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<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.operator.alternative"></a><a class="link" href="alternative.html" title="Alternative (a | b)"> Alternative
(<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.alternative.description"></a><h6>
<a name="id962335"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.description">Description</a>
</h6>
<p>
The alternative operator, <code class="computeroutput"><span class="identifier">a</span>
<span class="special">|</span> <span class="identifier">b</span></code>,
matches one of two or more operands (<code class="computeroutput"><span class="identifier">a</span></code>,
<code class="computeroutput"><span class="identifier">b</span></code>, ... etc.):
</p>
<pre class="programlisting"><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span> <span class="special">|</span> <span class="special">...</span>
</pre>
<p>
Alternative operands are tried one by one on a first-match-wins basis
starting from the leftmost operand. After a successfully matched alternative
is found, the parser concludes its search, essentially short-circuiting
the search for other potentially viable candidates. This short-circuiting
implicitly gives the highest priority to the leftmost alternative.
</p>
<p>
Short-circuiting is done in the same manner as C or C++'s logical expressions;
e.g. <code class="computeroutput"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">x</span> <span class="special"><</span> <span class="number">3</span> <span class="special">||</span> <span class="identifier">y</span>
<span class="special"><</span> <span class="number">2</span><span class="special">)</span></code> where, if <code class="computeroutput"><span class="identifier">x</span>
<span class="special"><</span> <span class="number">3</span></code>,
the <code class="computeroutput"><span class="identifier">y</span> <span class="special"><</span>
<span class="number">2</span></code> test is not done at all. In addition
to providing an implicit priority rule for alternatives which is necessary,
given its non-deterministic nature, short-circuiting improves the execution
time. If the order of your alternatives is logically irrelevant, strive
to put the (expected) most common choice first for maximum efficiency.
</p>
<a name="spirit.qi.reference.operator.alternative.header"></a><h6>
<a name="id962500"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/alternative.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_alternative</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.alternative.model_of"></a><h6>
<a name="id962574"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.model_of">Model
of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../parser_concepts/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</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.alternative.expression_semantics"></a><h6>
<a name="id962650"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.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/naryparser.html" title="NaryParser"><code class="computeroutput"><span class="identifier">NaryParser</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>
Match <code class="computeroutput"><span class="identifier">a</span></code> or
<code class="computeroutput"><span class="identifier">b</span></code>.
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.alternative.attributes"></a><h6>
<a name="id962763"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.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">variant</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="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> <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">optional</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span>
<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="identifier">c</span><span class="special">:</span> <span class="identifier">Unused</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">c</span><span class="special">):</span> <span class="identifier">optional</span><span class="special"><</span><span class="identifier">variant</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="special">></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">optional</span><span class="special"><</span><span class="identifier">B</span><span class="special">></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">Unused</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>
<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">A</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></pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.alternative.complexity"></a><h6>
<a name="id964970"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The overall complexity of the alternative parser is defined by the
sum of the complexities of its elements. The complexity of the alternative
parser itself is O(N), where N is the number of alternatives.
</p></blockquote></div>
<a name="spirit.qi.reference.operator.alternative.example"></a><h6>
<a name="id964990"></a>
<a class="link" href="alternative.html#spirit.qi.reference.operator.alternative.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">string</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">int_</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">_1</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Simple usage:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span>
<span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">);</span></pre>
<p>
</p>
<p>
Extracting the attribute variant (using <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">variant</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">,</span> <span class="keyword">int</span><span class="special">></span> <span class="identifier">attr</span><span class="special">;</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">,</span> <span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span></pre>
<p>
</p>
<p>
This should print <code class="computeroutput"><span class="string">"Hello"</span></code>.
Note: There are better ways to extract the value from the variant. See
<a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>
visitation. This code is solely for demonstration.
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">if</span> <span class="special">(</span><span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(&</span><span class="identifier">attr</span><span class="special">))</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="keyword">int</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
<span class="keyword">else</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">get</span><span class="special"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">)</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span></pre>
<p>
</p>
<p>
Extracting the attributes using Semantic Actions with <a href="../../../../../../phoenix/doc/html/index.html" target="_top">Phoenix</a>
(this should print <code class="computeroutput"><span class="number">123</span></code>):
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="special">(</span><span class="identifier">string</span><span class="special">(</span><span class="string">"Hello"</span><span class="special">)</span> <span class="special">|</span> <span class="identifier">int_</span><span class="special">)[</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">_1</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">]);</span>
</pre>
<p>
</p>
</div>
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<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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