106 lines
4.8 KiB
HTML
106 lines
4.8 KiB
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<html lang="en">
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<title>Basic Implementation - SBCL Internals</title>
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Slot_002dValue.html#Slot_002dValue" title="Slot-Value">
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<link rel="next" href="Compiler-Transformations.html#Compiler-Transformations" title="Compiler Transformations">
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This manual is part of the SBCL software system. See the `README'
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file for more information.
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This manual is in the public domain and is provided with
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absolutely no warranty. See the `COPYING' and `CREDITS' files for
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<body>
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<div class="node">
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<p>
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<a name="Basic-Implementation"></a>
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Next: <a rel="next" accesskey="n" href="Compiler-Transformations.html#Compiler-Transformations">Compiler Transformations</a>,
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Up: <a rel="up" accesskey="u" href="Slot_002dValue.html#Slot_002dValue">Slot-Value</a>
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<hr>
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</div>
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<!-- node-name, next, previous, up -->
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<h3 class="section">7.1 Basic Implementation</h3>
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<p>All of the following, while described in terms of <code>slot-value</code>,
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also applies to <code>(setf slot-value)</code> and to <code>slot-boundp</code>, and
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could in principle be extended to <code>slot-makunbound</code>.
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<p>The basic implementation of <code>slot-value</code>, following the suggestion
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in the standards document, is shown in <a href="ex_003aslot_002dvalue.html#ex_003aslot_002dvalue">ex:slot-value</a>; the
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implementation of the other slot operators is similar. The work to be
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done simply to arrive at the generic function call is already
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substantial: we need to look up the object's class and iterate over the
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class' slots to find a slot of the right name, only then are we in a
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position to call the generic function which implements the slot access
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directly.
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<div class="float">
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<a name="ex_003aslot_002dvalue"></a>
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<pre class="example"> (defun slot-value (object slot-name)
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(let* ((class (class-of object))
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(slot-definition (find-slot-definition class slot-name)))
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(if (null slot-definition)
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(values (slot-missing class object slot-name 'slot-value))
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(slot-value-using-class class object slot-definition))))
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</pre>
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<p><strong class="float-caption">Example 7.1</strong></p></div>
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<p>The basic implementation of <code>slot-value-using-class</code> specialized on
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the standard metaobject classes is shown in
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<a href="ex_003aslot_002dvalue_002dusing_002dclass.html#ex_003aslot_002dvalue_002dusing_002dclass">ex:slot-value-using-class</a>. First, we check for an obsolete
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instance (that is, one whose class has been redefined since the object
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was last accessed; if it has, the object must be updated by
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<code>update-instance-for-redefined-class</code>); then, we acquire the slot's
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storage location from the slot definition, the value from the instance's
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slot vector, and then after checking the value against the internal unbound
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marker, we return it.
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<div class="float">
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<a name="ex_003aslot_002dvalue_002dusing_002dclass"></a>
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<pre class="example"> (defmethod slot-value-using-class
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((class std-class)
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(object standard-object)
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(slotd standard-effective-slot-definition))
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(check-obsolete-instance object)
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(let* ((location (slot-definition-location slotd))
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(value
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(etypecase location
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(fixnum (clos-slots-ref (instance-slots object) location))
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(cons (cdr location)))))
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(if (eq value +slot-unbound+)
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(values (slot-unbound class object (slot-definition-name slotd)))
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value)))
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</pre>
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<p><strong class="float-caption">Example 7.2</strong></p></div>
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<p>Clearly, all of this activity will cause the performance of clos slot
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access to compare poorly with structure slot access; while there will be
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of necessity a slowdown between the slot accesses because the structure
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class need not be redefineable (while redefinition of standard-object
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classes is extremely common), the overhead presented in the above
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implementation is excessive.
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