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<TITLE>CLHS: Section 7.5.1</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="07_e.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="07_e.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="07_eb.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<H2>
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7.5.1 Introduction to Slots</H2> <P>
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An <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> of <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A> has zero or more named <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A>. The <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> of an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> are determined by the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of the <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. Each <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> can hold one value. The <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> is a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A> that is syntactically valid for use as a variable name. <P>
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When a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> does not have a value, the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is said to be <A REL=DEFINITION HREF="26_glo_u.htm#unbound"><I>unbound</I></A>. When an unbound <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is read, the <A REL=DEFINITION HREF="26_glo_g.htm#generic_function"><I>generic function</I></A> <A REL=DEFINITION HREF="f_slt_un.htm#slot-unbound"><B>slot-unbound</B></A> is invoked. The system-supplied primary <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> for <A REL=DEFINITION HREF="f_slt_un.htm#slot-unbound"><B>slot-unbound</B></A> on <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="t_t.htm#t"><B>t</B></A> signals an error. If <A REL=DEFINITION HREF="f_slt_un.htm#slot-unbound"><B>slot-unbound</B></A> returns, its <A REL=DEFINITION HREF="26_glo_p.htm#primary_value"><I>primary value</I></A> is used that time as the <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>value</I></A> of the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>. <P>
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The default initial value form for a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is defined by the <TT>:initform</TT> slot option. When the <TT>:initform</TT> form is used to supply a value, it is evaluated in the lexical environment in which the <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> form was evaluated. The <TT>:initform</TT> along with the lexical environment in which the <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> form was evaluated is called a <A REL=DEFINITION HREF="26_glo_c.htm#captured_initialization_form"><I>captured initialization form</I></A>. For more details, see <A REL=CHILD HREF="07_a.htm">Section 7.1 (Object Creation and Initialization)</A>. <P>
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A <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slot</I></A> is defined to be a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> that is <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> to exactly one <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>, namely the one in which the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is allocated. A <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> is defined to be a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> that is visible to more than one <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> of a given <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> and its <A REL=DEFINITION HREF="26_glo_s.htm#subclass"><I>subclasses</I></A>. <P>
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A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is said to define a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> with a given <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> when the <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> form for that <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> contains a <A REL=DEFINITION HREF="26_glo_s.htm#slot_specifier"><I>slot specifier</I></A> with that <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A>. Defining a <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slot</I></A> does not immediately create a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>; it causes a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> to be created each time an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is created. Defining a <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> immediately creates a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>. <P>
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The <TT>:allocation</TT> slot option to <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> controls the kind of <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> that is defined. If the value of the <TT>:allocation</TT> slot option is <TT>:instance</TT>, a <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slot</I></A> is created. If the value of <TT>:allocation</TT> is <TT>:class</TT>, a <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> is created. <P>
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A <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is said to be <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> in an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> of a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> if the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is defined by the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of the <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> or is inherited from a <A REL=DEFINITION HREF="26_glo_s.htm#superclass"><I>superclass</I></A> of that <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. At most one <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of a given <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>name</I></A> can be <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> in an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>. A <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> defined by a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> in all <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of that <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. A detailed explanation of the inheritance of <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> is given in <A REL=CHILD HREF="07_ec.htm">Section 7.5.3 (Inheritance of Slots and Slot Options)</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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<A REL=NAVIGATOR HREF="../Front/StartPts.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Starting Points]" SRC="../Graphics/StartPts.gif" ALIGN=Bottom></A><A REL=TOC HREF="../Front/Contents.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Contents]" SRC="../Graphics/Contents.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Master.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Index]" SRC="../Graphics/Index.gif" ALIGN=Bottom></A><A REL=INDEX HREF="../Front/X_Symbol.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Symbols]" SRC="../Graphics/Symbols.gif" ALIGN=Bottom></A><A REL=GLOSSARY HREF="../Body/26_a.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Glossary]" SRC="../Graphics/Glossary.gif" ALIGN=Bottom></A><A HREF="../Front/X3J13Iss.htm"><IMG WIDTH=80 HEIGHT=40 ALT="[Issues]" SRC="../Graphics/Issues.gif" ALIGN=Bottom></A><BR>
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<A REL=COPYRIGHT HREF="../Front/Help.htm#Legal"><I>Copyright 1996-2005, LispWorks Ltd. All rights reserved.</I></A><P>
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