1<html> 2<head> 3<title>The Select Parser</title> 4<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> 5<link rel="stylesheet" href="theme/style.css" type="text/css"> 6<style type="text/css"> 7<!-- 8.style1 {font-family: "Courier New", Courier, mono} 9.style2 {font-family: "Courier New", Courier, mono; font-style: italic; } 10.style3 {font-family: "Courier New", Courier, mono; color: #FF0000; } 11--> 12</style> 13</head> 14 15<body> 16<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 17 <tr> 18 <td width="10"> </td> 19 <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Select Parser </b></font></td> 20 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 21 </tr> 22</table> 23<br> 24<table border="0"> 25 <tr> 26 <td width="10"></td> 27 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 28 <td width="30"><a href="the_lazy_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 29 <td width="30"><a href="switch_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 30 </tr> 31</table> 32<p>Select parsers may be used to identify a single parser from a given list 33 of parsers, which successfully recognizes the current input sequence. Example:</p> 34<pre> rule<span class="special"><></span> rule_select <span class="special">=</span> 35 select_p<span class="special"> 36 (</span> 37 parser_a<span class="special"> 38 ,</span> parser_b<span class="special"> 39 <span class="comment">/* ... */</span> 40 ,</span> parser_n 41 <span class="special">)</span><span class="special">;</span></pre> 42<p>The parsers (parser_a, parser_b etc.) are tried sequentially from left to right until a parser matches the current input sequence. 43 If there is a matching parser found, the <tt>select_p</tt> parser returns 44 the parser's position (zero based index). For instance, in the example above, <tt>1</tt> is returned if parser_b 45 matches.</p> 46<p>There are two predefined parsers of the select parser family: <tt>select_p</tt> 47 and <tt>select_fail_p</tt>. These parsers differ in the way the no match 48 case is handled (when none of the parsers match the current input sequence). 49 While the <tt>select_p</tt> parser will return <tt>-1</tt> 50 if no matching parser is found, the <tt>select_fail_p</tt> parser will not match 51 at all.</p> 52<p>The following sample shows how the select parser may be used very conveniently 53 in conjunction with a <a href="switch_parser.html">switch parser</a>:</p> 54<pre> <span class="keyword">int</span> choice <span class="special">=</span> <span class="literal">-1</span><span class="special">;</span> 55 rule<span class="special"><></span> rule_select <span class="special">=</span> 56 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> 57 >> switch_p(var<span class="special">(</span>choice)) <span class="special"> 58 [</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> 59<p>This example shows a rule, which matches:</p> 60<ul> 61 <li><span class="literal"> 'a' </span>followed 62 by an integer</li> 63 <li><span class="literal">'b' </span>followed by a<span class="literal"> 64 ','</span></li> 65 <li><span class="literal">'c'</span> followed by <span class="string">"bcd"</span></li> 66 <li>a single <span class="literal">'d'</span>. </li> 67</ul> 68<p>For other input sequences the 69 give rule does not match at all.</p> 70<table width="80%" border="0" align="center"> 71 <tr> 72 <td class="note_box"><p><img src="theme/alert.gif" width="16" height="16"> <tt>BOOST_SPIRIT_SELECT_LIMIT</tt><br> 73 <br> 74 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> 75 <p class="style1"><span class="comment">// Define these before including anything else <br> 76 </span><span class="style3">#define</span> PHOENIX_LIMIT 10<br> 77 <span class="preprocessor">#define</span> BOOST_SPIRIT_SELECT_LIMIT 10 </p></td> 78 </tr> 79</table> 80<br> 81<table border="0"> 82 <tr> 83 <td width="10"></td> 84 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 85 <td width="30"><a href="the_lazy_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 86 <td width="30"><a href="switch_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 87 </tr> 88</table> 89<br> 90<hr size="1"> 91<p class="copyright">Copyright © 2003-2004 Hartmut Kaiser <br> 92 <br> 93 <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> 94<p> </p> 95</body> 96</html> 97
