180 lines
9.6 KiB
HTML
180 lines
9.6 KiB
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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>Meta-Vtables (Guile Reference Manual)</title>
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<meta name="description" content="Meta-Vtables (Guile Reference Manual)">
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<meta name="keywords" content="Meta-Vtables (Guile Reference Manual)">
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<meta name="distribution" content="global">
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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="Vtable-Example.html" rel="next" title="Vtable Example">
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<link href="Vtable-Contents.html" rel="prev" title="Vtable Contents">
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<style type="text/css">
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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="Meta_002dVtables">
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<div class="nav-panel">
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<p>
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Next: <a href="Vtable-Example.html" accesskey="n" rel="next">Vtable Example</a>, Previous: <a href="Vtable-Contents.html" accesskey="p" rel="prev">Vtable Contents</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="Meta_002dVtables-1"><span>6.6.18.4 Meta-Vtables<a class="copiable-link" href="#Meta_002dVtables-1"> ¶</a></span></h4>
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<p>As a structure, a vtable also has a vtable, which is also a structure.
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Structures, their vtables, the vtables of the vtables, and so on form a
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tree of structures. Making a new structure adds a leaf to the tree, and
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if that structure is a vtable, it may be used to create other leaves.
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</p>
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<p>If you traverse up the tree of vtables, via calling
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<code class="code">struct-vtable</code>, eventually you reach a root which is the vtable of
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itself:
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</p>
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<div class="example">
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<pre class="example-preformatted">scheme@(guile-user)> (current-module)
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$1 = #<directory (guile-user) 221b090>
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scheme@(guile-user)> (struct-vtable $1)
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$2 = #<record-type module>
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scheme@(guile-user)> (struct-vtable $2)
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$3 = #<<standard-vtable> 12c30a0>
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scheme@(guile-user)> (struct-vtable $3)
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$4 = #<<standard-vtable> 12c3fa0>
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scheme@(guile-user)> (struct-vtable $4)
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$5 = #<<standard-vtable> 12c3fa0>
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scheme@(guile-user)> <standard-vtable>
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$6 = #<<standard-vtable> 12c3fa0>
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</pre></div>
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<p>In this example, we can say that <code class="code">$1</code> is an instance of <code class="code">$2</code>,
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<code class="code">$2</code> is an instance of <code class="code">$3</code>, <code class="code">$3</code> is an instance of
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<code class="code">$4</code>, and <code class="code">$4</code>, strangely enough, is an instance of itself.
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The value bound to <code class="code">$4</code> in this console session also bound to
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<code class="code"><standard-vtable></code> in the default environment.
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</p>
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<dl class="first-defvr">
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<dt class="defvr" id="index-_003cstandard_002dvtable_003e"><span class="category-def">Scheme Variable: </span><span><strong class="def-name"><standard-vtable></strong><a class="copiable-link" href="#index-_003cstandard_002dvtable_003e"> ¶</a></span></dt>
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<dd><p>A meta-vtable, useful for making new vtables.
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</p></dd></dl>
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<p>All of these values are structures. All but <code class="code">$1</code> are vtables. As
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<code class="code">$2</code> is an instance of <code class="code">$3</code>, and <code class="code">$3</code> is a vtable, we can
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say that <code class="code">$3</code> is a <em class="dfn">meta-vtable</em>: a vtable that can create
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vtables.
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</p>
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<p>With this definition, we can specify more precisely what a vtable is: a
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vtable is a structure made from a meta-vtable. Making a structure from
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a meta-vtable runs some special checks to ensure that the first field of
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the structure is a valid layout. Additionally, if these checks see that
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the layout of the child vtable contains all the required fields of a
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vtable, in the correct order, then the child vtable will also be a
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meta-table, inheriting a magical bit from the parent.
