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| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.qi.reference.directive.lexeme"></a><a class="link" href="lexeme.html" title="Inhibiting Skipping (lexeme[])">Inhibiting |
| Skipping (<code class="computeroutput"><span class="identifier">lexeme</span><span class="special">[]</span></code>)</a> |
| </h5></div></div></div> |
| <a name="spirit.qi.reference.directive.lexeme.description"></a><h6> |
| <a name="id884434"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.description">Description</a> |
| </h6> |
| <p> |
| The <code class="computeroutput"><span class="identifier">lexeme</span></code> directive |
| makes its subject a primitive. In a logical point of view, lexemes (and |
| primitives) are minimal atomic units (e.g. words, numbers, identifiers, |
| etc). These are the things that you'd normally put in the lexer (hinting |
| at the term "lexeme"), but in a lexer-less world, you put these |
| in a lexeme. Seeing its subject as a primitive, the <code class="computeroutput"><span class="identifier">lexeme</span></code> |
| directive does an initial pre-skip (as all primitives do) and turns off |
| white space skipping. |
| </p> |
| <p> |
| At the phrase level, the parser ignores white spaces, possibly including |
| comments. Use <code class="computeroutput"><span class="identifier">lexeme</span></code> |
| in situations where you want to work at the character level instead of |
| the phrase level. Parsers can be made to work at the character level |
| by enclosing the pertinent parts inside the <code class="computeroutput"><span class="identifier">lexeme</span></code> |
| directive. For example, here's a rule that parses integers: |
| </p> |
| <pre class="programlisting"><span class="identifier">integer</span> <span class="special">=</span> <span class="identifier">lexeme</span><span class="special">[</span> <span class="special">-(</span><span class="identifier">lit</span><span class="special">(</span><span class="char">'+'</span><span class="special">)</span> <span class="special">|</span> <span class="char">'-'</span><span class="special">)</span> <span class="special">>></span> <span class="special">+</span><span class="identifier">digit</span> <span class="special">];</span> |
| </pre> |
| <p> |
| The <code class="computeroutput"><span class="identifier">lexeme</span></code> directive |
| instructs its subject parser to work on the character level. Without |
| it, the <code class="computeroutput"><span class="identifier">integer</span></code> rule |
| would have allowed erroneous embedded white spaces in inputs such as |
| <code class="computeroutput"><span class="string">"1 2 345"</span></code> which |
| will be parsed as <code class="computeroutput"><span class="string">"12345"</span></code>. |
| </p> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Keep in mind that <code class="computeroutput"><span class="identifier">lexeme</span><span class="special">[]</span></code> pre-skips spaces. If this is not |
| desired, use the <a class="link" href="no_skip.html" title="Inhibiting Skipping Without Pre-skip (no_skip[])"><code class="computeroutput"><span class="identifier">no_skip</span></code></a> directive instead. |
| </p></td></tr> |
| </table></div> |
| <a name="spirit.qi.reference.directive.lexeme.header"></a><h6> |
| <a name="id884615"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/directive/lexeme.hpp> |
| </span><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">spirit</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">qi_lexeme</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| <p> |
| Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. |
| </p> |
| <a name="spirit.qi.reference.directive.lexeme.namespace"></a><h6> |
| <a name="id884689"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.namespace">Namespace</a> |
| </h6> |
| <div class="informaltable"><table class="table"> |
| <colgroup><col></colgroup> |
| <thead><tr><th> |
| <p> |
| Name |
| </p> |
| </th></tr></thead> |
| <tbody><tr><td> |
| <p> |
| <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">lexeme</span> <span class="comment">// |
| alias: boost::spirit::qi::lexeme</span></code> |
| </p> |
| </td></tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.directive.lexeme.model_of"></a><h6> |
| <a name="id884768"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.model_of">Model of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a> |
| </p></blockquote></div> |
| <div class="variablelist"> |
| <p class="title"><b>Notation</b></p> |
| <dl> |
| <dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code></span></dt> |
| <dd><p> |
| A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a>. |
| </p></dd> |
| </dl> |
| </div> |
| <a name="spirit.qi.reference.directive.lexeme.expression_semantics"></a><h6> |
| <a name="id884837"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.expression_semantics">Expression |
| Semantics</a> |
| </h6> |
| <p> |
| Semantics of an expression is defined only where it differs from, or |
| is not defined in <a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a>. |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Semantics |
| </p> |
| </th> |
| </tr></thead> |
| <tbody><tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">lexeme</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Pre-skip and turn off white space skipping for the subject |
| parser, <code class="computeroutput"><span class="identifier">a</span></code> (and |
| all its children). |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.directive.lexeme.attributes"></a><h6> |
| <a name="id884944"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.attributes">Attributes</a> |
| </h6> |
| <p> |
| See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound |
| Attribute Notation</a>. |
| </p> |
| <div class="informaltable"><table class="table"> |
| <colgroup> |
| <col> |
| <col> |
| </colgroup> |
| <thead><tr> |
| <th> |
| <p> |
| Expression |
| </p> |
| </th> |
| <th> |
| <p> |
| Attribute |
| </p> |
| </th> |
| </tr></thead> |
| <tbody><tr> |
| <td> |
| <p> |
| <code class="computeroutput"><span class="identifier">lexeme</span><span class="special">[</span><span class="identifier">a</span><span class="special">]</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| |
| </p> |
| <pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="identifier">lexeme</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">A</span> |
| <span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span> <span class="special">--></span> <span class="identifier">lexeme</span><span class="special">[</span><span class="identifier">a</span><span class="special">]:</span> <span class="identifier">Unused</span></pre> |
| <p> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.directive.lexeme.complexity"></a><h6> |
| <a name="id886217"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The complexity is defined by the complexity of the subject parser, |
| <code class="computeroutput"><span class="identifier">a</span></code> |
| </p></blockquote></div> |
| <a name="spirit.qi.reference.directive.lexeme.example"></a><h6> |
| <a name="id886245"></a> |
| <a class="link" href="lexeme.html#spirit.qi.reference.directive.lexeme.example">Example</a> |
| </h6> |
| <div class="note"><table border="0" summary="Note"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> |
| <th align="left">Note</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a> |
| section. |
| </p></td></tr> |
| </table></div> |
| <p> |
| Some using declarations: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">lexeme</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">lit</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">digit</span><span class="special">;</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Simple usage of <code class="computeroutput"><span class="identifier">lexeme</span><span class="special">[]</span></code>: |
| </p> |
| <p> |
| The use of lexeme here will prevent skipping in between the digits and |
| the sign making inputs such as <code class="computeroutput"><span class="string">"1 2 |
| 345"</span></code> erroneous. |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">test_phrase_parser</span><span class="special">(</span><span class="string">"12345"</span><span class="special">,</span> <span class="identifier">lexeme</span><span class="special">[</span> <span class="special">-(</span><span class="identifier">lit</span><span class="special">(</span><span class="char">'+'</span><span class="special">)</span> <span class="special">|</span> <span class="char">'-'</span><span class="special">)</span> <span class="special">>></span> <span class="special">+</span><span class="identifier">digit</span> <span class="special">]);</span> |
| </pre> |
| <p> |
| </p> |
| </div> |
| <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> |
| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2001-2010 Joel de Guzman, Hartmut Kaiser<p> |
| Distributed under the Boost Software License, Version 1.0. (See accompanying |
| file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>) |
| </p> |
| </div></td> |
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