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<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<title>Bison 3.0.4: Printer Decl</title>

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<body lang="en">
<a name="Printer-Decl"></a>
<div class="header">
<p>
Next: <a href="Expect-Decl.html#Expect-Decl" accesskey="n" rel="next">Expect Decl</a>, Previous: <a href="Destructor-Decl.html#Destructor-Decl" accesskey="p" rel="prev">Destructor Decl</a>, Up: <a href="Declarations.html#Declarations" accesskey="u" rel="up">Declarations</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="Printing-Semantic-Values"></a>
<h4 class="subsection">3.7.7 Printing Semantic Values</h4>
<a name="index-printing-semantic-values"></a>
<a name="index-_0025printer"></a>
<a name="index-_003c_002a_003e-1"></a>
<a name="index-_003c_003e-1"></a>
<p>When run-time traces are enabled (see <a href="Tracing.html#Tracing">Tracing Your Parser</a>),
the parser reports its actions, such as reductions.  When a symbol involved
in an action is reported, only its kind is displayed, as the parser cannot
know how semantic values should be formatted.
</p>
<p>The <code>%printer</code> directive defines code that is called when a symbol is
reported.  Its syntax is the same as <code>%destructor</code> (see <a href="Destructor-Decl.html#Destructor-Decl">Freeing Discarded Symbols</a>).
</p>
<dl>
<dt><a name="index-_0025printer-1"></a>Directive: <strong>%printer</strong> <em>{ <var>code</var> } <var>symbols</var></em></dt>
<dd><a name="index-_0025printer-2"></a>
<a name="index-yyoutput"></a>
<p>Invoke the braced <var>code</var> whenever the parser displays one of the
<var>symbols</var>.  Within <var>code</var>, <code>yyoutput</code> denotes the output stream
(a <code>FILE*</code> in C, and an <code>std::ostream&amp;</code> in C++), <code>$$</code> (or
<code>$&lt;<var>tag</var>&gt;$</code>) designates the semantic value associated with the
symbol, and <code>@$</code> its location.  The additional parser parameters are
also available (see <a href="Parser-Function.html#Parser-Function">The Parser Function
<code>yyparse</code></a>).
</p>
<p>The <var>symbols</var> are defined as for <code>%destructor</code> (see <a href="Destructor-Decl.html#Destructor-Decl">Freeing Discarded Symbols</a>.): they can be per-type (e.g.,
&lsquo;<samp>&lt;ival&gt;</samp>&rsquo;), per-symbol (e.g., &lsquo;<samp>exp</samp>&rsquo;, &lsquo;<samp>NUM</samp>&rsquo;, &lsquo;<samp>&quot;float&quot;</samp>&rsquo;),
typed per-default (i.e., &lsquo;<samp>&lt;*&gt;</samp>&rsquo;, or untyped per-default (i.e.,
&lsquo;<samp>&lt;&gt;</samp>&rsquo;).
</p></dd></dl>

<p>For example:
</p>
<div class="example">
<pre class="example">%union { char *string; }
%token &lt;string&gt; STRING1 STRING2
%type  &lt;string&gt; string1 string2
%union { char character; }
%token &lt;character&gt; CHR
%type  &lt;character&gt; chr
%token TAGLESS

%printer { fprintf (yyoutput, &quot;'%c'&quot;, $$); } &lt;character&gt;
%printer { fprintf (yyoutput, &quot;&amp;%p&quot;, $$); } &lt;*&gt;
%printer { fprintf (yyoutput, &quot;\&quot;%s\&quot;&quot;, $$); } STRING1 string1
%printer { fprintf (yyoutput, &quot;&lt;&gt;&quot;); } &lt;&gt;
</pre></div>

<p>guarantees that, when the parser print any symbol that has a semantic type
tag other than <code>&lt;character&gt;</code>, it display the address of the semantic
value by default.  However, when the parser displays a <code>STRING1</code> or a
<code>string1</code>, it formats it as a string in double quotes.  It performs
only the second <code>%printer</code> in this case, so it prints only once.
Finally, the parser print &lsquo;<samp>&lt;&gt;</samp>&rsquo; for any symbol, such as <code>TAGLESS</code>,
that has no semantic type tag.  See <a href="Mfcalc-Traces.html#Mfcalc-Traces">Enabling Debug Traces
for <code>mfcalc</code></a>, for a complete example.
</p>


<hr>
<div class="header">
<p>
Next: <a href="Expect-Decl.html#Expect-Decl" accesskey="n" rel="next">Expect Decl</a>, Previous: <a href="Destructor-Decl.html#Destructor-Decl" accesskey="p" rel="prev">Destructor Decl</a>, Up: <a href="Declarations.html#Declarations" accesskey="u" rel="up">Declarations</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>
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