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137 lines
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<TITLE>28.1.7.2. Standard Method Combination</TITLE>
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<meta name="description" value=" Standard Method Combination">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5221 HREF="node286.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5219 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5213 HREF="node284.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5223 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5224 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5222 HREF="node286.html"> Declarative Method Combination</A>
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<B>Up:</B> <A NAME=tex2html5220 HREF="node283.html"> Method Selection and </A>
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<B> Previous:</B> <A NAME=tex2html5214 HREF="node284.html"> Determining the Effective </A>
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<HR> <P>
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<H3><A NAME=SECTION003217200000000000000>28.1.7.2. Standard Method Combination</A></H3>
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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<A NAME=StandardMethodCombinationSECTION>Standard</A>
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method combination is supported by the class
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<tt>standard-generic-function</tt>.
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It is used if no other type of method
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combination is specified or if the built-in method combination type
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<tt>standard</tt> is specified.
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<P>
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<i>Primary methods</i> define the main action of the effective method,
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while <i>auxiliary methods</i> modify that action in one of three ways.
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A primary method has no method qualifiers.
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<P>
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An auxiliary method is a method whose method qualifier is
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<tt>:before</tt>, <tt>:after</tt>, or <tt>:around</tt>. Standard method combination
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allows no more than one qualifier per method; if a method definition
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specifies more than one qualifier per method, an error is signaled.
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<P>
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<UL><LI>
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A <tt>:before</tt> method has the keyword <tt>:before</tt> as its
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only qualifier. A <tt>:before</tt> method specifies code that is to be
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run before any primary method.
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<P>
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<LI>
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An <tt>:after</tt> method has the keyword <tt>:after</tt> as its only
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qualifier. An <tt>:after</tt> method specifies code that is to be run
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after primary methods.
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<P>
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<LI>
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An <tt>:around</tt> method has the keyword <tt>:around</tt> as its only
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qualifier. An <tt>:around</tt> method specifies code that is to
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be run instead of other applicable methods but that is
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able to cause some of them to be run.
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<P>
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</UL>
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The semantics of standard method combination are as follows:
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<P>
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<UL><LI> If there are any <tt>:around</tt> methods, the most specific
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<tt>:around</tt> method is called. It supplies the value or values of the
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generic function.
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<P>
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<LI> Inside the body of an <tt>:around</tt> method,
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<tt>call-next-method</tt> can be used to call the next method. When the next
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method returns, the <tt>:around</tt> method can execute more code,
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perhaps based on the returned value or values. The generic function
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<tt>no-next-method</tt> is invoked if <tt>call-next-method</tt> is used and
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there is no applicable method to call. The function
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<tt>next-method-p</tt> may be used to determine whether a next method exists.
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<P>
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<LI>
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If an <tt>:around</tt> method invokes <tt>call-next-method</tt>, the next
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most specific <tt>:around</tt> method is called, if one is applicable.
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If there are no <tt>:around</tt> methods or if
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<tt>call-next-method</tt> is called by the least specific <tt>:around</tt>
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method, the other methods are called as follows:
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<P>
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<UL><LI> All the <tt>:before</tt> methods are called, in
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most-specific-first order. Their values are ignored.
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An error is signaled if <tt>call-next-method</tt> is used in a
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<tt>:before</tt> method.
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<P>
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<LI> The most specific primary method is called. Inside the
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body of a primary method, <tt>call-next-method</tt> may be used to call
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the next most specific primary method. When that method returns, the
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previous primary method can execute more code, perhaps based on the
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returned value or values. The generic function <tt>no-next-method</tt>
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is invoked if <tt>call-next-method</tt> is used and there are no more
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applicable primary methods. The function <tt>next-method-p</tt> may be
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used to determine whether a next method exists. If
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<tt>call-next-method</tt> is not used, only the most specific primary method
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is called.
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<P>
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<LI> All the <tt>:after</tt> methods are called in
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most-specific-last order. Their values are ignored.
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An error is signaled if <tt>call-next-method</tt> is used in an
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<tt>:after</tt> method.
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</UL>
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<P>
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<LI> If no <tt>:around</tt> methods were invoked, the most
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specific primary method supplies the value or values returned by the
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generic function. The value or values returned by the invocation of
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<tt>call-next-method</tt> in the least specific <tt>:around</tt> method are
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those returned by the most specific primary method.
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<P>
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</UL>
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<P>
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In standard method combination, if there is an applicable method
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but no applicable primary method, an error is signaled.
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<P>
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The <tt>:before</tt> methods are run in most-specific-first order and
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the <tt>:after</tt> methods are run in least-specific-first order. The
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design rationale for this difference can be illustrated with an
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example. Suppose class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif"> modifies the behavior of its
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superclass, <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif">, by adding <tt>:before</tt> and <tt>:after</tt>
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methods. Whether the behavior of the class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> is defined
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directly by methods on <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif"> or is inherited from its superclasses
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does not affect the relative order of invocation of methods on
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instances of the class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif">. Class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif">'s <tt>:before</tt>
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method runs before all of class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif">'s methods. Class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43387.gif">'s <tt>:after</tt> method runs after all of class <IMG ALIGN=BOTTOM ALT="" SRC="_24769_tex2html_wrap43591.gif">'s methods.
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<P>
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By contrast, all <tt>:around</tt> methods run before any other methods
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run. Thus a less specific <tt>:around</tt> method runs before a more
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specific primary method.
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<P>
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If only primary methods are used and if <tt>call-next-method</tt> is not
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used, only the most specific method is invoked; that is, more specific
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methods shadow more general ones.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR> <HR><A NAME=tex2html5221 HREF="node286.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5219 HREF="node283.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5213 HREF="node284.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5223 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5224 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5222 HREF="node286.html"> Declarative Method Combination</A>
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<B>Up:</B> <A NAME=tex2html5220 HREF="node283.html"> Method Selection and </A>
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<B> Previous:</B> <A NAME=tex2html5214 HREF="node284.html"> Determining the Effective </A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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