133 lines
6.2 KiB
HTML
133 lines
6.2 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>Vtables (Guile Reference Manual)</title>
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<meta name="description" content="Vtables (Guile Reference Manual)">
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<meta name="keywords" content="Vtables (Guile Reference Manual)">
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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="Structure-Basics.html" rel="next" title="Structure Basics">
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</head>
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<body lang="en">
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<div class="subsubsection-level-extent" id="Vtables">
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<div class="nav-panel">
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<p>
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Next: <a href="Structure-Basics.html" accesskey="n" rel="next">Structure Basics</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="Vtables-1"><span>6.6.18.1 Vtables<a class="copiable-link" href="#Vtables-1"> ¶</a></span></h4>
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<p>A vtable is a structure type, specifying its layout, and other
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information. A vtable is actually itself a structure, but there’s no
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need to worry about that initially (see <a class="pxref" href="Vtable-Contents.html">Vtable Contents</a>.)
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dvtable"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-vtable</strong> <var class="def-var-arguments">fields [print]</var><a class="copiable-link" href="#index-make_002dvtable"> ¶</a></span></dt>
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<dd><p>Create a new vtable.
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</p>
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<p><var class="var">fields</var> is a string describing the fields in the structures to be
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created. Each field is represented by two characters, a type letter
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and a permissions letter, for example <code class="code">"pw"</code>. The types are as
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follows.
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</p>
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<ul class="itemize mark-bullet">
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<li><code class="code">p</code> – a Scheme value. “p” stands for “protected” meaning
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it’s protected against garbage collection.
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</li><li><code class="code">u</code> – an arbitrary word of data (an <code class="code">scm_t_bits</code>). At the
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Scheme level it’s read and written as an unsigned integer. “u” stands
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for “unboxed”, as it’s stored as a raw value without additional type
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annotations.
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</li></ul>
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<p>It used to be that the second letter for each field was a permission
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code, such as <code class="code">w</code> for writable or <code class="code">r</code> for read-only. However
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over time structs have become more of a raw low-level facility; access
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control is better implemented as a layer on top. After all,
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<code class="code">struct-set!</code> is a cross-cutting operator that can bypass
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abstractions made by higher-level record facilities; it’s not generally
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safe (in the sense of abstraction-preserving) to expose
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<code class="code">struct-set!</code> to “untrusted” code, even if the fields happen to
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be writable. Additionally, permission checks added overhead to every
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structure access in a way that couldn’t be optimized out, hampering the
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ability of structs to act as a low-level building block. For all of
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these reasons, all fields in Guile structs are now writable; attempting
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to make a read-only field will now issue a deprecation warning, and the
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field will be writable regardless.
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</p>
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<div class="example">
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<pre class="example-preformatted">(make-vtable "pw") ;; one scheme field
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(make-vtable "pwuwuw") ;; one scheme and two unboxed fields
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</pre></div>
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<p>The optional <var class="var">print</var> argument is a function called by
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<code class="code">display</code> and <code class="code">write</code> (etc) to give a printed representation
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of a structure created from this vtable. It’s called
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<code class="code">(<var class="var">print</var> struct port)</code> and should look at <var class="var">struct</var> and
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write to <var class="var">port</var>. The default print merely gives a form like
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‘<samp class="samp">#<struct ADDR:ADDR></samp>’ with a pair of machine addresses.
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</p>
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<p>The following print function for example shows the two fields of its
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structure.
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</p>
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<div class="example">
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<pre class="example-preformatted">(make-vtable "pwpw"
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(lambda (struct port)
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(format port "#<~a and ~a>"
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(struct-ref struct 0)
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(struct-ref struct 1))))
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</pre></div>
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</dd></dl>
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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="Structure-Basics.html">Structure Basics</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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