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104 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 CHANGE-CLASS</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_upda_1.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_bo.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Next]" SRC="../Graphics/Next.gif" ALIGN=Bottom></A></H1>
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<HR>
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<A NAME="change-class"><I>Standard Generic Function</I> <B>CHANGE-CLASS</B></A> <P>
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<P><B>Syntax:</B><P>
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<P>
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<B>change-class</B> <I>instance new-class <TT>&key</TT> <TT>&allow-other-keys</TT></I> => <I>instance</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>change-class</B> <I>(<I>instance</I><B> <A REL=DEFINITION HREF="t_std_ob.htm#standard-object">standard-object</A></B>) (<I>new-class</I><B> <A REL=DEFINITION HREF="t_std_cl.htm#standard-class">standard-class</A></B>) <TT>&rest</TT> initargs</I><P>
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<P>
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<B>change-class</B> <I>(<I>instance</I><B> <A REL=DEFINITION HREF="t_t.htm#t">t</A></B>) (<I>new-class</I><B> <A REL=DEFINITION HREF="t_symbol.htm#symbol">symbol</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>instance</I>---an <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A>. <P>
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<I>new-class</I>---a <A REL=DEFINITION HREF="26_glo_c.htm#class_designator"><I>class designator</I></A>. <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 <A REL=DEFINITION HREF="26_glo_g.htm#generic_function"><I>generic function</I></A> <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> changes the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of an <I>instance</I> to <I>new-class</I>. It destructively modifies and returns the <I>instance</I>. <P>
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If in the old <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> there is any <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of the same name as a local <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> in the <I>new-class</I>, the value of that <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is retained. This means that if the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> has a value, the value returned by <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A> after <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> is invoked is <A REL=DEFINITION HREF="f_eql.htm#eql"><B>eql</B></A> to the value returned by <A REL=DEFINITION HREF="f_slt_va.htm#slot-value"><B>slot-value</B></A> before <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> is invoked. Similarly, if the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> was unbound, it remains unbound. The other <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> are initialized as described in <A REL=CHILD HREF="07_b.htm">Section 7.2 (Changing the Class of an Instance)</A>. <P>
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After completing all other actions, <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> invokes <A REL=DEFINITION HREF="f_update.htm#update-instance-for-different-class"><B>update-instance-for-different-class</B></A>. The generic function <A REL=DEFINITION HREF="f_update.htm#update-instance-for-different-class"><B>update-instance-for-different-class</B></A> can be used to assign values to slots in the transformed instance. See <A REL=CHILD HREF="07_bb.htm">Section 7.2.2 (Initializing Newly Added Local Slots)</A>. <P>
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If the second of the above <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> is selected, that <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> invokes <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> on <I>instance</I>, <TT>(find-class </TT><I>new-class</I><TT>)</TT>, and the <I>initargs</I>. <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 position () ())
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(defclass x-y-position (position)
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((x :initform 0 :initarg :x)
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(y :initform 0 :initarg :y)))
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(defclass rho-theta-position (position)
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((rho :initform 0)
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(theta :initform 0)))
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(defmethod update-instance-for-different-class :before ((old x-y-position)
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(new rho-theta-position)
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&key)
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;; Copy the position information from old to new to make new
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;; be a rho-theta-position at the same position as old.
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(let ((x (slot-value old 'x))
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(y (slot-value old 'y)))
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(setf (slot-value new 'rho) (sqrt (+ (* x x) (* y y)))
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(slot-value new 'theta) (atan y x))))
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;;; At this point an instance of the class x-y-position can be
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;;; changed to be an instance of the class rho-theta-position using
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;;; change-class:
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(setq p1 (make-instance 'x-y-position :x 2 :y 0))
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(change-class p1 'rho-theta-position)
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;;; The result is that the instance bound to p1 is now an instance of
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;;; the class rho-theta-position. The update-instance-for-different-class
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;;; method performed the initialization of the rho and theta slots based
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;;; on the value of the x and y slots, which were maintained by
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;;; the old instance.
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</PRE>
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</TT> <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> None.
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<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_update.htm#update-instance-for-different-class"><B>update-instance-for-different-class</B></A>, <A REL=CHILD HREF="07_b.htm">Section 7.2 (Changing the Class of an Instance)</A> <P>
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<P><B>Notes:</B><P>
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<P>
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The generic function <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> has several semantic difficulties. First, it performs a destructive operation that can be invoked within a <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> on an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> that was used to select that <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A>. When multiple <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> are involved because <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> are being combined, the <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> currently executing or about to be executed may no longer be applicable. Second, some implementations might use compiler optimizations of slot <A REL=DEFINITION HREF="26_glo_a.htm#access"><I>access</I></A>, and when the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> of an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> is changed the assumptions the compiler made might be violated. This implies that a programmer must not use <A REL=DEFINITION HREF="#change-class"><B>change-class</B></A> inside a <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> if any <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>methods</I></A> for that <A REL=DEFINITION HREF="26_glo_g.htm#generic_function"><I>generic function</I></A> <A REL=DEFINITION HREF="26_glo_a.htm#access"><I>access</I></A> any <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A>, or the results are undefined. <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/iss023.htm">CHANGE-CLASS-INITARGS:PERMIT</A><P></UL><HR>
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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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