170 lines
6.1 KiB
HTML
170 lines
6.1 KiB
HTML
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<!DOCTYPE html>
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<html>
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<!-- Created by GNU Texinfo 7.1, https://www.gnu.org/software/texinfo/ -->
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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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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under the terms of the GNU Free Documentation License, Version 1.3 or
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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>Vtable Example (Guile Reference Manual)</title>
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<meta name="description" content="Vtable Example (Guile Reference Manual)">
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<meta name="keywords" content="Vtable Example (Guile Reference Manual)">
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<meta name="resource-type" content="document">
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<meta name="distribution" content="global">
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<meta name="Generator" content=".texi2any-real">
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<meta name="viewport" content="width=device-width,initial-scale=1">
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<link href="index.html" rel="start" title="Top">
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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="Structures.html" rel="up" title="Structures">
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<link href="Meta_002dVtables.html" rel="prev" title="Meta-Vtables">
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<style type="text/css">
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<!--
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a.copiable-link {visibility: hidden; text-decoration: none; line-height: 0em}
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div.example {margin-left: 3.2em}
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span:hover a.copiable-link {visibility: visible}
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-->
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</style>
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<link rel="stylesheet" type="text/css" href="https://www.gnu.org/software/gnulib/manual.css">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Vtable-Example">
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<div class="nav-panel">
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<p>
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Previous: <a href="Meta_002dVtables.html" accesskey="p" rel="prev">Meta-Vtables</a>, Up: <a href="Structures.html" accesskey="u" rel="up">Structures</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="subsubsection" id="Vtable-Example-1"><span>6.6.18.5 Vtable Example<a class="copiable-link" href="#Vtable-Example-1"> ¶</a></span></h4>
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<p>Let us bring these points together with an example. Consider a simple
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object system with single inheritance. Objects will be normal
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structures, and classes will be vtables with three extra class fields:
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the name of the class, the parent class, and the list of fields.
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</p>
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<p>So, first we need a meta-vtable that allocates instances with these
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extra class fields.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define <class>
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(make-vtable
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(string-append standard-vtable-fields "pwpwpw")
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(lambda (x port)
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(format port "<<class> ~a>" (class-name x)))))
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(define (class? x)
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(and (struct? x)
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(eq? (struct-vtable x) <class>)))
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</pre></div>
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<p>To make a structure with a specific meta-vtable, we will use
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<code class="code">make-struct/no-tail</code>, passing it the computed instance layout and
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printer, as with <code class="code">make-vtable</code>, and additionally the extra three
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class fields.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define (make-class name parent fields)
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(let* ((fields (compute-fields parent fields))
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(layout (compute-layout fields)))
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(make-struct/no-tail <class>
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layout
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(lambda (x port)
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(print-instance x port))
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name
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parent
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fields)))
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</pre></div>
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<p>Instances will store their associated data in slots in the structure: as
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many slots as there are fields. The <code class="code">compute-layout</code> procedure
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below can compute a layout, and <code class="code">field-index</code> returns the slot
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corresponding to a field.
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax-rule (define-accessor name n)
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(define (name obj)
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(struct-ref obj n)))
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;; Accessors for classes
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(define-accessor class-name (+ vtable-offset-user 0))
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(define-accessor class-parent (+ vtable-offset-user 1))
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(define-accessor class-fields (+ vtable-offset-user 2))
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(define (compute-fields parent fields)
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(if parent
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(append (class-fields parent) fields)
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fields))
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(define (compute-layout fields)
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(make-struct-layout
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(string-concatenate (make-list (length fields) "pw"))))
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(define (field-index class field)
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(list-index (class-fields class) field))
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(define (print-instance x port)
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(format port "<~a" (class-name (struct-vtable x)))
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(for-each (lambda (field idx)
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(format port " ~a: ~a" field (struct-ref x idx)))
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(class-fields (struct-vtable x))
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(iota (length (class-fields (struct-vtable x)))))
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(format port ">"))
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</pre></div>
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<p>So, at this point we can actually make a few classes:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define-syntax-rule (define-class name parent field ...)
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(define name (make-class 'name parent '(field ...))))
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(define-class <surface> #f
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width height)
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(define-class <window> <surface>
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x y)
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</pre></div>
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<p>And finally, make an instance:
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</p>
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<div class="example">
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<pre class="example-preformatted">(make-struct/no-tail <window> 400 300 10 20)
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⇒ <<window> width: 400 height: 300 x: 10 y: 20>
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</pre></div>
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<p>And that’s that. Note that there are many possible optimizations and
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feature enhancements that can be made to this object system, and the
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included GOOPS system does make most of them. For more simple use
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cases, the records facility is usually sufficient. But sometimes you
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need to make new kinds of data abstractions, and for that purpose,
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structs are here.
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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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Previous: <a href="Meta_002dVtables.html">Meta-Vtables</a>, Up: <a href="Structures.html">Structures</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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