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| <div class="titlepage"><div><div><h5 class="title"> |
| <a name="spirit.qi.reference.operator.difference"></a><a class="link" href="difference.html" title="Difference (a - b)">Difference |
| (<code class="computeroutput"><span class="identifier">a</span> <span class="special">-</span> |
| <span class="identifier">b</span></code>)</a> |
| </h5></div></div></div> |
| <a name="spirit.qi.reference.operator.difference.description"></a><h6> |
| <a name="id923308"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.description">Description</a> |
| </h6> |
| <p> |
| The difference operator, <code class="computeroutput"><span class="identifier">a</span> |
| <span class="special">-</span> <span class="identifier">b</span></code>, |
| is a binary operator that matches the first (LHS) operand but not the |
| second (RHS). <sup>[<a name="id923337" href="#ftn.id923337" class="footnote">8</a>]</sup> |
| </p> |
| <a name="spirit.qi.reference.operator.difference.header"></a><h6> |
| <a name="id923377"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.header">Header</a> |
| </h6> |
| <pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/difference.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_difference</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.operator.difference.model_of"></a><h6> |
| <a name="id923451"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.model_of">Model |
| of</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| <a class="link" href="../parser_concepts/binaryparser.html" title="BinaryParser"><code class="computeroutput"><span class="identifier">BinaryParser</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>, <code class="computeroutput"><span class="identifier">b</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.operator.difference.expression_semantics"></a><h6> |
| <a name="id923527"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.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/binaryparser.html" title="BinaryParser"><code class="computeroutput"><span class="identifier">BinaryParser</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">a</span> <span class="special">-</span> |
| <span class="identifier">b</span></code> |
| </p> |
| </td> |
| <td> |
| <p> |
| Parse <code class="computeroutput"><span class="identifier">a</span></code> but |
| not <code class="computeroutput"><span class="identifier">b</span></code>. |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.operator.difference.attributes"></a><h6> |
| <a name="id923639"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.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">a</span> <span class="special">-</span> |
| <span class="identifier">b</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">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">-</span> <span class="identifier">b</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">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--></span> <span class="special">(</span><span class="identifier">a</span> <span class="special">-</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">Unused</span></pre> |
| <p> |
| </p> |
| </td> |
| </tr></tbody> |
| </table></div> |
| <a name="spirit.qi.reference.operator.difference.complexity"></a><h6> |
| <a name="id923858"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.complexity">Complexity</a> |
| </h6> |
| <div class="blockquote"><blockquote class="blockquote"><p> |
| The complexity of the difference parser is defined by the sum of the |
| complexities of both operands. |
| </p></blockquote></div> |
| <a name="spirit.qi.reference.operator.difference.example"></a><h6> |
| <a name="id923878"></a> |
| <a class="link" href="difference.html#spirit.qi.reference.operator.difference.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">ascii</span><span class="special">::</span><span class="identifier">char_</span><span class="special">;</span></pre> |
| <p> |
| </p> |
| <p> |
| Parse a C/C++ style comment: |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"/*A Comment*/"</span><span class="special">,</span> <span class="string">"/*"</span> <span class="special">>></span> <span class="special">*(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="string">"*/"</span><span class="special">)</span> <span class="special">>></span> <span class="string">"*/"</span><span class="special">);</span> |
| </pre> |
| <p> |
| </p> |
| <div class="footnotes"> |
| <br><hr width="100" align="left"> |
| <div class="footnote"><p><sup>[<a name="ftn.id923337" href="#id923337" class="para">8</a>] </sup> |
| Unlike classic Spirit, with Spirit2, the expression will always fail |
| if the RHS is a successful match regardless if the RHS matches less |
| characters. For example, the rule <code class="computeroutput"><span class="identifier">lit</span><span class="special">(</span><span class="string">"policeman"</span><span class="special">)</span> <span class="special">-</span> <span class="string">"police"</span></code> will always fail to |
| match. Spirit2 does not count the matching chars while parsing and |
| there is no reliable and fast way to check if the LHS matches more |
| than the RHS. |
| </p></div> |
| </div> |
| </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> |
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