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<a name="Basic-Implementation"></a>
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Next: <a href="Compiler-Transformations.html#Compiler-Transformations" accesskey="n" rel="next">Compiler Transformations</a>, Up: <a href="Slot_002dValue.html#Slot_002dValue" accesskey="u" rel="up">Slot-Value</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>]</p>
</div>
<hr>
<a name="Basic-Implementation-1"></a>
<h3 class="section">8.1 Basic Implementation</h3>

<p>All of the following, while described in terms of <code>slot-value</code>,
also applies to <code>(setf slot-value)</code> and to <code>slot-boundp</code>, and
could in principle be extended to <code>slot-makunbound</code>.
</p>
<p>The basic implementation of <code>slot-value</code>, following the suggestion
in the standards document, is shown in <a href="#ex_003aslot_002dvalue">Example 8.1</a>; the
implementation of the other slot operators is similar.  The work to be
done simply to arrive at the generic function call is already
substantial: we need to look up the object&rsquo;s class and iterate over the
class&rsquo; slots to find a slot of the right name, only then are we in a
position to call the generic function which implements the slot access
directly.
</p>
<div class="float"><a name="ex_003aslot_002dvalue"></a>
<div class="example">
<pre class="example">(defun slot-value (object slot-name)
  (let* ((class (class-of object))
         (slot-definition (find-slot-definition class slot-name)))
    (if (null slot-definition)
        (values (slot-missing class object slot-name 'slot-value))
        (slot-value-using-class class object slot-definition))))
</pre></div>
<div class="float-caption"><p><strong>Example 8.1
</strong></p></div></div>
<p>The basic implementation of <code>slot-value-using-class</code> specialized on
the standard metaobject classes is shown in
<a href="#ex_003aslot_002dvalue_002dusing_002dclass">Example 8.2</a>.  First, we check for an obsolete
instance (that is, one whose class has been redefined since the object
was last accessed; if it has, the object must be updated by
<code>update-instance-for-redefined-class</code>); then, we acquire the slot&rsquo;s
storage location from the slot definition, the value from the instance&rsquo;s
slot vector, and then after checking the value against the internal unbound
marker, we return it.
</p>
<div class="float"><a name="ex_003aslot_002dvalue_002dusing_002dclass"></a>
<div class="example">
<pre class="example">(defmethod slot-value-using-class
    ((class std-class)
     (object standard-object)
     (slotd standard-effective-slot-definition))
  (check-obsolete-instance object)
  (let* ((location (slot-definition-location slotd))
         (value
          (etypecase location
            (fixnum (clos-slots-ref (instance-slots object) location))
            (cons (cdr location)))))
    (if (eq value +slot-unbound+)
        (values (slot-unbound class object (slot-definition-name slotd)))
        value)))
</pre></div>
<div class="float-caption"><p><strong>Example 8.2
</strong></p></div></div>
<p>Clearly, all of this activity will cause the performance of clos slot
access to compare poorly with structure slot access; while there will be
of necessity a slowdown between the slot accesses because the structure
class need not be redefineable (while redefinition of standard-object
classes is extremely common), the overhead presented in the above
implementation is excessive.
</p>
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