487 lines
11 KiB
HTML
487 lines
11 KiB
HTML
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>change-class | Common Lisp Nova Spec</TITLE
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>Common Lisp Nova Spec</A
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> <SPAN CLASS="breadcrumb-item"
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> <SPAN CLASS="breadcrumb-item"
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>→ <A HREF="f_change-class.html"
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>change-class</A
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><TABLE WIDTH="100%" CELLSPACING="0" CELLPADDING="0"
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><TD ALIGN="LEFT" VALIGN="BASELINE" WIDTH="100%" CLASS="name"
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><SPAN CLASS="idx" DATA-KIND="idxref" DATA-TERM="change-class"
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></SPAN
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><B
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>change-class</B
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></TD
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><TD ALIGN="RIGHT" VALIGN="BASELINE" WIDTH="0" NOWRAP="NOWRAP" CLASS="ftype"
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><I
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>Standard</I
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> <I
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>Generic</I
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> <I
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>Function</I
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></TD
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></TR
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></TABLE
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><UL CLASS="subtoc"
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></UL
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><DL
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><DT
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><B
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>Syntax</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>change-class</B
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> <SPAN CLASS="cmssi"
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>instance</SPAN
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> <SPAN CLASS="cmssi"
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>new-class</SPAN
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> <SPAN CLASS="cmtt"
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>&key</SPAN
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> <SPAN CLASS="cmtt"
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>&allow-other-keys</SPAN
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> <SPAN CLASS="arrow"
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>→</SPAN
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> <SPAN CLASS="cmssi"
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>instance</SPAN
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></P
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></DD
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><DT
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><B
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>Method Signatures</B
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></DT
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><DD
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><P CLASS="j"
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><B
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>change-class</B
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> (<VAR CLASS="param"
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>instance</VAR
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> <A HREF="t_standard-object.html" CLASS="typeref"
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><B
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>standard-object</B
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></A
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>) (<VAR CLASS="param"
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>new-class</VAR
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> <A HREF="t_standard-class.html" CLASS="typeref"
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><B
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>standard-class</B
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></A
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>) <SPAN CLASS="cmtt"
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>&rest</SPAN
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> <SPAN CLASS="cmssi"
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>initargs</SPAN
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></P
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><P CLASS="j"
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><B
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>change-class</B
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> (<VAR CLASS="param"
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>instance</VAR
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> <A HREF="t_t.html" CLASS="typeref"
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><B
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>t</B
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></A
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>) (<VAR CLASS="param"
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>new-class</VAR
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> <A HREF="t_symbol.html" CLASS="typeref"
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><B
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>symbol</B
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></A
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>) <SPAN CLASS="cmtt"
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>&rest</SPAN
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> <SPAN CLASS="cmssi"
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>initargs</SPAN
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></P
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></DD
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><DT
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><B
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>Arguments and Values</B
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></DT
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><DD
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><P CLASS="j"
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><VAR CLASS="param"
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>instance</VAR
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> — an <A HREF="26_1_Glossary.html#object"
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><EM CLASS="term"
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>object</EM
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></A
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>new-class</VAR
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> — a <A HREF="26_1_Glossary.html#class_designator"
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><EM CLASS="term"
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>class designator</EM
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></A
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>. </P
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><P CLASS="j"
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><VAR CLASS="param"
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>initargs</VAR
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> — an <A HREF="26_1_Glossary.html#initialization_argument_list"
