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<TITLE>CLHS: Section 4.3.6</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="04_ceb.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Previous]" SRC="../Graphics/Prev.gif" ALIGN=Bottom></A><A REL=UP HREF="04_c.htm"><IMG WIDTH=40 HEIGHT=40 ALT="[Up]" SRC="../Graphics/Up.gif" ALIGN=Bottom></A><A REL=NEXT HREF="04_cfa.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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4.3.6 Redefining Classes</H2> <P>
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A <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> that is a <A REL=DEFINITION HREF="26_glo_d.htm#direct_instance"><I>direct instance</I></A> of <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A> can be redefined if the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is also a <A REL=DEFINITION HREF="26_glo_d.htm#direct_instance"><I>direct instance</I></A> of <A REL=DEFINITION HREF="t_std_cl.htm#standard-class"><B>standard-class</B></A>. Redefining a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> modifies the existing <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> to reflect the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> definition; it does not create a new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> for the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. Any <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>method</I></A> <A REL=DEFINITION HREF="26_glo_o.htm#object"><I>object</I></A> created by a <TT>:reader</TT>, <TT>:writer</TT>, or <TT>:accessor</TT> option specified by the old <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> form is removed from the corresponding <A REL=DEFINITION HREF="26_glo_g.htm#generic_function"><I>generic function</I></A>. <A REL=DEFINITION HREF="26_glo_m.htm#method"><I>Methods</I></A> specified by the new <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> form are added. <P>
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When the class C is redefined, changes are propagated to its <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> and to <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of any of its <A REL=DEFINITION HREF="26_glo_s.htm#subclass"><I>subclasses</I></A>. Updating such an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> occurs at an <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A> time, but no later than the next time a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> of that <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> is read or written. Updating an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> does not change its identity as defined by the <A REL=DEFINITION HREF="26_glo_f.htm#function"><I>function</I></A> <A REL=DEFINITION HREF="f_eq.htm#eq"><B>eq</B></A>. The updating process may change the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> of that particular <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>, but it does not create a new <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A>. Whether updating an <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> consumes storage is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A>. <P>
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Note that redefining a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> may cause <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> to be added or deleted. If a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is redefined in a way that changes the set of <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> in <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A>, the <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> are updated. It is <A REL=DEFINITION HREF="26_glo_i.htm#implementation-dependent"><I>implementation-dependent</I></A> whether <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> are updated if a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is redefined in a way that does not change the set of <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> <A REL=DEFINITION HREF="26_glo_a.htm#accessible"><I>accessible</I></A> in <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A>. <P>
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The value of a <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> that is specified as shared both in the old <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> and in the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is retained. If such a <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> was unbound in the old <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>, it is unbound in the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>Slots</I></A> that were local in the old <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> and that are shared in the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> are initialized. Newly added <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slots</I></A> are initialized. <P>
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Each newly added <A REL=DEFINITION HREF="26_glo_s.htm#shared_slot"><I>shared slot</I></A> is set to the result of evaluating the <A REL=DEFINITION HREF="26_glo_c.htm#captured_initialization_form"><I>captured initialization form</I></A> for the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> that was specified in the <A REL=DEFINITION HREF="m_defcla.htm#defclass"><B>defclass</B></A> <A REL=DEFINITION HREF="26_glo_f.htm#form"><I>form</I></A> for the new <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. If there was no <A REL=DEFINITION HREF="26_glo_i.htm#initialization_form"><I>initialization form</I></A>, the <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slot</I></A> is unbound. <P>
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If a <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is redefined in such a way that the set of <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> <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 the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> is changed, a two-step process of updating the <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instances</I></A> of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A> takes place. The process may be explicitly started by invoking the generic function <A REL=DEFINITION HREF="f_mk_i_1.htm#make-instances-obsolete"><B>make-instances-obsolete</B></A>. This two-step process can happen in other circumstances in some implementations. For example, in some implementations this two-step process is triggered if the order of <A REL=DEFINITION HREF="26_glo_s.htm#slot"><I>slots</I></A> in storage is changed. <P>
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The first step modifies the structure of the <A REL=DEFINITION HREF="26_glo_i.htm#instance"><I>instance</I></A> by adding new <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> and discarding <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> that are not defined in the new version of the <A REL=DEFINITION HREF="26_glo_c.htm#class"><I>class</I></A>. The second step initializes the newly-added <A REL=DEFINITION HREF="26_glo_l.htm#local_slot"><I>local slots</I></A> and performs any other user-defined actions. These two steps are further specified in the next two sections. <P>
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<A REL=CHILD HREF="04_cfa.htm"><H2>
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4.3.6.1 Modifying the Structure of Instances</H2></A><P><P>
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<A REL=CHILD HREF="04_cfb.htm"><H2>
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4.3.6.2 Initializing Newly Added Local Slots</H2></A><P>
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<A REL=CHILD HREF="04_cfc.htm"><H2>
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4.3.6.3 Customizing Class Redefinition</H2></A><P><P>
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