visus
/
toolchain-gcc-arm-embedded

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<!-- Copyright (C) 1988-2020 Free Software Foundation, Inc.

Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with the
Invariant Sections being "Funding Free Software", the Front-Cover
Texts being (a) (see below), and with the Back-Cover Texts being (b)
(see below).  A copy of the license is included in the section entitled
"GNU Free Documentation License".

(a) The FSF's Front-Cover Text is:

A GNU Manual

(b) The FSF's Back-Cover Text is:

You have freedom to copy and modify this GNU Manual, like GNU
     software.  Copies published by the Free Software Foundation raise
     funds for GNU development. -->
<!-- Created by GNU Texinfo 6.5, http://www.gnu.org/software/texinfo/ -->
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Inheritance and GTY (GNU Compiler Collection (GCC) Internals)</title>

<meta name="description" content="Inheritance and GTY (GNU Compiler Collection (GCC) Internals)">
<meta name="keywords" content="Inheritance and GTY (GNU Compiler Collection (GCC) Internals)">
<meta name="resource-type" content="document">
<meta name="distribution" content="global">
<meta name="Generator" content="makeinfo">
<link href="index.html#Top" rel="start" title="Top">
<link href="Option-Index.html#Option-Index" rel="index" title="Option Index">
<link href="index.html#SEC_Contents" rel="contents" title="Table of Contents">
<link href="Type-Information.html#Type-Information" rel="up" title="Type Information">
<link href="User-GC.html#User-GC" rel="next" title="User GC">
<link href="GTY-Options.html#GTY-Options" rel="prev" title="GTY Options">
<style type="text/css">
<!--
a.summary-letter {text-decoration: none}
blockquote.indentedblock {margin-right: 0em}
blockquote.smallindentedblock {margin-right: 0em; font-size: smaller}
blockquote.smallquotation {font-size: smaller}
div.display {margin-left: 3.2em}
div.example {margin-left: 3.2em}
div.lisp {margin-left: 3.2em}
div.smalldisplay {margin-left: 3.2em}
div.smallexample {margin-left: 3.2em}
div.smalllisp {margin-left: 3.2em}
kbd {font-style: oblique}
pre.display {font-family: inherit}
pre.format {font-family: inherit}
pre.menu-comment {font-family: serif}
pre.menu-preformatted {font-family: serif}
pre.smalldisplay {font-family: inherit; font-size: smaller}
pre.smallexample {font-size: smaller}
pre.smallformat {font-family: inherit; font-size: smaller}
pre.smalllisp {font-size: smaller}
span.nolinebreak {white-space: nowrap}
span.roman {font-family: initial; font-weight: normal}
span.sansserif {font-family: sans-serif; font-weight: normal}
ul.no-bullet {list-style: none}
-->
</style>


</head>

<body lang="en">
<a name="Inheritance-and-GTY"></a>
<div class="header">
<p>
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> &nbsp; [<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>
</div>
<hr>
<a name="Support-for-inheritance"></a>
<h3 class="section">23.2 Support for inheritance</h3>
<p>gengtype has some support for simple class hierarchies.  You can use
this to have gengtype autogenerate marking routines, provided:
</p>
<ul>
<li> There must be a concrete base class, with a discriminator expression
that can be used to identify which subclass an instance is.
</li><li> Only single inheritance is used.
</li><li> None of the classes within the hierarchy are templates.
</li></ul>

<p>If your class hierarchy does not fit in this pattern, you must use
<a href="User-GC.html#User-GC">User GC</a> instead.
</p>
<p>The base class and its discriminator must be identified using the &ldquo;desc&rdquo;
option.  Each concrete subclass must use the &ldquo;tag&rdquo; option to identify
which value of the discriminator it corresponds to.
</p>
<p>Every class in the hierarchy must have a <code>GTY(())</code> marker, as
gengtype will only attempt to parse classes that have such a marker
<a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.
</p>
<div class="smallexample">
<pre class="smallexample">class GTY((desc(&quot;%h.kind&quot;), tag(&quot;0&quot;))) example_base
{
public:
    int kind;
    tree a;
};

class GTY((tag(&quot;1&quot;))) some_subclass : public example_base
{
public:
    tree b;
};

class GTY((tag(&quot;2&quot;))) some_other_subclass : public example_base
{
public:
    tree c;
};
</pre></div>

<p>The generated marking routines for the above will contain a &ldquo;switch&rdquo;
on &ldquo;kind&rdquo;, visiting all appropriate fields.  For example, if kind is
2, it will cast to &ldquo;some_other_subclass&rdquo; and visit fields a, b, and c.
</p>
<div class="footnote">
<hr>
<h4 class="footnotes-heading">Footnotes</h4>

<h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
<p>Classes lacking such a marker will not be identified as being
part of the hierarchy, and so the marking routines will not handle them,
leading to a assertion failure within the marking routines due to an
unknown tag value (assuming that assertions are enabled).</p>
</div>


</body>
</html>