Inheritance-and-GTY.html 5.5 KB

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  64. <p>
  65. 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>
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  68. <a name="Support-for-inheritance"></a>
  69. <h3 class="section">22.2 Support for inheritance</h3>
  70. <p>gengtype has some support for simple class hierarchies. You can use
  71. this to have gengtype autogenerate marking routines, provided:
  72. </p>
  73. <ul>
  74. <li> There must be a concrete base class, with a discriminator expression
  75. that can be used to identify which subclass an instance is.
  76. </li><li> Only single inheritance is used.
  77. </li><li> None of the classes within the hierarchy are templates.
  78. </li></ul>
  79. <p>If your class hierarchy does not fit in this pattern, you must use
  80. <a href="User-GC.html#User-GC">User GC</a> instead.
  81. </p>
  82. <p>The base class and its discriminator must be identified using the &ldquo;desc&rdquo;
  83. option. Each concrete subclass must use the &ldquo;tag&rdquo; option to identify
  84. which value of the discriminator it corresponds to.
  85. </p>
  86. <p>Every class in the hierarchy must have a <code>GTY(())</code> marker, as
  87. gengtype will only attempt to parse classes that have such a marker
  88. <a name="DOCF6" href="#FOOT6"><sup>6</sup></a>.
  89. </p>
  90. <div class="smallexample">
  91. <pre class="smallexample">class GTY((desc(&quot;%h.kind&quot;), tag(&quot;0&quot;))) example_base
  92. {
  93. public:
  94. int kind;
  95. tree a;
  96. };
  97. class GTY((tag(&quot;1&quot;))) some_subclass : public example_base
  98. {
  99. public:
  100. tree b;
  101. };
  102. class GTY((tag(&quot;2&quot;))) some_other_subclass : public example_base
  103. {
  104. public:
  105. tree c;
  106. };
  107. </pre></div>
  108. <p>The generated marking routines for the above will contain a &ldquo;switch&rdquo;
  109. on &ldquo;kind&rdquo;, visiting all appropriate fields. For example, if kind is
  110. 2, it will cast to &ldquo;some_other_subclass&rdquo; and visit fields a, b, and c.
  111. </p>
  112. <div class="footnote">
  113. <hr>
  114. <h4 class="footnotes-heading">Footnotes</h4>
  115. <h3><a name="FOOT6" href="#DOCF6">(6)</a></h3>
  116. <p>Classes lacking such a marker will not be identified as being
  117. part of the hierarchy, and so the marking routines will not handle them,
  118. leading to a assertion failure within the marking routines due to an
  119. unknown tag value (assuming that assertions are enabled).</p>
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