97 lines
5.9 KiB
HTML
97 lines
5.9 KiB
HTML
<html>
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<title>The Select Parser</title>
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<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2">
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<td width="10"> </td>
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<td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Select Parser </b></font></td>
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<td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<p>Select parsers may be used to identify a single parser from a given list
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of parsers, which successfully recognizes the current input sequence. Example:</p>
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<pre> rule<span class="special"><></span> rule_select <span class="special">=</span>
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select_p<span class="special">
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(</span>
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parser_a<span class="special">
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,</span> parser_b<span class="special">
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<span class="comment">/* ... */</span>
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,</span> parser_n
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<span class="special">)</span><span class="special">;</span></pre>
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<p>The parsers (parser_a, parser_b etc.) are tried sequentially from left to right until a parser matches the current input sequence.
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If there is a matching parser found, the <tt>select_p</tt> parser returns
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the parser's position (zero based index). For instance, in the example above, <tt>1</tt> is returned if parser_b
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matches.</p>
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<p>There are two predefined parsers of the select parser family: <tt>select_p</tt>
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and <tt>select_fail_p</tt>. These parsers differ in the way the no match
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case is handled (when none of the parsers match the current input sequence).
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While the <tt>select_p</tt> parser will return <tt>-1</tt>
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if no matching parser is found, the <tt>select_fail_p</tt> parser will not match
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at all.</p>
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<p>The following sample shows how the select parser may be used very conveniently
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in conjunction with a <a href="switch_parser.html">switch parser</a>:</p>
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<pre> <span class="keyword">int</span> choice <span class="special">=</span> <span class="literal">-1</span><span class="special">;</span>
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rule<span class="special"><></span> rule_select <span class="special">=</span>
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select_fail_p<span class="special">(</span><span class="literal">'a'</span><span class="special">,</span> <span class="literal">'b'</span><span class="special">,</span> <span class="literal">'c'</span><span class="special">,</span> <span class="literal">'d'</span><span class="special">)[</span>assign_a<span class="special">(</span>choice<span class="special">)]</span>
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>> switch_p(var<span class="special">(</span>choice)) <span class="special">
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[</span><br> case_p<span class="special"><</span><span class="literal">0</span><span class="special">>(</span>int_p<span class="special">),</span><br> case_p<span class="special"><</span><span class="literal">1</span><span class="special">>(</span>ch_p<span class="special">(</span><span class="literal">','</span><span class="special">)),</span><br> case_p<span class="special"><</span><span class="literal">2</span><span class="special">>(</span>str_p<span class="special">(</span><span class="string">"bcd"</span><span class="special">)),</span><br> default_p<br> <span class="special">]</span><br><span class="special"> ;</span></pre>
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<p>This example shows a rule, which matches:</p>
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<ul>
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<li><span class="literal"> 'a' </span>followed
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by an integer</li>
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<li><span class="literal">'b' </span>followed by a<span class="literal">
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','</span></li>
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<li><span class="literal">'c'</span> followed by <span class="string">"bcd"</span></li>
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<li>a single <span class="literal">'d'</span>. </li>
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</ul>
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<p>For other input sequences the
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give rule does not match at all.</p>
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<table width="80%" border="0" align="center">
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<tr>
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<td class="note_box"><p><img src="theme/alert.gif" width="16" height="16"> <tt>BOOST_SPIRIT_SELECT_LIMIT</tt><br>
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<br>
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The number of possible entries inside the <tt>select_p</tt> parser is limited by the Spirit compile time constant <tt>BOOST_SPIRIT_SELECT_LIMIT</tt>, which defaults to 3. This value should not be greater than the compile time constant given by <tt>PHOENIX_LIMIT</tt> (see <a href="../phoenix/index.html">phoenix</a>). Example:</p>
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<p class="style1"><span class="comment">// Define these before including anything else <br>
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</span><span class="style3">#define</span> PHOENIX_LIMIT 10<br>
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<span class="preprocessor">#define</span> BOOST_SPIRIT_SELECT_LIMIT 10 </p></td>
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</tr>
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</table>
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<br>
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<table border="0">
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<tr>
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<td width="10"></td>
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<td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td>
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<td width="30"><a href="the_lazy_parser.html"><img src="theme/l_arr.gif" border="0"></a></td>
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<td width="30"><a href="switch_parser.html"><img src="theme/r_arr.gif" border="0"></a></td>
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</tr>
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</table>
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<br>
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<hr size="1">
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<p class="copyright">Copyright © 2003-2004 Hartmut Kaiser <br>
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<br>
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<font size="2">Use, modification and distribution is subject to the Boost Software License, Version 1.0. (See accompanying file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt) </font> </p>
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<p> </p>
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