122 lines
6.5 KiB
HTML
122 lines
6.5 KiB
HTML
|
<!DOCTYPE html>
|
||
|
|
<html>
|
||
|
|
<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
|
||
|
|
<head>
|
||
|
|
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
|
||
|
|
<!-- This manual documents Guile version 3.0.10.
|
||
|
|
|
||
|
|
Copyright (C) 1996-1997, 2000-2005, 2009-2023 Free Software Foundation,
|
||
|
|
Inc.
|
||
|
|
|
||
|
|
Copyright (C) 2021 Maxime Devos
|
||
|
|
|
||
|
|
Copyright (C) 2024 Tomas Volf
|
||
|
|
|
||
|
|
|
||
|
|
Permission is granted to copy, distribute and/or modify this document
|
||
|
|
under the terms of the GNU Free Documentation License, Version 1.3 or
|
||
|
|
any later version published by the Free Software Foundation; with no
|
||
|
|
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A
|
||
|
|
copy of the license is included in the section entitled "GNU Free
|
||
|
|
Documentation License." -->
|
||
|
|
<title>Changing the Class of an Instance (Guile Reference Manual)</title>
|
||
|
|
|
||
|
|
<meta name="description" content="Changing the Class of an Instance (Guile Reference Manual)">
|
||
|
|
<meta name="keywords" content="Changing the Class of an Instance (Guile Reference Manual)">
|
||
|
|
<meta name="resource-type" content="document">
|
||
|
|
<meta name="distribution" content="global">
|
||
|
|
<meta name="Generator" content=".texi2any-real">
|
||
|
|
<meta name="viewport" content="width=device-width,initial-scale=1">
|
||
|
|
|
||
|
|
<link href="index.html" rel="start" title="Top">
|
||
|
|
<link href="Concept-Index.html" rel="index" title="Concept Index">
|
||
|
|
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
|
||
|
|
<link href="GOOPS.html" rel="up" title="GOOPS">
|
||
|
|
<link href="Redefining-a-Class.html" rel="prev" title="Redefining a Class">
|
||
|
|
<style type="text/css">
|
||
|
|
<!--
|
||
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
||
|
|
span:hover a.copiable-link {visibility: visible}
|
||
|
|
strong.def-name {font-family: monospace; font-weight: bold; font-size: larger}
|
||
|
|
-->
|
||
|
|
</style>
|
||
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
||
|
|
|
||
|
|
|
||
|
|
</head>
|
||
|
|
|
||
|
|
<body lang="en">
|
||
|
|
<div class="section-level-extent" id="Changing-the-Class-of-an-Instance">
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<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>
|
||
|
|
|
||
|
|
<p>When a redefinable class is redefined, any existing instance of the
|
||
|
|
redefined class will be modified for the new class definition before the
|
||
|
|
next time that any of the instance’s slots is referenced or set. GOOPS
|
||
|
|
modifies each instance by calling the generic function
|
||
|
|
<code class="code">change-class</code>.
|
||
|
|
</p>
|
||
|
|
<p>More generally, you can change the class of an existing instance at any
|
||
|
|
time by invoking the generic function <code class="code">change-class</code> with two
|
||
|
|
arguments: the instance and the new class.
|
||
|
|
</p>
|
||
|
|
<p>The default method for <code class="code">change-class</code> decides how to implement the
|
||
|
|
change of class by looking at the slot definitions for the instance’s
|
||
|
|
existing class and for the new class. If the new class has slots with
|
||
|
|
the same name as slots in the existing class, the values for those slots
|
||
|
|
are preserved. Slots that are present only in the existing class are
|
||
|
|
discarded. Slots that are present only in the new class are initialized
|
||
|
|
using the corresponding slot definition’s init function (see <a class="pxref" href="Classes.html">slot-init-function</a>).
|
||
|
|
</p>
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<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>
|
||
|
|
</dl>
|
||
|
|
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<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>
|
||
|
|
<dd><p>Modify instance <var class="var">obj</var> to make it an instance of class <var class="var">new</var>.
|
||
|
|
<var class="var">obj</var> itself must already be an instance of a redefinable class.
|
||
|
|
</p>
|
||
|
|
<p>The value of each of <var class="var">obj</var>’s slots is preserved only if a similarly named
|
||
|
|
slot exists in <var class="var">new</var>; any other slot values are discarded.
|
||
|
|
</p>
|
||
|
|
<p>The slots in <var class="var">new</var> that do not correspond to any of <var class="var">obj</var>’s
|
||
|
|
pre-existing slots are initialized according to <var class="var">new</var>’s slot definitions’
|
||
|
|
init functions.
|
||
|
|
</p></dd></dl>
|
||
|
|
|
||
|
|
<p>The default <code class="code">change-class</code> method also invokes another generic
|
||
|
|
function, <code class="code">update-instance-for-different-class</code>, as the last thing
|
||
|
|
that it does before returning. The applied
|
||
|
|
<code class="code">update-instance-for-different-class</code> method can make any further
|
||
|
|
adjustments to <var class="var">new-instance</var> that are required to complete or
|
||
|
|
modify the change of class. The return value from the applied method is
|
||
|
|
ignored.
|
||
|
|
</p>
|
||
|
|
<dl class="first-deffn">
|
||
|
|
<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>
|
||
|
|
<dd><p>A generic function that can be customized to put finishing touches to an
|
||
|
|
instance whose class has just been changed. The default
|
||
|
|
<code class="code">update-instance-for-different-class</code> method does nothing.
|
||
|
|
</p></dd></dl>
|
||
|
|
|
||
|
|
<p>Customized change of class behavior can be implemented by defining
|
||
|
|
<code class="code">change-class</code> methods that are specialized either by the class
|
||
|
|
of the instances to be modified or by the metaclass of the new class.
|
||
|
|
</p>
|
||
|
|
</div>
|
||
|
|
<hr>
|
||
|
|
<div class="nav-panel">
|
||
|
|
<p>
|
||
|
|
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>
|
||
|
|
</div>
|
||
|
|
|
||
|
|
|
||
|
|
|
||
|
|
</body>
|
||
|
|
</html>
|