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<title>CLiki: change-class dangers</title>
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<span class="hidden">CLiki - change-class dangers</span>
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<div id="content"><div id="content-area"><div id="article-title">change-class dangers</div><div id="article"><a href="https://www.cliki.net/site/HyperSpec/Body/stagenfun_change-class.html" class="hyperspec">change-class</a> changes the class of the <a href="CLOS.html" class="category">CLOS</a> object involved; in
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the specification you'll see a "Notes" section that points
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out the problems of changing the class of an object in the presence
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of a smart compiler.<p>Experience has shown that sometimes you'll get into problems using
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<a href="https://www.cliki.net/site/HyperSpec/Body/fun_slot-value.html" class="hyperspec">slot-value</a> or variables introduced with <a href="https://www.cliki.net/site/HyperSpec/Body/mac_with-slots.html" class="hyperspec">with-slots</a>. It may
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appear the the EQ-ness of objects in not preserved because the
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compiler was "smart enough" to optimize these features, so for instance: if it's possible
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to access the same object (EQ) with another symbol, the compiler can
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return different slot-value's for the original or the other symbol. Even
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if they are EQ! This can be very confusing.<p>For instance:<p><div class="code"><span class="nonparen"><span class="paren1">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/mac_defparametercm_defvar.html" class="symbol"><i><span class="symbol">defvar</span></i></a> <span class="special">*A*</span> <span class="paren2">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/stagenfun_make-instance.html" class="symbol">make-instance</a> 'bar</span>)</span></span>)</span>
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<span class="paren1">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/mac_defmethod.html" class="symbol"><i><span class="symbol">defmethod</span></i></a> foo <span class="paren2">(<span class="nonparen"><span class="paren3">(<span class="nonparen">x bar</span>)</span></span>)</span>
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<span class="paren2">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/stagenfun_change-class.html" class="symbol">change-class</a> x 'baz</span>)</span>
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<span class="paren2">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/fun_format.html" class="symbol">format</a> <a href="https://www.cliki.net/site/HyperSpec/Body/any_t.html" class="symbol">t</a> <span class="string">"~&from x: ~S from *A*: ~S"</span>
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<span class="paren3">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/fun_slot-value.html" class="symbol">slot-value</a> x 'quux</span>)</span>
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<span class="paren3">(<span class="nonparen"><a href="https://www.cliki.net/site/HyperSpec/Body/fun_slot-value.html" class="symbol">slot-value</a> <span class="special">*A*</span> 'quux</span>)</span></span>)</span></span>)</span>
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<span class="paren1">(<span class="nonparen">foo <span class="special">*A*</span></span>)</span></span></div><p>Can produce output like: "from x: 1 from *A*: 2", depending
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on the compiler and/or optimization qualities.<p>To solve this just call a helper function just after
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changing the class. The compiler will not optimize over function calls
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so the slot-value and with-slots features work again.<p><hr>
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