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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 UPDATE-INSTANCE-FOR-DIFFERENT-CLASS</TITLE>
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<A NAME="update-instance-for-different-class"><I>Standard Generic Function</I> <B>UPDATE-INSTANCE-FOR-DIFFERENT-CLASS</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>update-instance-for-different-class</B> <I>previous current <TT>&rest</TT> initargs <TT>&key</TT> <TT>&allow-other-keys</TT></I> => <I><A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A></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>update-instance-for-different-class</B> <I>(<I>previous</I><B> <A REL=DEFINITION HREF="t_std_ob.htm#standard-object">standard-object</A></B>) (<I>current</I><B> <A REL=DEFINITION HREF="t_std_ob.htm#standard-object">standard-object</A></B>) <TT>&rest</TT> initargs</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>previous</I>---a copy of the original <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>. <P>
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<I>current</I>---the original <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> (altered). <P>
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<I>initargs</I>---an <A REL=DEFINITION HREF="26_glo_i.htm#initialization_argument_list"><I>initialization argument list</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="#update-instance-for-different-class"><B>update-instance-for-different-class</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. The <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> is called only by the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A>. <P>
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The system-supplied primary <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> on <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> checks the validity of <I>initargs</I> and signals an error if an <I>initarg</I> is supplied that is not declared as valid. This <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> then initializes <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> with values according to the <I>initargs</I>, and initializes the newly added <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> with values according to their <TT>:initform</TT> forms. It does this by calling the generic function <A REL=DEFINITION HREF="f_shared.htm#shared-initialize"><B>shared-initialize</B></A> with the following arguments: the instance (<I>current</I>), a list of <A REL=DEFINITION HREF="26_glo_n.htm#name"><I>names</I></A> of the newly added <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A>, and the <I>initargs</I> it received. Newly added <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> are those <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> for which no <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of the same name exists in the <I>previous</I> class. <P>
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<A REL=DEFINITION HREF="26_glo_m.htm#method"><I>Methods</I></A> for <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> can be defined to specify actions to be taken when an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> is updated. If only <A REL=DEFINITION HREF="26_glo_a.htm#after_method"><I>after methods</I></A> for <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> are defined, they will be run after the system-supplied primary <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> for initialization and therefore will not interfere with the default behavior of <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A>. <P>
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<A REL=DEFINITION HREF="26_glo_m.htm#method"><I>Methods</I></A> on <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> can be defined to initialize <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> differently from <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A>. The default behavior of <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A> is described in <A REL=CHILD HREF="07_b.htm">Section 7.2 (Changing the Class of an Instance)</A>. <P>
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The arguments to <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> are computed by <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A>. When <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A> is invoked on an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>, a copy of that <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> is made; <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A> then destructively alters the original <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>. The first argument to <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A>, <I>previous</I>, is that copy; it holds the old <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> values temporarily. This argument has dynamic extent within <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A>; if it is referenced in any way once <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> returns, the results are undefined. The second argument to <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A>, <I>current</I>, is the altered original <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>. The intended use of <I>previous</I> is to extract old <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> values by using <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A> or <A REL=DEFINITION HREF="m_w_slts.htm#with-slots"><B>with-slots</B></A> or by invoking a reader generic function, or to run other <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> that were applicable to <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of the original <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. <P>
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<P><B>Examples:</B><P>
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<P>
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See the example for the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</B></A>. <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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The system-supplied primary <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> on <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> signals an error if an initialization argument is supplied that is not declared as valid. <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_chg_cl.htm#change-class"><B>change-class</B></A>, <A REL=DEFINITION HREF="f_shared.htm#shared-initialize"><B>shared-initialize</B></A>, <A REL=CHILD HREF="07_b.htm">Section 7.2 (Changing the Class of an Instance)</A>, <A REL=CHILD HREF="07_ad.htm">Section 7.1.4 (Rules for Initialization Arguments)</A>, <A REL=CHILD HREF="07_ab.htm">Section 7.1.2 (Declaring the Validity of Initialization Arguments)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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<I>Initargs</I> are declared as valid by using the <TT>:initarg</TT> option to <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A>, or by defining <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> for <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> or <A REL=DEFINITION HREF="f_shared.htm#shared-initialize"><B>shared-initialize</B></A>. The keyword name of each keyword parameter specifier in the <A REL=DEFINITION HREF="26_glo_l.htm#lambda_list"><I>lambda list</I></A> of any <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> defined on <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> or <A REL=DEFINITION HREF="f_shared.htm#shared-initialize"><B>shared-initialize</B></A> is declared as a valid <I>initarg</I> name for all <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>classes</I></A> for which that <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> is applicable. <P>
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The value returned by <A REL=DEFINITION HREF="#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> is ignored by <A REL=DEFINITION HREF="f_chg_cl.htm#change-class"><B>change-class</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/iss197.htm">INITIALIZATION-FUNCTION-KEYWORD-CHECKING</A><P></UL><HR>
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