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<title>GNU Compiler Collection (GCC) Internals: Inheritance and GTY</title>
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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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<hr>
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<a name="Support-for-inheritance"></a>
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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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</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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