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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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| <p>Select parsers may be used to identify a single parser from a given list |
| of parsers, which successfully recognizes the current input sequence. Example:</p> |
| <pre> rule<span class="special"><></span> rule_select <span class="special">=</span> |
| select_p<span class="special"> |
| (</span> |
| parser_a<span class="special"> |
| ,</span> parser_b<span class="special"> |
| <span class="comment">/* ... */</span> |
| ,</span> parser_n |
| <span class="special">)</span><span class="special">;</span></pre> |
| <p>The parsers (parser_a, parser_b etc.) are tried sequentially from left to right until a parser matches the current input sequence. |
| If there is a matching parser found, the <tt>select_p</tt> parser returns |
| the parser's position (zero based index). For instance, in the example above, <tt>1</tt> is returned if parser_b |
| matches.</p> |
| <p>There are two predefined parsers of the select parser family: <tt>select_p</tt> |
| and <tt>select_fail_p</tt>. These parsers differ in the way the no match |
| case is handled (when none of the parsers match the current input sequence). |
| While the <tt>select_p</tt> parser will return <tt>-1</tt> |
| if no matching parser is found, the <tt>select_fail_p</tt> parser will not match |
| at all.</p> |
| <p>The following sample shows how the select parser may be used very conveniently |
| in conjunction with a <a href="switch_parser.html">switch parser</a>:</p> |
| <pre> <span class="keyword">int</span> choice <span class="special">=</span> <span class="literal">-1</span><span class="special">;</span> |
| rule<span class="special"><></span> rule_select <span class="special">=</span> |
| 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> |
| >> switch_p(var<span class="special">(</span>choice)) <span class="special"> |
| [</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> |
| <p>This example shows a rule, which matches:</p> |
| <ul> |
| <li><span class="literal"> 'a' </span>followed |
| by an integer</li> |
| <li><span class="literal">'b' </span>followed by a<span class="literal"> |
| ','</span></li> |
| <li><span class="literal">'c'</span> followed by <span class="string">"bcd"</span></li> |
| <li>a single <span class="literal">'d'</span>. </li> |
| </ul> |
| <p>For other input sequences the |
| give rule does not match at all.</p> |
| <table width="80%" border="0" align="center"> |
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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> |
| <br> |
| 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> |
| <p class="style1"><span class="comment">// Define these before including anything else <br> |
| </span><span class="style3">#define</span> PHOENIX_LIMIT 10<br> |
| <span class="preprocessor">#define</span> BOOST_SPIRIT_SELECT_LIMIT 10 </p></td> |
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| <p class="copyright">Copyright © 2003-2004 Hartmut Kaiser <br> |
| <br> |
| <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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