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<title>Sequential Or (a || b)</title>
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<a name="spirit.qi.reference.operator.sequential_or"></a><a class="link" href="sequential_or.html" title="Sequential Or (a || b)">Sequential
Or (<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.sequential_or.description"></a><h6>
<a name="id943155"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.description">Description</a>
</h6>
<p>
The sequential-or operator, <code class="computeroutput"><span class="identifier">a</span>
<span class="special">||</span> <span class="identifier">b</span></code>,
matches <code class="computeroutput"><span class="identifier">a</span></code> or <code class="computeroutput"><span class="identifier">b</span></code> or <code class="computeroutput"><span class="identifier">a</span></code>
followed by <code class="computeroutput"><span class="identifier">b</span></code>. That is,
if both <code class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code> match, it must be in sequence; this
is equivalent to <code class="computeroutput"><span class="identifier">a</span> <span class="special">&gt;&gt;</span>
<span class="special">-</span><span class="identifier">b</span>
<span class="special">|</span> <span class="identifier">b</span></code>:
</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>
<a name="spirit.qi.reference.operator.sequential_or.header"></a><h6>
<a name="id943290"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to &lt;boost/spirit/home/qi/operator/sequential_or.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">qi_sequential_or</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.qi.reference.operator.sequential_or.model_of"></a><h6>
<a name="id943364"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.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.sequential_or.expression_semantics"></a><h6>
<a name="id943440"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.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> in sequence.
equivalent to <code class="computeroutput"><span class="identifier">a</span> <span class="special">&gt;&gt;</span> <span class="special">-</span><span class="identifier">b</span> <span class="special">|</span>
<span class="identifier">b</span></code>
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.sequential_or.attributes"></a><h6>
<a name="id943582"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.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">--&gt;</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">tuple</span><span class="special">&lt;</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;,</span> <span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">B</span><span class="special">&gt;</span> <span class="special">&gt;</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">--&gt;</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">&lt;</span><span class="identifier">A</span><span class="special">&gt;</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">--&gt;</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">&lt;</span><span class="identifier">B</span><span class="special">&gt;</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">--&gt;</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">--&gt;</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">vector</span><span class="special">&lt;</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">&gt;</span> <span class="special">&gt;</span></pre>
<p>
</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 sequential-or parser behaves attribute-wise very similar to the
plain sequence parser (<code class="computeroutput"><span class="identifier">a</span>
<span class="special">&gt;&gt;</span> <span class="identifier">b</span></code>)
in the sense that it exposes the attributes of its elements separately.
For instance, if you attach a semantic action to the whole sequential-or:
</p>
<pre class="programlisting"><span class="special">(</span><span class="identifier">int_</span> <span class="special">||</span> <span class="identifier">int_</span><span class="special">)[</span><span class="identifier">print_pair</span><span class="special">(</span><span class="identifier">_1</span><span class="special">,</span> <span class="identifier">_2</span><span class="special">)]</span>
</pre>
<p>
the function object <code class="computeroutput"><span class="identifier">print_pair</span></code>
would be invoked with the attribute of the first <code class="computeroutput"><span class="identifier">int_</span></code>
(<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code>)
as its first parameter and the attribute of the second <code class="computeroutput"><span class="identifier">int_</span></code> (<code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">&lt;</span><span class="keyword">int</span><span class="special">&gt;</span></code> as well) as its second parameter.
</p>
</td></tr>
</table></div>
<a name="spirit.qi.reference.operator.sequential_or.complexity"></a><h6>
<a name="id944211"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The overall complexity of the sequential-or parser is defined by the
sum of the complexities of its elements. The complexity of the sequential-or
itself is O(N), where N is the number of elements in the sequence.
</p></blockquote></div>
<a name="spirit.qi.reference.operator.sequential_or.example"></a><h6>
<a name="id944232"></a>
<a class="link" href="sequential_or.html#spirit.qi.reference.operator.sequential_or.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">int_</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
Correctly parsing a number with optional fractional digits:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123.456"</span><span class="special">,</span> <span class="identifier">int_</span> <span class="special">||</span> <span class="special">(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">));</span> <span class="comment">// full
</span><span class="identifier">test_parser</span><span class="special">(</span><span class="string">"123"</span><span class="special">,</span> <span class="identifier">int_</span> <span class="special">||</span> <span class="special">(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">));</span> <span class="comment">// just the whole number
</span><span class="identifier">test_parser</span><span class="special">(</span><span class="string">".456"</span><span class="special">,</span> <span class="identifier">int_</span> <span class="special">||</span> <span class="special">(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">));</span> <span class="comment">// just the fraction</span></pre>
<p>
</p>
<p>
A naive but incorrect solution would try to do this using optionals (e.g.):
</p>
<pre class="programlisting"><span class="identifier">int_</span> <span class="special">&gt;&gt;</span> <span class="special">-(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">)</span> <span class="comment">// will not match ".456"
</span><span class="special">-</span><span class="identifier">int_</span> <span class="special">&gt;&gt;</span> <span class="special">(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">)</span> <span class="comment">// will not match "123"
</span><span class="special">-</span><span class="identifier">int_</span> <span class="special">&gt;&gt;</span> <span class="special">-(</span><span class="char">'.'</span> <span class="special">&gt;&gt;</span> <span class="identifier">int_</span><span class="special">)</span> <span class="comment">// will match empty strings! Ooops.
</span></pre>
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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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