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><EM CLASS="term"
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>initialization argument list</EM
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></A
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>.</P
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></DD
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><DT
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><B
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>Description</B
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></DT
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><DD
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><P CLASS="j"
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>The <A HREF="26_1_Glossary.html#generic_function"
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><EM CLASS="term"
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>generic function</EM
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></A
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> <A HREF="f_change-class.html" CLASS="funref"
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><B
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>change-class</B
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></A
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> changes the <A HREF="26_1_Glossary.html#class"
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><EM CLASS="term"
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>class</EM
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></A
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> of an <VAR CLASS="param"
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>instance</VAR
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> to <VAR CLASS="param"
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>new-class</VAR
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>. It destructively modifies and returns the <VAR CLASS="param"
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>instance</VAR
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>. </P
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><P CLASS="j"
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>If in the old <A HREF="26_1_Glossary.html#class"
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><EM CLASS="term"
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>class</EM
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></A
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> there is any <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slot</EM
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></A
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> of the same name as a local <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slot</EM
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></A
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> in the <VAR CLASS="param"
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>new-class</VAR
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>, the value of that <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slot</EM
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></A
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> is retained. This means that if the <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slot</EM
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></A
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> has a value, the value returned by <A HREF="f_slot-value.html" CLASS="funref"
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><B
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>slot-value</B
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></A
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> after <A HREF="f_change-class.html" CLASS="funref"
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><B
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>change-class</B
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></A
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> is invoked is <A HREF="f_eql.html" CLASS="funref"
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><B
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>eql</B
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></A
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> to the value returned by <A HREF="f_slot-value.html" CLASS="funref"
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><B
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>slot-value</B
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></A
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> before <A HREF="f_change-class.html" CLASS="funref"
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><B
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>change-class</B
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></A
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> is invoked. Similarly, if the <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slot</EM
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></A
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> was unbound, it remains unbound. The other <A HREF="26_1_Glossary.html#slot"
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><EM CLASS="term"
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>slots</EM
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></A
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> are initialized as described in <A HREF="7_2_Changing_the_Class_of_an_Instance.html#sec_7_2" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>7.2</SPAN
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> <SPAN CLASS="cmr"
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>(Changing</SPAN
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> <SPAN CLASS="cmr"
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>the</SPAN
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> <SPAN CLASS="cmr"
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>Class</SPAN
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> <SPAN CLASS="cmr"
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>of</SPAN
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> <SPAN CLASS="cmr"
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>an</SPAN
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> <SPAN CLASS="cmr"
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>Instance)</SPAN
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></A
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>. </P
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><P CLASS="j"
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>After completing all other actions, <A HREF="f_change-class.html" CLASS="funref"
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><B
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>change-class</B
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></A
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> invokes <A HREF="f_update-instance-for-different-class.html" CLASS="funref"
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><B
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>update-instance-for-different-class</B
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></A
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>. The generic function <A HREF="f_update-instance-for-different-class.html" CLASS="funref"
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><B
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>update-instance-for-different-class</B
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></A
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> can be used to assign values to slots in the transformed instance. See <A HREF="7_2_Changing_the_Class_of_an_Instance.html#sec_7_2_2" CLASS="secref"
