1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Optional Parser (-a)</title> 5<link rel="stylesheet" href="../../../../../../../../doc/src/boostbook.css" type="text/css"> 6<meta name="generator" content="DocBook XSL Stylesheets V1.79.1"> 7<link rel="home" href="../../../../index.html" title="Spirit 2.5.8"> 8<link rel="up" href="../operator.html" title="Parser Operators"> 9<link rel="prev" href="not_predicate.html" title="Not-Predicate Parser (!a)"> 10<link rel="next" href="permutation.html" title="Permutation Parser (a ^ b)"> 11</head> 12<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> 13<table cellpadding="2" width="100%"><tr> 14<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../../boost.png"></td> 15<td align="center"><a href="../../../../../../../../index.html">Home</a></td> 16<td align="center"><a 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href="optional.html" title="Optional Parser (-a)">Optional Parser 28 (<code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code>)</a> 29</h5></div></div></div> 30<h6> 31<a name="spirit.qi.reference.operator.optional.h0"></a> 32 <span class="phrase"><a name="spirit.qi.reference.operator.optional.description"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.description">Description</a> 33 </h6> 34<p> 35 The optional operator, <code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code>, is a unary operator that matches 36 its operand zero or one time. 37 </p> 38<h6> 39<a name="spirit.qi.reference.operator.optional.h1"></a> 40 <span class="phrase"><a name="spirit.qi.reference.operator.optional.header"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.header">Header</a> 41 </h6> 42<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/operator/optional.hpp></span> 43<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> 44</pre> 45<p> 46 Also, see <a class="link" href="../../../structure/include.html" title="Include">Include Structure</a>. 47 </p> 48<h6> 49<a name="spirit.qi.reference.operator.optional.h2"></a> 50 <span class="phrase"><a name="spirit.qi.reference.operator.optional.model_of"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.model_of">Model 51 of</a> 52 </h6> 53<div class="blockquote"><blockquote class="blockquote"><p> 54 <a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a> 55 </p></blockquote></div> 56<div class="variablelist"> 57<p class="title"><b>Notation</b></p> 58<dl class="variablelist"> 59<dt><span class="term"><code class="computeroutput"><span class="identifier">a</span></code></span></dt> 60<dd><p> 61 A <a class="link" href="../parser_concepts/parser.html" title="Parser"><code class="computeroutput"><span class="identifier">Parser</span></code></a> 62 </p></dd> 63</dl> 64</div> 65<h6> 66<a name="spirit.qi.reference.operator.optional.h3"></a> 67 <span class="phrase"><a name="spirit.qi.reference.operator.optional.expression_semantics"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.expression_semantics">Expression 68 Semantics</a> 69 </h6> 70<p> 71 Semantics of an expression is defined only where it differs from, or 72 is not defined in <a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a>. 73 </p> 74<div class="informaltable"><table class="table"> 75<colgroup> 76<col> 77<col> 78</colgroup> 79<thead><tr> 80<th> 81 <p> 82 Expression 83 </p> 84 </th> 85<th> 86 <p> 87 Semantics 88 </p> 89 </th> 90</tr></thead> 91<tbody><tr> 92<td> 93 <p> 94 <code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code> 95 </p> 96 </td> 97<td> 98 <p> 99 Match <code class="computeroutput"><span class="identifier">a</span></code> zero 100 or one time. 101 </p> 102 </td> 103</tr></tbody> 104</table></div> 105<h6> 106<a name="spirit.qi.reference.operator.optional.h4"></a> 107 <span class="phrase"><a name="spirit.qi.reference.operator.optional.attributes"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.attributes">Attributes</a> 108 </h6> 109<p> 110 See <a class="link" href="../../quick_reference/compound_attribute_rules.html#spirit.qi.quick_reference.compound_attribute_rules.notation">Compound 111 Attribute Notation</a>. 112 </p> 113<div class="informaltable"><table class="table"> 114<colgroup> 115<col> 116<col> 117</colgroup> 118<thead><tr> 119<th> 120 <p> 121 Expression 122 </p> 123 </th> 124<th> 125 <p> 126 Attribute 127 </p> 128 </th> 129</tr></thead> 130<tbody><tr> 131<td> 132 <p> 133 <code class="computeroutput"><span class="special">-</span><span class="identifier">a</span></code> 134 </p> 135 </td> 136<td> 137 <p> 138</p> 139<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> 140<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> 141<p> 142 </p> 143 </td> 144</tr></tbody> 145</table></div> 146<h6> 147<a name="spirit.qi.reference.operator.optional.h5"></a> 148 <span class="phrase"><a name="spirit.qi.reference.operator.optional.complexity"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.complexity">Complexity</a> 149 </h6> 150<div class="blockquote"><blockquote class="blockquote"><p> 151 The complexity is defined by the complexity of the operand, <code class="computeroutput"><span class="identifier">a</span></code> 152 </p></blockquote></div> 153<h6> 154<a name="spirit.qi.reference.operator.optional.h6"></a> 155 <span class="phrase"><a name="spirit.qi.reference.operator.optional.example"></a></span><a class="link" href="optional.html#spirit.qi.reference.operator.optional.example">Example</a> 156 </h6> 157<div class="note"><table border="0" summary="Note"> 158<tr> 159<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../../images/note.png"></td> 160<th align="left">Note</th> 161</tr> 162<tr><td align="left" valign="top"><p> 163 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> 164 section. 165 </p></td></tr> 166</table></div> 167<p> 168 Some using declarations: 169 </p> 170<p> 171</p> 172<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> 173<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> 174<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> 175<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> 176<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> 177<span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special">;</span> 178</pre> 179<p> 180 </p> 181<p> 182 Parse a person info with name (in quotes) optional age <a href="#ftn.spirit.qi.reference.operator.optional.f0" class="footnote" name="spirit.qi.reference.operator.optional.f0"><sup class="footnote">[9]</sup></a> and optional sex, all separated by comma. 183 </p> 184<p> 185</p> 186<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> 187 188<span class="identifier">test_phrase_parser_attr</span><span class="special">(</span> 189 <span class="string">"\"James Bond\", M"</span> 190 <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> 191 <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> 192 <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> 193 <span class="special">,</span> <span class="identifier">attr</span><span class="special">);</span> 194 195<span class="comment">// Should print: James Bond,M</span> 196<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> 197<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> 198 <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> 199<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> 200 <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> 201<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> 202</pre> 203<p> 204 </p> 205<div class="footnotes"> 206<br><hr style="width:100; text-align:left;margin-left: 0"> 207<div id="ftn.spirit.qi.reference.operator.optional.f0" class="footnote"><p><a href="#spirit.qi.reference.operator.optional.f0" class="para"><sup class="para">[9] </sup></a> 208 James Bond is shy about his age :-) 209 </p></div> 210</div> 211</div> 212<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 213<td align="left"></td> 214<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 215 Distributed under the Boost Software License, Version 1.0. (See accompanying 216 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>) 217 </p> 218</div></td> 219</tr></table> 220<hr> 221<div class="spirit-nav"> 222<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> 223</div> 224</body> 225</html> 226
