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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="id966266"></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="id966295" href="#ftn.id966295" class="footnote">8</a>]</sup>
</p>
<a name="spirit.qi.reference.operator.difference.header"></a><h6>
<a name="id966334"></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="id966409"></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="id966484"></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="id966597"></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="id972722"></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="id972742"></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.id966295" href="#id966295" 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>
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<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>)
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