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<title>The Switch Parser</title>
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    <td width="10"> </td>
    <td width="85%"> <font size="6" face="Verdana, Arial, Helvetica, sans-serif"><b>The Switch Parser </b></font></td>
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<p>Switch parsers may be used to simplify certain alternation constructs. Consider the following code:</p>
<pre>    rule<span class="special">&lt;&gt;</span> rule_overall <span class="special">=</span>
            ch_p<span class="special">(</span><span class="literal">'a'</span><span class="special">)</span> <span class="special">&gt;&gt;</span> parser_a
        <span class="special">|</span>   ch_p<span class="special">(</span><span class="literal">'b'</span><span class="special">)</span> <span class="special">&gt;&gt;</span> parser_b
            <span class="comment">// ...</span>
        <span class="special">|</span>   ch_p<span class="special">(</span><span class="literal">'n'</span><span class="special">)</span> <span class="special">&gt;&gt;</span> parser_n
        <span class="special">;</span></pre>
<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>
<pre>    rule<span class="special">&lt;&gt;</span> rule_overall <span class="special">=</span>
            switch_p
            <span class="special">[</span>
                case_p<span class="special">&lt;</span><span class="literal">'a'</span><span class="special">&gt;(</span>parser_a<span class="special">),</span>
                case_p<span class="special">&lt;</span><span class="literal">'b'</span><span class="special">&gt;(</span>parser_b<span class="special">),</span>
            <span class="comment">    // ...</span>
                case_p<span class="special">&lt;</span><span class="literal">'n'</span><span class="special">&gt;(</span>parser_n<span class="special">)</span>
            ]
        ;</pre>
<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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    <td class="note_box"><div align="justify"><img src="theme/bulb.gif" width="13" height="18"><strong> Nabialek trick </strong><br>
            <br>
        The <strong><em><a href="techniques.html#nabialek_trick">&quot;Nabialek trick&quot; </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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<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>
<pre>    rule<span class="special">&lt;&gt;</span> rule_overall <span class="special">=</span>
            switch_p
            <span class="special">[</span>
                case_p<span class="special">&lt;</span><span class="literal">'a'</span><span class="special">&gt;(</span>parser_a<span class="special">),</span>
                case_p<span class="special">&lt;</span><span class="literal">'b'</span><span class="special">&gt;(</span>parser_b<span class="special">),</span>
            <span class="comment">    // ...</span>
                case_p<span class="special">&lt;</span><span class="literal">'n'</span><span class="special">&gt;(</span>parser_n<span class="special">),</span>
                default_p<span class="special">(</span>parser_default<span class="special">)</span>
            <span class="special">]
        ;</span></pre>
<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>
<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>
<pre>    rule<span class="special">&lt;&gt;</span> rule_overall <span class="special">=</span>
            switch_p
            <span class="special">[</span>
                case_p<span class="special">&lt;</span><span class="literal">'a'</span><span class="special">&gt;(</span>parser_a<span class="special">),</span>
                case_p<span class="special">&lt;</span><span class="literal">'b'</span><span class="special">&gt;(</span>parser_b<span class="special">),</span>
                <span class="comment">// ...</span>
                case_p<span class="special">&lt;</span><span class="literal">'n'</span><span class="special">&gt;(</span>parser_n<span class="special">),</span>
                default_p
            <span class="special">]</span>
        ;</pre>
<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>
<pre>    rule<span class="special">&lt;&gt;</span> rule_overall <span class="special">=</span>
            switch_p<span class="special">(</span>cond<span class="special">)</span>
            <span class="special">[</span>
                case_p<span class="special">&lt;</span><span class="literal">'a'</span><span class="special">&gt;(</span>parser_a<span class="special">),</span>
                case_p<span class="special">&lt;</span><span class="literal">'b'</span><span class="special">&gt;(</span>parser_b<span class="special">),</span>
            <span class="comment">    // ...</span>
                case_p<span class="special">&lt;</span><span class="literal">'n'</span><span class="special">&gt;(</span>parser_n<span class="special">)</span>
            <span class="special">]</span>
        ;</pre>
<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>
<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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    <td class="note_box"><p><img src="theme/alert.gif" width="16" height="16"> <tt>BOOST_SPIRIT_SWITCH_CASE_LIMIT</tt><br>
        <br>
    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>
    <p>Example:</p>
    <p class="style1"><span class="comment">// Define these before including switch.hpp <br>
      </span><span class="preprocessor">#define</span> BOOST_SPIRIT_SWITCH_CASE_LIMIT 10 </p></td>
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<p class="copyright">Copyright &copy; 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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