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><SPAN CLASS="cmr"
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>Section</SPAN
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> <SPAN CLASS="cmr"
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>7.2.2</SPAN
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> <SPAN CLASS="cmr"
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>(Initializing</SPAN
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> <SPAN CLASS="cmr"
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>Newly</SPAN
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> <SPAN CLASS="cmr"
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>Added</SPAN
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> <SPAN CLASS="cmr"
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>Local</SPAN
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> <SPAN CLASS="cmr"
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>Slots)</SPAN
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></A
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>. </P
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><P CLASS="j"
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>If the second of the above <A HREF="26_1_Glossary.html#method"
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><EM CLASS="term"
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>methods</EM
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></A
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> is selected, that <A HREF="26_1_Glossary.html#method"
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><EM CLASS="term"
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>method</EM
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></A
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> invokes <A HREF="f_change-class.html" CLASS="funref"
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><B
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>change-class</B
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></A
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> on <VAR CLASS="param"
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>instance</VAR
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>, <CODE CLASS="f"
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>(find-class <VAR CLASS="param"
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>new-class</VAR
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>)</CODE
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>, and the <VAR CLASS="param"
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>initargs</VAR
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>.</P
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></DD
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><DT
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><B
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>Examples</B
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></DT
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><DD
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><PRE CLASS="screen"
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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.</PRE
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></DD
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><DT
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><B
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>See Also</B
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></DT
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||
|
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><DD
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||
|
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><P CLASS="j"
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||
|
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><A HREF="f_update-instance-for-different-class.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>update-instance-for-different-class</B
|
||
|
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></A
|
||
|
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>, <A HREF="7_2_Changing_the_Class_of_an_Instance.html#sec_7_2" CLASS="secref"
|
||
|
|
><SPAN CLASS="cmr"
|
||
|
|
>Section</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>7.2</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>(Changing</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>the</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>Class</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>of</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>an</SPAN
|
||
|
|
> <SPAN CLASS="cmr"
|
||
|
|
>Instance)</SPAN
|
||
|
|
></A
|
||
|
|
></P
|
||
|
|
></DD
|
||
|
|
><DT
|
||
|
|
><B
|
||
|
|
>Notes</B
|
||
|
|
></DT
|
||
|
|
><DD
|
||
|
|
><P CLASS="j"
|
||
|
|
>The generic function <A HREF="f_change-class.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>change-class</B
|
||
|
|
></A
|
||
|
|
> has several semantic difficulties. First, it performs a destructive operation that can be invoked within a <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>method</EM
|
||
|
|
></A
|
||
|
|
> on an <A HREF="26_1_Glossary.html#instance"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>instance</EM
|
||
|
|
></A
|
||
|
|
> that was used to select that <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>method</EM
|
||
|
|
></A
|
||
|
|
>. When multiple <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>methods</EM
|
||
|
|
></A
|
||
|
|
> are involved because <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>methods</EM
|
||
|
|
></A
|
||
|
|
> are being combined, the <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>methods</EM
|
||
|
|
></A
|
||
|
|
> currently executing or about to be executed may no longer be applicable. Second, some implementations might use compiler optimizations of slot <A HREF="26_1_Glossary.html#access"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>access</EM
|
||
|
|
></A
|
||
|
|
>, and when the <A HREF="26_1_Glossary.html#class"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>class</EM
|
||
|
|
></A
|
||
|
|
> of an <A HREF="26_1_Glossary.html#instance"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>instance</EM
|
||
|
|
></A
|
||
|
|
> is changed the assumptions the compiler made might be violated. This implies that a programmer must not use <A HREF="f_change-class.html" CLASS="funref"
|
||
|
|
><B
|
||
|
|
>change-class</B
|
||
|
|
></A
|
||
|
|
> inside a <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>method</EM
|
||
|
|
></A
|
||
|
|
> if any <A HREF="26_1_Glossary.html#method"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>methods</EM
|
||
|
|
></A
|
||
|
|
> for that <A HREF="26_1_Glossary.html#generic_function"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>generic function</EM
|
||
|
|
></A
|
||
|
|
> <A HREF="26_1_Glossary.html#access"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>access</EM
|
||
|
|
></A
|
||
|
|
> any <A HREF="26_1_Glossary.html#slot"
|
||
|
|
><EM CLASS="term"
|
||
|
|
>slots</EM
|
||
|
|
></A
|
||
|
|
>, or the results are undefined.</P
|
||
|
|
></DD
|
||
|
|
></DL
|
||
|
|
></DIV
|
||
|
|
></DIV
|
||
|
|
><DIV CLASS="footer"
|
||
|
|
><DIV CLASS="btmnav"
|
||
|
|
><A HREF="f_update-instance-for-redefined-class.html" CLASS="prev"
|
||
|
|
>←</A
|
||
|
|
><A HREF="f_slot-boundp.html" CLASS="next"
|
||
|
|
>→</A
|
||
|
|
></DIV
|
||
|
|
><DIV CLASS="trail"
|
||
|
|
>Conversion to HTML copyright 2023 by Gilbert Baumann</DIV
|
||
|
|
></DIV
|
||
|
|
></DIV
|
||
|
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|
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|
>domReady();</SCRIPT
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