1<html> 2<head> 3<title>The Switch 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.style3 {font-family: "Courier New", Courier, mono; color: #FF0000; } 10--> 11</style> 12</head> 13 14<body> 15<table width="100%" border="0" background="theme/bkd2.gif" cellspacing="2"> 16 <tr> 17 <td width="10"> </td> 18 <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Switch Parser </b></font></td> 19 <td width="112"><a href="http://spirit.sf.net"><img src="theme/spirit.gif" width="112" height="48" align="right" border="0"></a></td> 20 </tr> 21</table> 22<br> 23<table border="0"> 24 <tr> 25 <td width="10"></td> 26 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 27 <td width="30"><a href="select_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 28 <td width="30"><a href="escape_char_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 29 </tr> 30</table> 31<p>Switch parsers may be used to simplify certain alternation constructs. Consider the following code:</p> 32<pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> 33 ch_p<span class="special">(</span><span class="literal">'a'</span><span class="special">)</span> <span class="special">>></span> parser_a 34 <span class="special">|</span> ch_p<span class="special">(</span><span class="literal">'b'</span><span class="special">)</span> <span class="special">>></span> parser_b 35 <span class="comment">// ...</span> 36 <span class="special">|</span> ch_p<span class="special">(</span><span class="literal">'n'</span><span class="special">)</span> <span class="special">>></span> parser_n 37 <span class="special">;</span></pre> 38<p>Each of the alternatives are evaluated normally in a sequential manner. This tend to be inefficient, especially for a large number of alternatives. To avoid this inefficiency and to make it possible to write such constructs in a more readable form, Spirit contains the <tt>switch_p</tt> family of parsers. The switch_p parser allows us to rewrite the previous construct as:</p> 39<pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> 40 switch_p 41 <span class="special">[</span> 42 case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> 43 case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> 44 <span class="comment"> // ...</span> 45 case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">)</span> 46 ] 47 ;</pre> 48<p>This <tt>switch_p</tt> parser takes the next character (or token) from the input stream and tries to match it against the given integral compile time constants supplied as the template parameters to the <tt>case_p</tt> parsers. If this character matches one of the <tt>case_p</tt> branches, the associated parser is executed (i.e. if 'a' is matched, <tt>parser_a</tt> is executed, if 'b' is matched, <tt>parser_b</tt> is executed and so on) . If no <tt>case_p</tt> branch matches the next input character, the overall construct does not match at all. </p> 49<table width="80%" border="0" align="center"> 50 <tr> 51 <td class="note_box"><div align="justify"><img src="theme/bulb.gif" width="13" height="18"><strong> Nabialek trick </strong><br> 52 <br> 53 The <strong><em><a href="techniques.html#nabialek_trick">"Nabialek trick" </a></em></strong>(from the name of its inventor, Sam Nabialek), can also improve the rule dispatch from linear non-deterministic to deterministic. This is similar to the <tt>switch_p</tt> parser, yet, can handle grammars where a keyword (operator, etc), instead of a single character or token, precedes a production.</div></td> 54 </tr> 55</table> 56<p>Sometimes it is desirable to add handling of the default case (none of the <tt>case_p</tt> branches matched). This may be achieved with the help of a <tt>default_p</tt> branch:</p> 57<pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> 58 switch_p 59 <span class="special">[</span> 60 case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> 61 case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> 62 <span class="comment"> // ...</span> 63 case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">),</span> 64 default_p<span class="special">(</span>parser_default<span class="special">)</span> 65 <span class="special">] 66 ;</span></pre> 67<p>This form chooses the <tt>parser_default</tt> parser if none of the cases matches the next character from the input stream. Please note that, obviously, only one <tt>default_p</tt> branch may be added to the <tt>switch_p</tt> parser construct. </p> 68<p>Moreover, it is possible to omit the parentheses and body from the <tt>default_p</tt> construct, in which case, no additional parser is executed and the overall <tt>switch_p</tt> construct simply returns a match on any character of the input stream, which does not match any of the <tt>case_p</tt> branches:</p> 69<pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> 70 switch_p 71 <span class="special">[</span> 72 case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> 73 case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> 74 <span class="comment">// ...</span> 75 case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">),</span> 76 default_p 77 <span class="special">]</span> 78 ;</pre> 79<p>There is another form of the switch_p construct. This form allows us to explicitly specify the value to be used for matching against the <tt>case_p</tt> branches: </p> 80<pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> 81 switch_p<span class="special">(</span>cond<span class="special">)</span> 82 <span class="special">[</span> 83 case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> 84 case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> 85 <span class="comment"> // ...</span> 86 case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">)</span> 87 <span class="special">]</span> 88 ;</pre> 89<p>where <tt>cond</tt> is a parser or a nullary function or function object (functor). If it is a parser, then it is tried and its return value is used to match against the <tt>case_p</tt> branches. If it is a nullary function or functor, then its return value will be used. </p> 90<p>Please note that during its compilation, the <tt>switch_p</tt> construct is transformed into a real C++ <tt>switch</tt> statement. This makes the runtime execution very efficient. </p> 91<table width="80%" border="0" align="center"> 92 <tr> 93 <td class="note_box"><p><img src="theme/alert.gif" width="16" height="16"> <tt>BOOST_SPIRIT_SWITCH_CASE_LIMIT</tt><br> 94 <br> 95 The number of possible <tt>case_p</tt>/<tt>default_p</tt> branches is limited by the Spirit compile time constant <tt>BOOST_SPIRIT_SWITCH_CASE_LIMIT</tt>, which defaults to 3. There is no theoretical upper limit for this constant, but most compilers won't allow you to specify a very large number.</p> 96 <p>Example:</p> 97 <p class="style1"><span class="comment">// Define these before including switch.hpp <br> 98 </span><span class="preprocessor">#define</span> BOOST_SPIRIT_SWITCH_CASE_LIMIT 10 </p></td> 99 </tr> 100</table><br> 101<table border="0"> 102 <tr> 103 <td width="10"></td> 104 <td width="30"><a href="../index.html"><img src="theme/u_arr.gif" border="0"></a></td> 105 <td width="30"><a href="select_parser.html"><img src="theme/l_arr.gif" border="0"></a></td> 106 <td width="30"><a href="escape_char_parser.html"><img src="theme/r_arr.gif" border="0"></a></td> 107 </tr> 108</table> 109<br> 110<hr size="1"> 111<p class="copyright">Copyright © 2003-2004 Hartmut Kaiser <br> 112 <br> 113 <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> 114</body> 115</html> 116
