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112 lines
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112 lines
4.3 KiB
HTML
<html lang="en">
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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="GNU Compiler Collection (GCC) Internals">
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<link title="Top" rel="start" href="index.html#Top">
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<link rel="up" href="Type-Information.html#Type-Information" title="Type Information">
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<link rel="prev" href="GTY-Options.html#GTY-Options" title="GTY Options">
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<link rel="next" href="User-GC.html#User-GC" title="User GC">
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Copyright (C) 1988-2015 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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</head>
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<body>
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<div class="node">
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<a name="Inheritance-and-GTY"></a>
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<p>
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Next: <a rel="next" accesskey="n" href="User-GC.html#User-GC">User GC</a>,
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Previous: <a rel="previous" accesskey="p" href="GTY-Options.html#GTY-Options">GTY Options</a>,
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Up: <a rel="up" accesskey="u" href="Type-Information.html#Type-Information">Type Information</a>
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<hr>
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</div>
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<h3 class="section">22.2 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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<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>Only single inheritance is used.
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<li>None of the classes within the hierarchy are templates.
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</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>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>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 rel="footnote" href="#fn-1" name="fnd-1"><sup>1</sup></a>.
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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>
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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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<div class="footnote">
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<hr>
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<h4>Footnotes</h4><p class="footnote"><small>[<a name="fn-1" href="#fnd-1">1</a>]</small> 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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<hr></div>
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</body></html>
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