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<title>Inheritance and GTY (GNU Compiler Collection (GCC) Internals)</title>

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<a name="Inheritance-and-GTY"></a>
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<p>
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<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>
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