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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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| 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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| <a name="Support-for-inheritance"></a> |
| <h3 class="section">22.2 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 “desc” |
| option. Each concrete subclass must use the “tag” 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("%h.kind"), tag("0"))) example_base |
| { |
| public: |
| int kind; |
| tree a; |
| }; |
| |
| class GTY((tag("1")) some_subclass : public example_base |
| { |
| public: |
| tree b; |
| }; |
| |
| class GTY((tag("2")) some_other_subclass : public example_base |
| { |
| public: |
| tree c; |
| }; |
| </pre></div> |
| |
| <p>The generated marking routines for the above will contain a “switch” |
| on “kind”, visiting all appropriate fields. For example, if kind is |
| 2, it will cast to “some_other_subclass” 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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