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| 3 | <title>The Switch Parser</title> |
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| 17 | <td width="10"> </td> |
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| 18 | <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Switch Parser </b></font></td> |
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| 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> |
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| 30 | </table> |
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| 31 | <p>Switch parsers may be used to simplify certain alternation constructs. Consider the following code:</p> |
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| 32 | <pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> |
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| 33 | ch_p<span class="special">(</span><span class="literal">'a'</span><span class="special">)</span> <span class="special">>></span> parser_a |
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| 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 |
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| 35 | <span class="comment">// ...</span> |
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| 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 |
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| 37 | <span class="special">;</span></pre> |
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| 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> |
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| 39 | <pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> |
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| 40 | switch_p |
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| 41 | <span class="special">[</span> |
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| 42 | case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> |
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| 43 | case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> |
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| 44 | <span class="comment"> // ...</span> |
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| 45 | case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">)</span> |
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| 46 | ] |
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| 47 | ;</pre> |
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| 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> |
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| 49 | <table width="80%" border="0" align="center"> |
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| 50 | <tr> |
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| 51 | <td class="note_box"><div align="justify"><img src="theme/bulb.gif" width="13" height="18"><strong> Nabialek trick </strong><br> |
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| 52 | <br> |
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| 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> |
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| 54 | </tr> |
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| 55 | </table> |
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| 56 | <p>Sometimes it is desireable 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> |
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| 57 | <pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> |
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| 58 | switch_p |
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| 59 | <span class="special">[</span> |
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| 60 | case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> |
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| 61 | case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> |
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| 62 | <span class="comment"> // ...</span> |
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| 63 | case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">),</span> |
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| 64 | default_p<span class="special">(</span>parser_default<span class="special">)</span> |
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| 65 | <span class="special">] |
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| 66 | ;</span></pre> |
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| 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> |
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| 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> |
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| 69 | <pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> |
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| 70 | switch_p |
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| 71 | <span class="special">[</span> |
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| 72 | case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> |
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| 73 | case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> |
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| 74 | <span class="comment">// ...</span> |
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| 75 | case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">),</span> |
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| 76 | default_p |
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| 77 | <span class="special">]</span> |
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| 78 | ;</pre> |
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| 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> |
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| 80 | <pre> rule<span class="special"><></span> rule_overall <span class="special">=</span> |
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| 81 | switch_p<span class="special">(</span>cond<span class="special">)</span> |
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| 82 | <span class="special">[</span> |
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| 83 | case_p<span class="special"><</span><span class="literal">'a'</span><span class="special">>(</span>parser_a<span class="special">),</span> |
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| 84 | case_p<span class="special"><</span><span class="literal">'b'</span><span class="special">>(</span>parser_b<span class="special">),</span> |
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| 85 | <span class="comment"> // ...</span> |
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| 86 | case_p<span class="special"><</span><span class="literal">'n'</span><span class="special">>(</span>parser_n<span class="special">)</span> |
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| 87 | <span class="special">]</span> |
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| 88 | ;</pre> |
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| 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> |
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| 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> |
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| 91 | <table width="80%" border="0" align="center"> |
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| 92 | <tr> |
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| 93 | <td class="note_box"><p><img src="theme/alert.gif" width="16" height="16"> <tt>BOOST_SPIRIT_SWITCH_CASE_LIMIT</tt><br> |
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| 94 | <br> |
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| 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> |
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| 96 | <p>Example:</p> |
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| 97 | <p class="style1"><span class="comment">// Define these before including switch.hpp <br> |
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| 98 | </span><span class="preprocessor">#define</span> BOOST_SPIRIT_SWITCH_CASE_LIMIT 10 </p></td> |
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| 99 | </tr> |
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| 100 | </table><br> |
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| 101 | <table border="0"> |
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| 103 | <td width="10"></td> |
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| 109 | <br> |
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| 110 | <hr size="1"> |
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| 111 | <p class="copyright">Copyright © 2003-2004 Hartmut Kaiser <br> |
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| 112 | <br> |
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| 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> |
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