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<title>And-Predicate (&amp;a)</title>
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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.karma.reference.operator.and_predicate"></a><a class="link" href="and_predicate.html" title="And-Predicate (&amp;a)">And-Predicate
(<code class="computeroutput"><span class="special">&amp;</span><span class="identifier">a</span></code>)</a>
</h5></div></div></div>
<a name="spirit.karma.reference.operator.and_predicate.description"></a><h6>
<a name="id1096972"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.description">Description</a>
</h6>
<p>
The and-predicate generator is used to test, whether the embedded generator
succeeds without generating any output. It succeeds if the embedded generator
succeeds.
</p>
<a name="spirit.karma.reference.operator.and_predicate.header"></a><h6>
<a name="id1096991"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/karma/operator/and_predicate.hpp&gt;
</span><span class="preprocessor">#include</span> <span class="special">&lt;</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">karma_and_predicate</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
</pre>
<p>
Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>.
</p>
<a name="spirit.karma.reference.operator.and_predicate.model_of"></a><h6>
<a name="id1097066"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.model_of">Model
of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</span></code></a>
</p></blockquote></div>
<a name="spirit.karma.reference.operator.and_predicate.expression_semantics"></a><h6>
<a name="id1097098"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.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="../generator_concepts/unarygenerator.html" title="UnaryGenerator"><code class="computeroutput"><span class="identifier">UnaryGenerator</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="special">&amp;</span><span class="identifier">a</span></code>
</p>
</td>
<td>
<p>
The generator <code class="computeroutput"><span class="identifier">a</span></code>
is executed for the sole purpose of testing whether it succeeds.
The and-predicate generator succeeds if its embedded generator
succeeds (unless the underlying output stream reports an error).
The and-predicate never produces any output.
</p>
</td>
</tr></tbody>
</table></div>
<p>
The and generator is implemented by redirecting all output produced by
its embedded generator into a discarding device.
</p>
<a name="spirit.karma.reference.operator.and_predicate.attributes"></a><h6>
<a name="id1097204"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.attributes">Attributes</a>
</h6>
<p>
See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.karma.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="special">&amp;</span><span class="identifier">a</span></code>
(and-predicate, unary <code class="computeroutput"><span class="special">&amp;</span></code>)
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">:</span>
<span class="identifier">A</span> <span class="special">--&gt;</span>
<span class="special">&amp;</span><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span></code>
</p>
</td>
</tr></tbody>
</table></div>
<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 attribute of the and-predicate is not always <code class="computeroutput"><span class="identifier">unused_type</span></code>,
which is different from Qi's and-predicate. This is necessary as the
generator the and predicate is attached to most of the time needs an
attribute.
</p></td></tr>
</table></div>
<a name="spirit.karma.reference.operator.and_predicate.complexity"></a><h6>
<a name="id1097355"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The overall complexity of the and-predicate generator is defined by
the complexity of its embedded generator. The complexity of the and-predicate
generator itself is O(1).
</p></blockquote></div>
<a name="spirit.karma.reference.operator.and_predicate.example"></a><h6>
<a name="id1097378"></a>
<a class="link" href="and_predicate.html#spirit.karma.reference.operator.and_predicate.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="../karma_basics.html#spirit.karma.reference.karma_basics.examples">Basics Examples</a>
section.
</p></td></tr>
</table></div>
<p>
Some includes:
</p>
<p>
</p>
<pre class="programlisting"><span class="preprocessor">#include</span> <span class="special">&lt;</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">karma</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">phoenix_core</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</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">phoenix_operator</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">std_pair</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">string</span><span class="special">&gt;</span>
</pre>
<p>
</p>
<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">karma</span><span class="special">::</span><span class="identifier">double_</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">karma</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</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">karma</span><span class="special">::</span><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">string</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">phoenix</span><span class="special">::</span><span class="identifier">ref</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Basic usage of an and predicate generator:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"b"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">char_</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="char">'b'</span> <span class="special">|</span> <span class="char">'c'</span><span class="special">,</span> <span class="char">'a'</span><span class="special">);</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"c"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">char_</span><span class="special">(</span><span class="char">'a'</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="char">'b'</span> <span class="special">|</span> <span class="char">'c'</span><span class="special">,</span> <span class="char">'x'</span><span class="special">);</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"abc"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">string</span><span class="special">(</span><span class="string">"123"</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"abc"</span> <span class="special">|</span> <span class="string">"def"</span><span class="special">,</span> <span class="string">"123"</span><span class="special">);</span>
<span class="identifier">test_generator_attr</span><span class="special">(</span><span class="string">"def"</span><span class="special">,</span> <span class="special">&amp;</span><span class="identifier">string</span><span class="special">(</span><span class="string">"123"</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="string">"abc"</span> <span class="special">|</span> <span class="string">"def"</span><span class="special">,</span> <span class="string">"456"</span><span class="special">);</span>
</pre>
<p>
</p>
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<td align="right"><div class="copyright-footer">Copyright &#169; 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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