122 lines
6 KiB
HTML
122 lines
6 KiB
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<!DOCTYPE html>
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<!-- This manual documents Guile version 3.0.10.
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Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
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Inc.
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Copyright (C) 2021 Maxime Devos
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Copyright (C) 2024 Tomas Volf
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Permission is granted to copy, distribute and/or modify this document
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any later version published by the Free Software Foundation; with no
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Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
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copy of the license is included in the section entitled "GNU Free
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Documentation License." -->
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<title>Default Class Redefinition Behavior (Guile Reference Manual)</title>
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<link href="Concept-Index.html" rel="index" title="Concept Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Redefining-a-Class.html" rel="up" title="Redefining a Class">
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<link href="Customizing-Class-Redefinition.html" rel="next" title="Customizing Class Redefinition">
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</head>
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<body lang="en">
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<div class="subsection-level-extent" id="Default-Class-Redefinition-Behavior">
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<div class="nav-panel">
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<p>
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Next: <a href="Customizing-Class-Redefinition.html" accesskey="n" rel="next">Customizing Class Redefinition</a>, Previous: <a href="Redefinable-Classes.html" accesskey="p" rel="prev">Redefinable Classes</a>, Up: <a href="Redefining-a-Class.html" accesskey="u" rel="up">Redefining a Class</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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</div>
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<hr>
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<h4 class="subsection" id="Default-Class-Redefinition-Behavior-1"><span>8.12.2 Default Class Redefinition Behavior<a class="copiable-link" href="#Default-Class-Redefinition-Behavior-1"> ¶</a></span></h4>
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<p>When a class is defined using <code class="code">define-class</code> and the class name was
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previously defined, by default the new binding just replaces the old
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binding. This is the normal behavior for <code class="code">define</code>. However if
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both the old and new bindings are redefinable classes (instances of
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<code class="code"><redefinable-class></code>), then the class will be updated in place,
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and its instances lazily migrated over.
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</p>
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<p>The way that the class is updated and the way that the instances migrate
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over are of course part of the meta-object protocol. However the
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default behavior usually suffices, and it goes as follows.
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</p>
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<ul class="itemize mark-bullet">
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<li>All existing direct instances of <code class="code"><my-class></code> are converted to be
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instances of the new class. This is achieved by preserving the values
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of slots that exist in both the old and new definitions, and
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initializing the values of new slots in the usual way (see <a class="pxref" href="Instance-Creation.html">make</a>).
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</li><li>All existing subclasses of <code class="code"><my-class></code> are redefined, as though
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the <code class="code">define-class</code> expressions that defined them were re-evaluated
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following the redefinition of <code class="code"><my-class></code>, and the class
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redefinition process described here is applied recursively to the
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redefined subclasses.
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</li><li>Once all of its instances and subclasses have been updated, the class
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metaobject previously bound to the variable <code class="code"><my-class></code> is no
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longer needed and so can be allowed to be garbage collected.
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</li></ul>
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<p>To keep things tidy, GOOPS also needs to do a little housekeeping on
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methods that are associated with the redefined class.
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</p>
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<ul class="itemize mark-bullet">
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<li>Slot accessor methods for slots in the old definition should be removed
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from their generic functions. They will be replaced by accessor methods
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for the slots of the new class definition.
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</li><li>Any generic function method that uses the old <code class="code"><my-class></code> metaobject
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as one of its formal parameter specializers must be updated to refer to
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the new <code class="code"><my-class></code> metaobject. (Whenever a new generic function
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method is defined, <code class="code">define-method</code> adds the method to a list stored
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in the class metaobject for each class used as a formal parameter
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specializer, so it is easy to identify all the methods that must be
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updated when a class is redefined.)
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</li></ul>
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<p>If this class redefinition strategy strikes you as rather counter-intuitive,
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bear in mind that it is derived from similar behavior in other object
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systems such as CLOS, and that experience in those systems has shown it to be
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very useful in practice.
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</p>
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<p>Also bear in mind that, like most of GOOPS’ default behavior, it can
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be customized…
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</p>
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</div>
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<hr>
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<div class="nav-panel">
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<p>
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Next: <a href="Customizing-Class-Redefinition.html">Customizing Class Redefinition</a>, Previous: <a href="Redefinable-Classes.html">Redefinable Classes</a>, Up: <a href="Redefining-a-Class.html">Redefining a Class</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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</div>
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</body>
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</html>
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