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<TITLE>Brief Guide to CLOS: Instances</TITLE>
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<H2><A NAME="s3">3. Instances</A></H2>
<P>You can make an instance of a class with <CODE>MAKE-INSTANCE</CODE>. It's
similar to the <CODE>MAKE-x</CODE> functions defined by <CODE>DEFSTRUCT</CODE> but lets you
pass the class to instantiate as an argument:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(MAKE-INSTANCE class {initarg value}*)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Instead of the class object itself, you can use its name.
For example:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(make-instance 'person :age 100)
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>This person object would have age <CODE>100</CODE> and name <CODE>BILL</CODE>, the default.</P>
<P>It's often a good idea to define your own constructor functions,
rather than call <CODE>MAKE-INSTANCE</CODE> directly, because you can hide
implementation details and don't have to use keyword parameters
for everything. For instance, you might want to define</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
(defun make-person (name age)
(make-instance 'person :name name :age age))
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>if you wanted the name and age to be required, positional parameters,
rather than keyword parameters.</P>
<P>The accessor functions can be used to get and set slot values:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
<cl> (setq p1 (make-instance 'person :name 'jill :age 100))
#<person @ #x7bf826>
<cl> (person-name p1)
jill
<cl> (person-age p1)
100
<cl> (setf (person-age p1) 101)
101
<cl> (person-age p1)
101
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>Note that when you use <CODE>DEFCLASS</CODE>, the instances are printed using
the <CODE>#<...></CODE> notation, rather than
as <CODE>#s(person :name jill :age 100)</CODE> .
But you can change the way instances are printed by defining methods
on the generic function <CODE>PRINT-OBJECT</CODE>.</P>
<P>Slots can also be accessed by name using <CODE>(SLOT-VALUE instance slot-name)</CODE>:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
<cl> (slot-value p1 'name)
jill
<cl> (setf (slot-value p1 'name) 'jillian)
jillian
<cl> (person-name p1)
jillian
</PRE>
</CODE></BLOCKQUOTE>
</P>
<P>You can find out various things about an instance by calling
<CODE>DESCRIBE</CODE>:</P>
<P>
<BLOCKQUOTE><CODE>
<PRE>
<cl> (describe p1)
#<person @ #x7bf826> is an instance
of class #<clos:standard-class person @ #x7ad8ae>:
The following slots have :INSTANCE allocation:
age 101
name jillian
</PRE>
</CODE></BLOCKQUOTE>
</P>
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