The default <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> on <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A> signals an error of <A REL=DEFINITION HREF="26_glo_t.htm#type"><I>type</I></A> <A REL=DEFINITION HREF="e_error.htm#error"><B>error</B></A>. <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A>, <A REL=DEFINITION HREF="f_slt_ex.htm#slot-exists-p"><B>slot-exists-p</B></A>, <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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The set of arguments (including the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of the instance) facilitates defining methods on the metaclass for <A REL=DEFINITION HREF="#slot-missing"><B>slot-missing</B></A>. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issue</A>, <I>not part of the specification</I>, applies to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss319.htm">SLOT-MISSING-VALUES:SPECIFY</A><P></UL><HR>
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