1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Re-Initialize an Attribute Value before Parsing (Qi)</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="../customize.html" title="Customization of Spirit's Attribute Handling"> 9<link rel="prev" href="store_value/push_back.html" title="Store a Parsed Attribute Value into a Container (Qi)"> 10<link rel="next" href="extract_from.html" title="Extract an Attribute Value to Generate Output (Karma)"> 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 href="../../../../../../../libs/libraries.htm">Libraries</a></td> 17<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> 18<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> 19<td align="center"><a href="../../../../../../../more/index.htm">More</a></td> 20</tr></table> 21<hr> 22<div class="spirit-nav"> 23<a accesskey="p" href="store_value/push_back.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../customize.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="extract_from.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 24</div> 25<div class="section"> 26<div class="titlepage"><div><div><h4 class="title"> 27<a name="spirit.advanced.customize.clear_value"></a><a class="link" href="clear_value.html" title="Re-Initialize an Attribute Value before Parsing (Qi)">Re-Initialize 28 an Attribute Value before Parsing (Qi)</a> 29</h4></div></div></div> 30<h6> 31<a name="spirit.advanced.customize.clear_value.h0"></a> 32 <span class="phrase"><a name="spirit.advanced.customize.clear_value.clear_value"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.clear_value">clear_value</a> 33 </h6> 34<p> 35 The template <code class="computeroutput"><span class="identifier">clear_value</span></code> 36 is a type used as an attribute customization point. It is invoked by the 37 <span class="emphasis"><em>Qi</em></span> repetitive parsers (<a class="link" href="../../qi/reference/operator/kleene.html" title="Kleene Parser (*a)">Kleene</a>, 38 <a class="link" href="../../qi/reference/operator/plus.html" title="Plus Parser (+a)">Plus</a>, <a class="link" href="../../qi/reference/operator/list.html" title="List Parser (a % b)">List</a>, 39 and <a class="link" href="../../qi/reference/directive/repeat.html" title="Repetition Parser Directive (repeat[])">Repeat</a>) 40 in order to re-initialize the attribute instance passed to the embedded 41 parser after it has been stored in the provided container. This re-initialized 42 attribute instance is reused during the next iteration of the repetitive 43 parser. 44 </p> 45<h6> 46<a name="spirit.advanced.customize.clear_value.h1"></a> 47 <span class="phrase"><a name="spirit.advanced.customize.clear_value.module_headers"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.module_headers">Module Headers</a> 48 </h6> 49<pre class="programlisting"><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">home</span><span class="special">/</span><span class="identifier">support</span><span class="special">/</span><span class="identifier">attributes</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 50</pre> 51<p> 52 Also, see <a class="link" href="../../structure/include.html" title="Include">Include Structure</a>. 53 </p> 54<div class="note"><table border="0" summary="Note"> 55<tr> 56<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td> 57<th align="left">Note</th> 58</tr> 59<tr><td align="left" valign="top"><p> 60 This header file does not need to be included directly by any user program 61 as it is normally included by other Spirit header files relying on its 62 content. 63 </p></td></tr> 64</table></div> 65<h6> 66<a name="spirit.advanced.customize.clear_value.h2"></a> 67 <span class="phrase"><a name="spirit.advanced.customize.clear_value.namespace"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.namespace">Namespace</a> 68 </h6> 69<div class="informaltable"><table class="table"> 70<colgroup><col></colgroup> 71<thead><tr><th> 72 <p> 73 Name 74 </p> 75 </th></tr></thead> 76<tbody><tr><td> 77 <p> 78 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">traits</span></code> 79 </p> 80 </td></tr></tbody> 81</table></div> 82<h6> 83<a name="spirit.advanced.customize.clear_value.h3"></a> 84 <span class="phrase"><a name="spirit.advanced.customize.clear_value.synopsis"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.synopsis">Synopsis</a> 