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139 lines
5.4 KiB
HTML
139 lines
5.4 KiB
HTML
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html>
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<!-- Copyright (C) 1988-2018 Free Software Foundation, Inc.
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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 the
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Invariant Sections being "Funding Free Software", the Front-Cover
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Texts being (a) (see below), and with the Back-Cover Texts being (b)
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(see below). A copy of the license is included in the section entitled
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"GNU Free Documentation License".
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(a) The FSF's Front-Cover Text is:
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A GNU Manual
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(b) The FSF's Back-Cover Text is:
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You have freedom to copy and modify this GNU Manual, like GNU
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software. Copies published by the Free Software Foundation raise
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funds for GNU development. -->
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<!-- Created by GNU Texinfo 6.4, http://www.gnu.org/software/texinfo/ -->
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<head>
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<title>Inheritance and GTY (GNU Compiler Collection (GCC) Internals)</title>
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<meta name="description" content="Inheritance and GTY (GNU Compiler Collection (GCC) Internals)">
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<meta name="keywords" content="Inheritance and GTY (GNU Compiler Collection (GCC) Internals)">
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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="makeinfo">
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<link href="index.html#Top" rel="start" title="Top">
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<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
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<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
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<link href="Type-Information.html#Type-Information" rel="up" title="Type Information">
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<link href="User-GC.html#User-GC" rel="next" title="User GC">
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<link href="GTY-Options.html#GTY-Options" rel="prev" title="GTY Options">
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<style type="text/css">
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-->
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</style>
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</head>
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<body lang="en">
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<a name="Inheritance-and-GTY"></a>
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<div class="header">
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<p>
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Next: <a href="User-GC.html#User-GC" accesskey="n" rel="next">User GC</a>, Previous: <a href="GTY-Options.html#GTY-Options" accesskey="p" rel="prev">GTY Options</a>, Up: <a href="Type-Information.html#Type-Information" accesskey="u" rel="up">Type Information</a> [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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</div>
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<hr>
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<a name="Support-for-inheritance"></a>
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<h3 class="section">23.2 Support for inheritance</h3>
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<p>gengtype has some support for simple class hierarchies. You can use
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this to have gengtype autogenerate marking routines, provided:
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</p>
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<ul>
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<li> There must be a concrete base class, with a discriminator expression
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that can be used to identify which subclass an instance is.
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</li><li> Only single inheritance is used.
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</li><li> None of the classes within the hierarchy are templates.
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</li></ul>
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<p>If your class hierarchy does not fit in this pattern, you must use
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<a href="User-GC.html#User-GC">User GC</a> instead.
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</p>
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<p>The base class and its discriminator must be identified using the “desc”
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option. Each concrete subclass must use the “tag” option to identify
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which value of the discriminator it corresponds to.
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</p>
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<p>Every class in the hierarchy must have a <code>GTY(())</code> marker, as
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gengtype will only attempt to parse classes that have such a marker
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<a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.
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</p>
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<div class="smallexample">
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<pre class="smallexample">class GTY((desc("%h.kind"), tag("0"))) example_base
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{
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public:
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int kind;
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tree a;
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};
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class GTY((tag("1"))) some_subclass : public example_base
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{
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public:
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tree b;
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};
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class GTY((tag("2"))) some_other_subclass : public example_base
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{
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public:
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tree c;
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};
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</pre></div>
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<p>The generated marking routines for the above will contain a “switch”
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on “kind”, visiting all appropriate fields. For example, if kind is
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2, it will cast to “some_other_subclass” and visit fields a, b, and c.
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</p>
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<div class="footnote">
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<hr>
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<h4 class="footnotes-heading">Footnotes</h4>
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<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
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<p>Classes lacking such a marker will not be identified as being
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part of the hierarchy, and so the marking routines will not handle them,
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leading to a assertion failure within the marking routines due to an
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unknown tag value (assuming that assertions are enabled).</p>
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</div>
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</body>
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</html>
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