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</p>
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<dl class="first-deffn">
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<dt class="deffn" id="index-struct_002dvtable_003f"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">struct-vtable?</strong> <var class="def-var-arguments">obj</var><a class="copiable-link" href="#index-struct_002dvtable_003f"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fstruct_005fvtable_005fp"><span class="category-def">C Function: </span><span><strong class="def-name">scm_struct_vtable_p</strong> <var class="def-var-arguments">(obj)</var><a class="copiable-link" href="#index-scm_005fstruct_005fvtable_005fp"> ¶</a></span></dt>
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<dd><p>Return <code class="code">#t</code> if <var class="var">obj</var> is a vtable structure: an instance of a
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meta-vtable.
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</p></dd></dl>
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<p><code class="code"><standard-vtable></code> is a root of the vtable tree. (Normally there
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is only one root in a given Guile process, but due to some legacy
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interfaces there may be more than one.)
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</p>
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<p>The set of required fields of a vtable is the set of fields in the
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<code class="code"><standard-vtable></code>, and is bound to <code class="code">standard-vtable-fields</code>
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in the default environment. It is possible to create a meta-vtable that
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with additional fields in its layout, which can be used to create
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vtables with additional data:
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</p>
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<div class="example">
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<pre class="example-preformatted">scheme@(guile-user)> (struct-ref $3 vtable-index-layout)
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$6 = pwuhuhpwphuhuhpwpwpw
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scheme@(guile-user)> (struct-ref $4 vtable-index-layout)
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$7 = pwuhuhpwphuhuh
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scheme@(guile-user)> standard-vtable-fields
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$8 = "pwuhuhpwphuhuh"
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scheme@(guile-user)> (struct-ref $2 vtable-offset-user)
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$9 = module
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</pre></div>
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<p>In this continuation of our earlier example, <code class="code">$2</code> is a vtable that
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has extra fields, because its vtable, <code class="code">$3</code>, was made from a
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meta-vtable with an extended layout. <code class="code">vtable-offset-user</code> is a
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convenient definition that indicates the number of fields in
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<code class="code">standard-vtable-fields</code>.
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</p>
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<dl class="first-defvr">
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<dt class="defvr" id="index-standard_002dvtable_002dfields"><span class="category-def">Scheme Variable: </span><span><strong class="def-name">standard-vtable-fields</strong><a class="copiable-link" href="#index-standard_002dvtable_002dfields"> ¶</a></span></dt>
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<dd><p>A string containing the ordered set of fields that a vtable must have.
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</p></dd></dl>
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<dl class="first-defvr">
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<dt class="defvr" id="index-vtable_002doffset_002duser"><span class="category-def">Scheme Variable: </span><span><strong class="def-name">vtable-offset-user</strong><a class="copiable-link" href="#index-vtable_002doffset_002duser"> ¶</a></span></dt>
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<dd><p>The first index in a vtable that is available for a user.
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</p></dd></dl>
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<dl class="first-deffn">
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<dt class="deffn" id="index-make_002dstruct_002dlayout"><span class="category-def">Scheme Procedure: </span><span><strong class="def-name">make-struct-layout</strong> <var class="def-var-arguments">fields</var><a class="copiable-link" href="#index-make_002dstruct_002dlayout"> ¶</a></span></dt>
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<dt class="deffnx def-cmd-deffn" id="index-scm_005fmake_005fstruct_005flayout"><span class="category-def">C Function: </span><span><strong class="def-name">scm_make_struct_layout</strong> <var class="def-var-arguments">(fields)</var><a class="copiable-link" href="#index-scm_005fmake_005fstruct_005flayout"> ¶</a></span></dt>
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<dd><p>Return a structure layout symbol, from a <var class="var">fields</var> string.
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<var class="var">fields</var> is as described under <code class="code">make-vtable</code>
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(see <a class="pxref" href="Vtables.html">Vtables</a>). An invalid <var class="var">fields</var> string is an error.
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</p></dd></dl>
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<p>With these definitions, one can define <code class="code">make-vtable</code> in this way:
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</p>
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<div class="example">
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<pre class="example-preformatted">(define* (make-vtable fields #:optional printer)
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(make-struct/no-tail <standard-vtable>
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(make-struct-layout fields)
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printer))
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</pre></div>
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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="Vtable-Example.html">Vtable Example</a>, Previous: <a href="Vtable-Contents.html">Vtable Contents</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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