projects / ceph.git / blame
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1 | <html> |
| 2 | <head> | |
| 3 | <title>The Select Parser</title> | |
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| 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> |