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<title>Bison 3.0.4: Union Decl</title>

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<a name="Union-Decl"></a>
<div class="header">
<p>
Next: <a href="Structured-Value-Type.html#Structured-Value-Type" accesskey="n" rel="next">Structured Value Type</a>, Previous: <a href="Type-Generation.html#Type-Generation" accesskey="p" rel="prev">Type Generation</a>, Up: <a href="Semantics.html#Semantics" accesskey="u" rel="up">Semantics</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Index-of-Terms.html#Index-of-Terms" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="The-Union-Declaration"></a>
<h4 class="subsection">3.4.4 The Union Declaration</h4>
<a name="index-declaring-value-types-1"></a>
<a name="index-value-types_002c-declaring-1"></a>
<a name="index-_0025union"></a>

<p>The <code>%union</code> declaration specifies the entire collection of possible
data types for semantic values.  The keyword <code>%union</code> is followed by
braced code containing the same thing that goes inside a <code>union</code> in C.
</p>
<p>For example:
</p>
<div class="example">
<pre class="example">%union {
  double val;
  symrec *tptr;
}
</pre></div>

<p>This says that the two alternative types are <code>double</code> and <code>symrec
*</code>.  They are given names <code>val</code> and <code>tptr</code>; these names are used
in the <code>%token</code> and <code>%type</code> declarations to pick one of the types
for a terminal or nonterminal symbol (see <a href="Type-Decl.html#Type-Decl">Nonterminal Symbols</a>).
</p>
<p>As an extension to POSIX, a tag is allowed after the <code>%union</code>.  For
example:
</p>
<div class="example">
<pre class="example">%union value {
  double val;
  symrec *tptr;
}
</pre></div>

<p>specifies the union tag <code>value</code>, so the corresponding C type is
<code>union value</code>.  If you do not specify a tag, it defaults to
<code>YYSTYPE</code> (see <a href="_0025define-Summary.html#g_t_0025define-Summary">api.value.union.name</a>).
</p>
<p>As another extension to POSIX, you may specify multiple <code>%union</code>
declarations; their contents are concatenated.  However, only the first
<code>%union</code> declaration can specify a tag.
</p>
<p>Note that, unlike making a <code>union</code> declaration in C, you need not write
a semicolon after the closing brace.
</p>



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