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105 lines
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<TITLE>28.1.9. Object Creation and Initialization</TITLE>
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<meta name="description" value=" Object Creation and Initialization">
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html5304 HREF="node293.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5302 HREF="node261.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5296 HREF="node291.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5306 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5307 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5305 HREF="node293.html"> Initialization Arguments</A>
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<B>Up:</B> <A NAME=tex2html5303 HREF="node261.html"> Programmer Interface Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5297 HREF="node291.html"> Standard Meta-objects</A>
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<HR> <P>
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<H2><A NAME=SECTION003219000000000000000>28.1.9. Object Creation and Initialization</A></H2>
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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=ObjectCreationandInitializationSECTION>The</A>
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generic function <tt>make-instance</tt> creates and returns a new
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instance of a class. The first argument is a class or the name of a
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class, and the remaining arguments form an <i>initialization argument</i>
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list.
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<P>
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The initialization of a new instance consists of several distinct
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steps, including the following: combining the explicitly supplied
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initialization arguments with default values for the unsupplied
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initialization arguments, checking the validity of the initialization
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arguments, allocating storage for the instance, filling slots with
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values, and executing user-supplied methods that perform additional
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initialization. Each step of <tt>make-instance</tt> is implemented by a
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generic function to provide a mechanism for customizing that step. In
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addition, <tt>make-instance</tt> is itself a generic function and thus
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also can be customized.
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<P>
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The Object System specifies system-supplied primary methods for each step and
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thus specifies a well-defined standard behavior for the entire
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initialization process. The standard behavior provides four simple
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mechanisms for controlling initialization:
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<P>
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<UL><LI> Declaring a symbol to be an initialization argument for a
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slot. An initialization argument is declared by using the
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<tt>:initarg</tt> slot option to <tt>defclass</tt>. This provides a mechanism
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for supplying a value for a slot in a call to <tt>make-instance</tt>.
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<P>
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<LI> Supplying a default value form for an initialization
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argument. Default value forms for initialization arguments are
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defined by using the <tt>:default-initargs</tt> class option to
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<tt>defclass</tt>. If an initialization argument is not explicitly provided
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as an argument to <tt>make-instance</tt>, the default value form is
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evaluated in the lexical environment of the <tt>defclass</tt> form that
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defined it, and the resulting value is used as the value of the
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initialization argument.
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<P>
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<LI> Supplying a default initial value form for a slot. A
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default initial value form for a slot is defined by using the
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<tt>:initform</tt> slot option to <tt>defclass</tt>. If no initialization
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argument associated with that slot is given as an argument to
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<tt>make-instance</tt> or is defaulted by <tt>:default-initargs</tt>, this
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default initial value form is evaluated in the lexical environment of
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the <tt>defclass</tt> form that defined it, and the resulting value is
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stored in the slot. The <tt>:initform</tt> form for a local slot may be
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used when creating an instance, when updating an instance to conform
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to a redefined class, or when updating an instance to conform to the
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definition of a different class. The <tt>:initform</tt> form for a shared
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slot may be used when defining or re-defining the class.
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<P>
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<LI> Defining methods for <tt>initialize-instance</tt> and
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<tt>shared-initialize</tt>. The slot-filling behavior described above is
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implemented by a system-supplied primary method for
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<tt>initialize-instance</tt> which invokes <tt>shared-initialize</tt>. The
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generic function <tt>shared-initialize</tt> implements the parts of
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initialization shared by these four situations: when making an
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instance, when re-initializing an instance, when updating an instance
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to conform to a redefined class, and when updating an instance to
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conform to the definition of a different class. The system-supplied
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primary method for <tt>shared-initialize</tt> directly implements the
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slot-filling behavior described above, and <tt>initialize-instance</tt>
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simply invokes <tt>shared-initialize</tt>.
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<P>
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</UL>
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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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<HR>
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<UL>
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<LI> <A NAME=tex2html5308 HREF="node293.html#SECTION003219100000000000000"> Initialization Arguments</A>
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<LI> <A NAME=tex2html5309 HREF="node294.html#SECTION003219200000000000000"> Declaring the Validity of Initialization Arguments</A>
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<LI> <A NAME=tex2html5310 HREF="node295.html#SECTION003219300000000000000"> Defaulting of Initialization Arguments</A>
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<LI> <A NAME=tex2html5311 HREF="node296.html#SECTION003219400000000000000"> Rules for Initialization Arguments</A>
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<LI> <A NAME=tex2html5312 HREF="node297.html#SECTION003219500000000000000"> Shared-Initialize</A>
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<LI> <A NAME=tex2html5313 HREF="node298.html#SECTION003219600000000000000"> Initialize-Instance</A>
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<LI> <A NAME=tex2html5314 HREF="node299.html#SECTION003219700000000000000"> Definitions of Make-Instance and Initialize-Instance</A>
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</UL>
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<BR> <HR><A NAME=tex2html5304 HREF="node293.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html5302 HREF="node261.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html5296 HREF="node291.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html5306 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html5307 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html5305 HREF="node293.html"> Initialization Arguments</A>
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<B>Up:</B> <A NAME=tex2html5303 HREF="node261.html"> Programmer Interface Concepts</A>
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<B> Previous:</B> <A NAME=tex2html5297 HREF="node291.html"> Standard Meta-objects</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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