84 lines
4.2 KiB
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84 lines
4.2 KiB
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<title>Accessor Discriminating Functions - SBCL Internals</title>
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<link rel="up" href="Discriminating-Functions.html#Discriminating-Functions" title="Discriminating Functions">
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<link rel="prev" href="Method_002dBased-Discriminating-Functions.html#Method_002dBased-Discriminating-Functions" title="Method-Based Discriminating Functions">
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<a name="Accessor-Discriminating-Functions"></a>
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Next: <a rel="next" accesskey="n" href="Cacheing-and-Dispatch-Functions.html#Cacheing-and-Dispatch-Functions">Cacheing and Dispatch Functions</a>,
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Previous: <a rel="previous" accesskey="p" href="Method_002dBased-Discriminating-Functions.html#Method_002dBased-Discriminating-Functions">Method-Based Discriminating Functions</a>,
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Up: <a rel="up" accesskey="u" href="Discriminating-Functions.html#Discriminating-Functions">Discriminating Functions</a>
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<!-- node-name, next, previous, up -->
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<h3 class="section">3.3 Accessor Discriminating Functions</h3>
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<p>Accessor Discriminating Functions are used when the effective method of
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all calls is an access to a slot, either reading, writing or checking
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boundness<a rel="footnote" href="Accessor-Discriminating-Functions.html#fn-1" name="fnd-1"><sup>1</sup></a>; for this path to apply, there must be no non-standard
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methods on <code>SB-MOP:SLOT-VALUE-USING-CLASS</code> and its siblings. The
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first state is <code>SB-PCL::ONE-CLASS</code>, entered when one class of
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instance has been accessed; the discriminating function here closes over
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the wrapper of the class and the slot index, and accesses the slot of
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the instance directly.
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<p>If a direct instance of another class is passed to the generic function
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for slot access, then another accessor discriminating function is
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created: if the index of the slot in the slots vector of each instance
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is the same, then a <code>SB-PCL::TWO-CLASS</code> function is created,
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closing over the two class wrappers and the index and performing the
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simple dispatch. If the slot indexes are not the same, then we go to
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the <code>SB-PCL::N-N</code> state.
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<p>For slot accesses for more than two classes with the same index, we move
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to the <code>SB-PCL::ONE-INDEX</code> state which maintains a cache of
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wrappers for which the slot index is the same. If at any point the slot
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index for an instance is not the same, the state moves to
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<code>SB-PCL::N-N</code>, which maintains a cache of wrappers and their
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associated indexes; if at any point an effective method which is not a
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simple slot access is encountered, then the discriminating function
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moves into the <code>SB-PCL::CHECKING</code>, <code>SB-PCL::CACHING</code> or
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<code>SB-PCL::DISPATCH</code> states.
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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="Accessor-Discriminating-Functions.html#fnd-1">1</a>]</small> Although there is ordinarily no way for a user to
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define a boundp method, some automatically generated generic functions
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have them.</p>
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<hr></div>
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</body></html>
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