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93 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: Function SLOT-VALUE</TITLE>
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<A NAME="slot-value"><I>Function</I> <B>SLOT-VALUE</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>slot-value</B> <I>object slot-name</I> => <I>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>object</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>name</I>---a <A REL=DEFINITION HREF="26_glo_s.htm#symbol"><I>symbol</I></A>. <P>
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<I>value</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 <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="#slot-value"><B>slot-value</B></A> returns 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> named <I>slot-name</I> in the <I>object</I>. If there is no <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> named <I>slot-name</I>, <A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A> is called. If the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is unbound, <A REL=DEFINITION HREF="f_slt_un.htm#slot-unbound"><B>slot-unbound</B></A> is called. <P>
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The macro <A REL=DEFINITION HREF="m_setf_.htm#setf"><B>setf</B></A> can be used with <A REL=DEFINITION HREF="#slot-value"><B>slot-value</B></A> to change the value of a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A>. <P>
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<P><B>Examples:</B><P>
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<P>
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<PRE>
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(defclass foo ()
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((a :accessor foo-a :initarg :a :initform 1)
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(b :accessor foo-b :initarg :b)
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(c :accessor foo-c :initform 3)))
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=> #<STANDARD-CLASS FOO 244020371>
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(setq foo1 (make-instance 'foo :a 'one :b 'two))
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=> #<FOO 36325624>
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(slot-value foo1 'a) => ONE
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(slot-value foo1 'b) => TWO
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(slot-value foo1 'c) => 3
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(setf (slot-value foo1 'a) 'uno) => UNO
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(slot-value foo1 'a) => UNO
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(defmethod foo-method ((x foo))
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(slot-value x 'a))
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=> #<STANDARD-METHOD FOO-METHOD (FOO) 42720573>
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(foo-method foo1) => UNO
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</PRE>
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</TT> <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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If an attempt is made to read a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> and no <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of the name <I>slot-name</I> exists in the <I>object</I>, <A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A> is called as follows: <P>
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<PRE>
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(slot-missing (class-of instance)
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instance
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slot-name
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'slot-value)
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</PRE>
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</TT> <P>
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(If <A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A> is invoked, its <A REL=DEFINITION HREF="26_glo_p.htm#primary_value"><I>primary value</I></A> is returned by <A REL=DEFINITION HREF="#slot-value"><B>slot-value</B></A>.) <P>
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If an attempt is made to write a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> and no <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of the name <I>slot-name</I> exists in the <I>object</I>, <A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A> is called as follows: <P>
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<PRE>
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(slot-missing (class-of instance)
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instance
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slot-name
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'setf
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new-value)
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</PRE>
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</TT> <P>
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(If <A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A> returns in this case, any <A REL=DEFINITION HREF="26_glo_v.htm#value"><I>values</I></A> are ignored.) <P>
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The specific behavior depends on <I>object</I>'s <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A>. An error is never signaled if <I>object</I> has <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>. An error is always signaled if <I>object</I> has <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A> <A REL=DEFINITION HREF="t_built_.htm#built-in-class"><B>built-in-class</B></A>. The consequences are unspecified if <I>object</I> has any other <A REL=DEFINITION HREF="26_glo_m.htm#metaclass"><I>metaclass</I></A>--an error might or might not be signaled in this situation. Note in particular that the behavior for <A REL=DEFINITION HREF="26_glo_c.htm#condition"><I>conditions</I></A> and <A REL=DEFINITION HREF="26_glo_s.htm#structure"><I>structures</I></A> is not specified. <P>
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<P><B>See Also:</B><P>
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<P>
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<A REL=DEFINITION HREF="f_slt_mi.htm#slot-missing"><B>slot-missing</B></A>, <A REL=DEFINITION HREF="f_slt_un.htm#slot-unbound"><B>slot-unbound</B></A>, <A REL=DEFINITION HREF="m_w_slts.htm#with-slots"><B>with-slots</B></A> <P>
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<P><B>Notes:</B><P>
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<P>
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Although no <A REL=DEFINITION HREF="26_glo_i.htm#implementation"><I>implementation</I></A> is required to do so, implementors are strongly encouraged to implement the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="#slot-value"><B>slot-value</B></A> using the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <TT>slot-value-using-class</TT> described in the <A REL=DEFINITION HREF="26_glo_m.htm#metaobject_protocol"><I>Metaobject Protocol</I></A>. <P>
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Implementations may optimize <A REL=DEFINITION HREF="#slot-value"><B>slot-value</B></A> by compiling it inline. <P>
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<P><HR>The following <A REL=META HREF="../Front/X3J13Iss.htm">X3J13 cleanup issues</A>, <I>not part of the specification</I>, apply to this section:<P><UL><LI> <A REL=CHILD HREF="../Issues/iss320.htm">SLOT-VALUE-METACLASSES:LESS-MINIMAL</A><LI> <A REL=CHILD HREF="../Issues/iss319.htm">SLOT-MISSING-VALUES:SPECIFY</A><P></UL><HR>
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