85 </h6> 86<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Attrib</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Enable</span><span class="special">></span> 87<span class="keyword">struct</span> <span class="identifier">clear_value</span> 88<span class="special">{</span> 89 <span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">call</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&</span> <span class="identifier">val</span><span class="special">);</span> 90<span class="special">};</span> 91</pre> 92<h6> 93<a name="spirit.advanced.customize.clear_value.h4"></a> 94 <span class="phrase"><a name="spirit.advanced.customize.clear_value.template_parameters"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.template_parameters">Template 95 parameters</a> 96 </h6> 97<div class="informaltable"><table class="table"> 98<colgroup> 99<col> 100<col> 101<col> 102</colgroup> 103<thead><tr> 104<th> 105 <p> 106 Parameter 107 </p> 108 </th> 109<th> 110 <p> 111 Description 112 </p> 113 </th> 114<th> 115 <p> 116 Default 117 </p> 118 </th> 119</tr></thead> 120<tbody> 121<tr> 122<td> 123 <p> 124 <code class="computeroutput"><span class="identifier">Attrib</span></code> 125 </p> 126 </td> 127<td> 128 <p> 129 The type, <code class="computeroutput"><span class="identifier">Attrib</span></code> 130 of the attribute to be re-initialized. 131 </p> 132 </td> 133<td> 134 <p> 135 none 136 </p> 137 </td> 138</tr> 139<tr> 140<td> 141 <p> 142 <code class="computeroutput"><span class="identifier">Enable</span></code> 143 </p> 144 </td> 145<td> 146 <p> 147 Helper template parameter usable to selectively enable or disable 148 certain specializations of <code class="computeroutput"><span class="identifier">clear_value</span></code> 149 utilizing SFINAE (i.e. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code> 150 or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span></code>). 151 </p> 152 </td> 153<td> 154 <p> 155 <code class="computeroutput"><span class="keyword">void</span></code> 156 </p> 157 </td> 158</tr> 159</tbody> 160</table></div> 161<div class="variablelist"> 162<p class="title"><b>Notation</b></p> 163<dl class="variablelist"> 164<dt><span class="term"><code class="computeroutput"><span class="identifier">Attrib</span></code></span></dt> 165<dd><p> 166 A type to be used as a container to store attribute values in. 167 </p></dd> 168<dt><span class="term"><code class="computeroutput"><span class="identifier">attr</span></code></span></dt> 169<dd><p> 170 An attribute instance of type <code class="computeroutput"><span class="identifier">Attrib</span></code>. 171 </p></dd> 172<dt><span class="term"><code class="computeroutput"><span class="identifier">T1</span></code>, <code class="computeroutput"><span class="identifier">T2</span></code>, ...</span></dt> 173<dd><p> 174 Arbitrary types 175 </p></dd> 176</dl> 177</div> 178<h6> 179<a name="spirit.advanced.customize.clear_value.h5"></a> 180 <span class="phrase"><a name="spirit.advanced.customize.clear_value.expression_semantics"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.expression_semantics">Expression 181 Semantics</a> 182 </h6> 183<div class="informaltable"><table class="table"> 184<colgroup> 185<col> 186<col> 187</colgroup> 188<thead><tr> 189<th> 190 <p> 191 Expression 192 </p> 193 </th> 194<th> 195 <p> 196 Semantics 197 </p> 198 </th> 199</tr></thead> 200<tbody><tr> 201<td> 202 <p> 203</p> 204<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">clear_value</span><span class="special"><</span><span class="identifier">Attrib</span><span class="special">>::</span><span class="identifier">call</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">)</span></pre> 205<p> 206 </p> 207 </td> 208<td> 209 <p> 210 Re-initialize the instance referred to by <code class="computeroutput"><span class="identifier">attr</span></code> 211 in the most efficient way. 212 </p> 213 </td> 214</tr></tbody> 215</table></div> 216<h6> 217<a name="spirit.advanced.customize.clear_value.h6"></a> 218 <span class="phrase"><a name="spirit.advanced.customize.clear_value.predefined_specializations"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.predefined_specializations">Predefined 219 Specializations</a> 220 </h6> 221<p> 222 <a href="http://boost-spirit.com" target="_top">Spirit</a> predefines specializations 223 of this customization point for several types. The following table lists 224 those types together with the types exposed and the corresponding semantics: 225 </p> 226<div class="informaltable"><table class="table"> 227<colgroup> 228<col> 229<col> 230</colgroup> 231<thead><tr> 232<th> 233 <p> 234 Template Parameters 235 </p> 236 </th> 237<th> 238 <p> 239 Semantics 240 </p> 241 </th> 242</tr></thead> 243<tbody> 244<tr> 245<td> 246 <p> 247 <code class="computeroutput"><span class="identifier">Attrib</span></code> 248 </p> 249 </td> 250<td> 251 <p> 252 Re-initialize using assignment of default constructed value. 