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75 lines
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<a name="Discriminating-Functions"></a>
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Next: <a rel="next" accesskey="n" href="Foreign-Linkage.html#Foreign-Linkage">Foreign Linkage</a>,
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Previous: <a rel="previous" accesskey="p" href="Calling-Convention.html#Calling-Convention">Calling Convention</a>,
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<h2 class="chapter">3 Discriminating Functions</h2>
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<ul class="menu">
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<li><a accesskey="1" href="The-Initial-Discriminating-Function.html#The-Initial-Discriminating-Function">The Initial Discriminating Function</a>
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<li><a accesskey="2" href="Method_002dBased-Discriminating-Functions.html#Method_002dBased-Discriminating-Functions">Method-Based Discriminating Functions</a>
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<li><a accesskey="3" href="Accessor-Discriminating-Functions.html#Accessor-Discriminating-Functions">Accessor Discriminating Functions</a>
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<li><a accesskey="4" href="Cacheing-and-Dispatch-Functions.html#Cacheing-and-Dispatch-Functions">Cacheing and Dispatch Functions</a>
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<li><a accesskey="5" href="The-Cacheing-Mechanism.html#The-Cacheing-Mechanism">The Cacheing Mechanism</a>
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</ul>
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<p>The Common Lisp Object System specifies a great deal of run-time
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customizeability, such as class redefinition, generic function and
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method redefinition, addition and removal of methods and redefinitions
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of method combinations. The additional flexibility defined by the
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Metaobject Protocol, specifying the generic functions called to achieve
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the effects of CLOS operations (and allowing many of them to be
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overridden by the user) makes any form of optimization seem intractable.
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And yet such optimization is necessary to achieve reasonable
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performance: the MOP specifies that a slot access looks up the class of
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the object, and the slot definition from that class and the slot name,
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and then invokes a generic function specialized on those three
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arguments. This is clearly going to act against the user's intuition
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that a slot access given an instance should be relatively fast.
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<p>The optimizations performed cannot be done wholly at compile-time,
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however, thanks to all of these possibilities for run-time redefinition
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and extensibility. This section describes the optimizations performed
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in SBCL's CLOS implementation in computing and calling the effective
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method for generic functions.
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