149 lines
6.9 KiB
HTML
149 lines
6.9 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>Metaclasses (Guile Reference Manual)</title>
|
|
|
|
<meta name="description" content="Metaclasses (Guile Reference Manual)">
|
|
<meta name="keywords" content="Metaclasses (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="The-Metaobject-Protocol.html" rel="up" title="The Metaobject Protocol">
|
|
<link href="MOP-Specification.html" rel="next" title="MOP Specification">
|
|
<link href="Metaobjects-and-the-Metaobject-Protocol.html" rel="prev" title="Metaobjects and the Metaobject Protocol">
|
|
<style type="text/css">
|
|
<!--
|
|
a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
|
|
div.example {margin-left: 3.2em}
|
|
span:hover a.copiable-link {visibility: visible}
|
|
ul.mark-bullet {list-style-type: disc}
|
|
-->
|
|
</style>
|
|
<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
|
|
|
|
|
|
</head>
|
|
|
|
<body lang="en">
|
|
<div class="subsection-level-extent" id="Metaclasses">
|
|
<div class="nav-panel">
|
|
<p>
|
|
Next: <a href="MOP-Specification.html" accesskey="n" rel="next">MOP Specification</a>, Previous: <a href="Metaobjects-and-the-Metaobject-Protocol.html" accesskey="p" rel="prev">Metaobjects and the Metaobject Protocol</a>, Up: <a href="The-Metaobject-Protocol.html" accesskey="u" rel="up">The Metaobject Protocol</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>
|
|
<h4 class="subsection" id="Metaclasses-1"><span>8.11.2 Metaclasses<a class="copiable-link" href="#Metaclasses-1"> ¶</a></span></h4>
|
|
|
|
<p>A <em class="dfn">metaclass</em> is the class of an object which represents a GOOPS
|
|
class. Put more succinctly, a metaclass is a class’s class.
|
|
</p>
|
|
<p>Most GOOPS classes have the metaclass <code class="code"><class></code> and, by default,
|
|
any new class that is created using <code class="code">define-class</code> has the
|
|
metaclass <code class="code"><class></code>.
|
|
</p>
|
|
<p>But what does this really mean? To find out, let’s look in more detail
|
|
at what happens when a new class is created using <code class="code">define-class</code>:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define-class <my-class> (<object>) . slots)
|
|
</pre></div>
|
|
|
|
<p>Guile expands this to something like:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define <my-class> (class (<object>) . slots))
|
|
</pre></div>
|
|
|
|
<p>which in turn expands to:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define <my-class>
|
|
(make <class> #:dsupers (list <object>) #:slots slots))
|
|
</pre></div>
|
|
|
|
<p>As this expansion makes clear, the resulting value of <code class="code"><my-class></code>
|
|
is an instance of the class <code class="code"><class></code> with slot values specifying
|
|
the superclasses and slot definitions for the class <code class="code"><my-class></code>.
|
|
(<code class="code">#:dsupers</code> and <code class="code">#:slots</code> are initialization keywords for the
|
|
<code class="code">dsupers</code> and <code class="code">dslots</code> slots of the <code class="code"><class></code> class.)
|
|
</p>
|
|
<p>Now suppose that you want to define a new class with a metaclass other
|
|
than the default <code class="code"><class></code>. This is done by writing:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define-class <my-class2> (<object>)
|
|
slot ...
|
|
#:metaclass <my-metaclass>)
|
|
</pre></div>
|
|
|
|
<p>and Guile expands <em class="emph">this</em> to something like:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define <my-class2>
|
|
(make <my-metaclass> #:dsupers (list <object>) #:slots slots))
|
|
</pre></div>
|
|
|
|
<p>In this case, the value of <code class="code"><my-class2></code> is an instance of the more
|
|
specialized class <code class="code"><my-metaclass></code>. Note that
|
|
<code class="code"><my-metaclass></code> itself must previously have been defined as a
|
|
subclass of <code class="code"><class></code>. For a full discussion of when and how it is
|
|
useful to define new metaclasses, see <a class="ref" href="MOP-Specification.html">MOP Specification</a>.
|
|
</p>
|
|
<p>Now let’s make an instance of <code class="code"><my-class2></code>:
|
|
</p>
|
|
<div class="example">
|
|
<pre class="example-preformatted">(define my-object (make <my-class2> ...))
|
|
</pre></div>
|
|
|
|
<p>All of the following statements are correct expressions of the
|
|
relationships between <code class="code">my-object</code>, <code class="code"><my-class2></code>,
|
|
<code class="code"><my-metaclass></code> and <code class="code"><class></code>.
|
|
</p>
|
|
<ul class="itemize mark-bullet">
|
|
<li><code class="code">my-object</code> is an instance of the class <code class="code"><my-class2></code>.
|
|
|
|
</li><li><code class="code"><my-class2></code> is an instance of the class <code class="code"><my-metaclass></code>.
|
|
|
|
</li><li><code class="code"><my-metaclass></code> is an instance of the class <code class="code"><class></code>.
|
|
|
|
</li><li>The class of <code class="code">my-object</code> is <code class="code"><my-class2></code>.
|
|
|
|
</li><li>The class of <code class="code"><my-class2></code> is <code class="code"><my-metaclass></code>.
|
|
|
|
</li><li>The class of <code class="code"><my-metaclass></code> is <code class="code"><class></code>.
|
|
</li></ul>
|
|
|
|
|
|
</div>
|
|
<hr>
|
|
<div class="nav-panel">
|
|
<p>
|
|
Next: <a href="MOP-Specification.html">MOP Specification</a>, Previous: <a href="Metaobjects-and-the-Metaobject-Protocol.html">Metaobjects and the Metaobject Protocol</a>, Up: <a href="The-Metaobject-Protocol.html">The Metaobject Protocol</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>
|