253 </p> 254 </td> 255</tr> 256<tr> 257<td> 258 <p> 259 Any type <code class="computeroutput"><span class="identifier">T</span></code> for 260 which <code class="computeroutput"><span class="identifier">is_container</span><span class="special"><>::</span><span class="identifier">type</span></code> 261 is <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> 262 </p> 263 </td> 264<td> 265 <p> 266 Call the member function <code class="computeroutput"><span class="identifier">attr</span><span class="special">.</span><span class="identifier">clear</span><span class="special">()</span></code> for the passed attribute instance. 267 </p> 268 </td> 269</tr> 270<tr> 271<td> 272 <p> 273 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">optional</span><span class="special"><</span><span class="identifier">Attrib</span><span class="special">></span></code> 274 </p> 275 </td> 276<td> 277 <p> 278 Clear the <code class="computeroutput"><span class="identifier">optional</span></code> 279 instance and leave it uninitialized. 280 </p> 281 </td> 282</tr> 283<tr> 284<td> 285 <p> 286 <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">variant</span><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> 287 <span class="identifier">T2</span><span class="special">,</span> 288 <span class="special">...></span></code> 289 </p> 290 </td> 291<td> 292 <p> 293 Invoke the <code class="computeroutput"><span class="identifier">clear_value</span></code> 294 customization point for the currently held value. 295 </p> 296 </td> 297</tr> 298<tr> 299<td> 300 <p> 301 <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">T1</span><span class="special">,</span> 302 <span class="identifier">T2</span><span class="special">,</span> 303 <span class="special">...></span></code> 304 </p> 305 </td> 306<td> 307 <p> 308 Invoke the <code class="computeroutput"><span class="identifier">clear_value</span></code> 309 customization point for all elements of the tuple. 310 </p> 311 </td> 312</tr> 313<tr> 314<td> 315 <p> 316 <code class="computeroutput"><span class="identifier">unused_type</span></code> 317 </p> 318 </td> 319<td> 320 <p> 321 Do nothing. 322 </p> 323 </td> 324</tr> 325</tbody> 326</table></div> 327<h6> 328<a name="spirit.advanced.customize.clear_value.h7"></a> 329 <span class="phrase"><a name="spirit.advanced.customize.clear_value.when_to_implement"></a></span><a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.when_to_implement">When 330 to Implement</a> 331 </h6> 332<p> 333 The customization point <code class="computeroutput"><span class="identifier">clear_value</span></code> 334 needs to be implemented for a specific type whenever this type is to be 335 used as an attribute to be stored into a STL container and if the type 336 cannot be re-initialized using one of the specializations listed above. 337 Examples for this might be types not being default constructible or container 338 types not exposing a member function <code class="computeroutput"><span class="identifier">clear</span><span class="special">()</span></code>. 339 </p> 340</div> 341<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 342<td align="left"></td> 343<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 344 Distributed under the Boost Software License, Version 1.0. (See accompanying 345 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>) 346 </p> 347</div></td> 348</tr></table> 349<hr> 350<div class="spirit-nav"> 351<a accesskey="p" href="store_value/push_back.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../customize.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="extract_from.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 352</div> 353</body> 354</html> 355
