<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Optional (-a)</title>
<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css">
<meta name="generator" content="DocBook XSL Stylesheets V1.75.0">
<link rel="home" href="../../../../index.html" title="Spirit 2.4">
<link rel="up" href="../operator.html" title="Operator">
<link rel="prev" href="not_predicate.html" title="Not-Predicate (!a)">
<link rel="next" href="permutation.html" title="Permutation (a ^ b)">
</head>
<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td>
<td align="center"><a href="../../../../../../../../index.html">Home</a></td>
<td align="center"><a href="../../../../../../../../libs/libraries.htm">Libraries</a></td>
<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
<td align="center"><a href="../../../../../../../../more/index.htm">More</a></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="not_predicate.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="permutation.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.operator.optional"></a><a class="link" href="optional.html" title="Optional (-a)"> Optional
(<code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code>)</a>
</h5></div></div></div>
<a name="spirit.qi.reference.operator.optional.description"></a><h6>
<a name="id978806"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.description">Description</a>
</h6>
<p>
The optional operator, <code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code>, is a unary operator that matches
its operand zero or one time.
</p>
<a name="spirit.qi.reference.operator.optional.header"></a><h6>
<a name="id978834"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/optional.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_optional</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.optional.model_of"></a><h6>
<a name="id978908"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.model_of">Model
of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</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></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.optional.expression_semantics"></a><h6>
<a name="id978977"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.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/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</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">-</span><span class="identifier">a</span></code>
</p>
</td>
<td>
<p>
Match <code class="computeroutput"><span class="identifier">a</span></code> zero
or one time.
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.optional.attributes"></a><h6>
<a name="id979078"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.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="special">-</span><span class="identifier">a</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="special">-</span><span class="identifier">a</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">Unused</span> <span class="special">--></span> <span class="special">-</span><span class="identifier">a</span><span class="special">:</span> <span class="identifier">Unused</span></pre>
<p>
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.qi.reference.operator.optional.complexity"></a><h6>
<a name="id979248"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The complexity is defined by the complexity of the operand, <code class="computeroutput"><span class="identifier">a</span></code>
</p></blockquote></div>
<a name="spirit.qi.reference.operator.optional.example"></a><h6>
<a name="id979276"></a>
<a class="link" href="optional.html#spirit.qi.reference.operator.optional.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>
<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">lexeme</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">fusion</span><span class="special">::</span><span class="identifier">vector</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">at_c</span><span class="special">;</span>
<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">;</span></pre>
<p>
</p>
<p>
Parse a person info with name (in quotes) optional age <sup>[<a name="id979489" href="#ftn.id979489" class="footnote">9</a>]</sup> and optional sex, all separated by comma.
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">vector</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">optional</span><span class="special"><</span><span class="keyword">int</span><span class="special">>,</span> <span class="identifier">optional</span><span class="special"><</span><span class="keyword">char</span><span class="special">></span> <span class="special">></span> <span class="identifier">attr</span><span class="special">;</span>
<span class="identifier">test_phrase_parser_attr</span><span class="special">(</span>
<span class="string">"\"James Bond\", M"</span>
<span class="special">,</span> <span class="identifier">lexeme</span><span class="special">[</span><span class="char">'"'</span> <span class="special">>></span> <span class="special">+(</span><span class="identifier">char_</span> <span class="special">-</span> <span class="char">'"'</span><span class="special">)</span> <span class="special">>></span> <span class="char">'"'</span><span class="special">]</span> <span class="comment">// name
</span> <span class="special">>></span> <span class="special">-(</span><span class="char">','</span> <span class="special">>></span> <span class="identifier">int_</span><span class="special">)</span> <span class="comment">// optional age
</span> <span class="special">>></span> <span class="special">-(</span><span class="char">','</span> <span class="special">>></span> <span class="identifier">char_</span><span class="special">)</span> <span class="comment">// optional sex
</span> <span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span>
<span class="comment">// Should print: James Bond,M
</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">at_c</span><span class="special"><</span><span class="number">0</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">);</span> <span class="comment">// print name
</span><span class="keyword">if</span> <span class="special">(</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">))</span> <span class="comment">// print optional age
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="special">*</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">1</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">);</span>
<span class="keyword">if</span> <span class="special">(</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">2</span><span class="special">>(</span><span class="identifier">attr</span><span class="special">))</span> <span class="comment">// print optional sex
</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="char">','</span> <span class="special"><<</span> <span class="special">*</span><span class="identifier">at_c</span><span class="special"><</span><span class="number">2</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">std</span><span class="special">::</span><span class="identifier">endl</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.id979489" href="#id979489" class="para">9</a>] </sup>
James Bond is shy about his age :-)
</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>
</div></td>
</tr></table>
<hr>
<div class="spirit-nav">
<a accesskey="p" href="not_predicate.html"><img src="../../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../operator.html"><img src="../../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../../index.html"><img src="../../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="permutation.html"><img src="../../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
