121 lines
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121 lines
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HTML
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Copyright (C) 2024 Tomas Volf
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<title>Changing the Class of an Instance (Guile Reference Manual)</title>
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<div class="section-level-extent" id="Changing-the-Class-of-an-Instance">
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<p>
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Previous: <a href="Redefining-a-Class.html" accesskey="p" rel="prev">Redefining a Class</a>, Up: <a href="GOOPS.html" accesskey="u" rel="up">GOOPS</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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<h3 class="section" id="Changing-the-Class-of-an-Instance-1"><span>8.13 Changing the Class of an Instance<a class="copiable-link" href="#Changing-the-Class-of-an-Instance-1"> ¶</a></span></h3>
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<p>When a redefinable class is redefined, any existing instance of the
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redefined class will be modified for the new class definition before the
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next time that any of the instance’s slots is referenced or set. GOOPS
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modifies each instance by calling the generic function
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<code class="code">change-class</code>.
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</p>
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<p>More generally, you can change the class of an existing instance at any
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time by invoking the generic function <code class="code">change-class</code> with two
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arguments: the instance and the new class.
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</p>
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<p>The default method for <code class="code">change-class</code> decides how to implement the
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change of class by looking at the slot definitions for the instance’s
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existing class and for the new class. If the new class has slots with
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the same name as slots in the existing class, the values for those slots
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are preserved. Slots that are present only in the existing class are
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discarded. Slots that are present only in the new class are initialized
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using the corresponding slot definition’s init function (see <a class="pxref" href="Classes.html">slot-init-function</a>).
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-change_002dclass"><span class="category-def">generic: </span><span><strong class="def-name">change-class</strong> <var class="def-var-arguments">instance new-class</var><a class="copiable-link" href="#index-change_002dclass"> ¶</a></span></dt>
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</dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-change_002dclass-1"><span class="category-def">method: </span><span><strong class="def-name">change-class</strong> <var class="def-var-arguments">(obj <object>) (new <redefinable-class>)</var><a class="copiable-link" href="#index-change_002dclass-1"> ¶</a></span></dt>
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<dd><p>Modify instance <var class="var">obj</var> to make it an instance of class <var class="var">new</var>.
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<var class="var">obj</var> itself must already be an instance of a redefinable class.
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</p>
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<p>The value of each of <var class="var">obj</var>’s slots is preserved only if a similarly named
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slot exists in <var class="var">new</var>; any other slot values are discarded.
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</p>
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<p>The slots in <var class="var">new</var> that do not correspond to any of <var class="var">obj</var>’s
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pre-existing slots are initialized according to <var class="var">new</var>’s slot definitions’
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init functions.
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</p></dd></dl>
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<p>The default <code class="code">change-class</code> method also invokes another generic
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function, <code class="code">update-instance-for-different-class</code>, as the last thing
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that it does before returning. The applied
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<code class="code">update-instance-for-different-class</code> method can make any further
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adjustments to <var class="var">new-instance</var> that are required to complete or
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modify the change of class. The return value from the applied method is
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ignored.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-update_002dinstance_002dfor_002ddifferent_002dclass"><span class="category-def">generic: </span><span><strong class="def-name">update-instance-for-different-class</strong> <var class="def-var-arguments">old-instance new-instance</var><a class="copiable-link" href="#index-update_002dinstance_002dfor_002ddifferent_002dclass"> ¶</a></span></dt>
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<dd><p>A generic function that can be customized to put finishing touches to an
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instance whose class has just been changed. The default
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<code class="code">update-instance-for-different-class</code> method does nothing.
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</p></dd></dl>
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<p>Customized change of class behavior can be implemented by defining
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<code class="code">change-class</code> methods that are specialized either by the class
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of the instances to be modified or by the metaclass of the new class.
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</p>
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</div>
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<hr>
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<p>
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Previous: <a href="Redefining-a-Class.html">Redefining a Class</a>, Up: <a href="GOOPS.html">GOOPS</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Concept-Index.html" title="Index" rel="index">Index</a>]</p>
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