1<html> 2<head> 3<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> 4<title>Atomically Extract an Attribute Value from a Container (Karma)</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="string_traits/extract_c_string.html" title="Get a C-style String from a String Type"> 10<link rel="next" href="auto.html" title="Create Components from Attributes (Qi and 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="string_traits/extract_c_string.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="auto.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.attribute_as"></a><a class="link" href="attribute_as.html" title="Atomically Extract an Attribute Value from a Container (Karma)">Atomically 28 Extract an Attribute Value from a Container (Karma)</a> 29</h4></div></div></div> 30<h6> 31<a name="spirit.advanced.customize.attribute_as.h0"></a> 32 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.attribute_as"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.attribute_as">attribute_as</a> 33 </h6> 34<p> 35 <code class="computeroutput"><span class="identifier">attribute_as</span></code> atomically 36 extracts an instance of a type from another type. This customization point 37 is used by the <a class="link" href="../../karma/reference/directive/as.html" title="Generator Directives Forcing Atomic Extraction (as<T>, as_string[], as_wstring[])"><code class="computeroutput"><span class="identifier">as</span><span class="special"><</span><span class="identifier">T</span><span class="special">>[]</span></code></a> 38 directive. 39 </p> 40<h6> 41<a name="spirit.advanced.customize.attribute_as.h1"></a> 42 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.module_headers"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.module_headers">Module 43 Headers</a> 44 </h6> 45<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_fwd</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> 46</pre> 47<p> 48 Also, see <a class="link" href="../../structure/include.html" title="Include">Include Structure</a>. 49 </p> 50<div class="note"><table border="0" summary="Note"> 51<tr> 52<td rowspan="2" align="center" valign="top" width="25"><img alt="[Note]" src="../../../images/note.png"></td> 53<th align="left">Note</th> 54</tr> 55<tr><td align="left" valign="top"><p> 56 This header file does not need to be included directly by any user program 57 as it is normally included by other Spirit header files relying on its 58 content. 59 </p></td></tr> 60</table></div> 61<h6> 62<a name="spirit.advanced.customize.attribute_as.h2"></a> 63 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.namespace"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.namespace">Namespace</a> 64 </h6> 65<div class="informaltable"><table class="table"> 66<colgroup><col></colgroup> 67<thead><tr><th> 68 <p> 69 Name 70 </p> 71 </th></tr></thead> 72<tbody><tr><td> 73 <p> 74 <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> 75 </p> 76 </td></tr></tbody> 77</table></div> 78<h6> 79<a name="spirit.advanced.customize.attribute_as.h3"></a> 80 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.synopsis"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.synopsis">Synopsis</a> 81 </h6> 82<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Attribute</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Enable</span> <span class="special">=</span> <span class="keyword">void</span><span class="special">></span> 83<span class="keyword">struct</span> <span class="identifier">attribute_as</span><span class="special">;</span> 84</pre> 85<h6> 86<a name="spirit.advanced.customize.attribute_as.h4"></a> 87 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.template_parameters"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.template_parameters">Template 88 parameters</a> 89 </h6> 90<div class="informaltable"><table class="table"> 91<colgroup> 92<col> 93<col> 94<col> 95</colgroup> 96<thead><tr> 97<th> 98 <p> 99 Parameter 100 </p> 101 </th> 102<th> 103 <p> 104 Description 105 </p> 106 </th> 107<th> 108 <p> 109 Default 110 </p> 111 </th> 112</tr></thead> 113<tbody> 114<tr> 115<td> 116 <p> 117 <code class="computeroutput"><span class="identifier">T</span></code> 118 </p> 119 </td> 120<td> 121 <p> 122 The type of the attribute natively exposed by the component the 123 <code class="computeroutput"><span class="identifier">attribute_as</span></code> 124 is invoked from. 125 </p> 126 </td> 127<td> 128 <p> 129 none 130 </p> 131 </td> 132</tr> 133<tr> 134<td> 135 <p> 136 <code class="computeroutput"><span class="identifier">Attribute</span></code> 137 </p> 138 </td> 139<td> 140 <p> 141 The type of the attribute to be used to generate output from. 142 </p> 143 </td> 144<td> 145 <p> 146 none 147 </p> 148 </td> 149</tr> 150<tr> 151<td> 152 <p> 153 <code class="computeroutput"><span class="identifier">Enable</span></code> 154 </p> 155 </td> 156<td> 157 <p> 158 Helper template parameter usable to selectively enable or disable 159 certain specializations of <code class="computeroutput"><span class="identifier">attribute_as</span></code> 160 utilizing SFINAE (i.e. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code> 161 or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span></code>). 162 </p> 163 </td> 164<td> 165 <p> 166 <code class="computeroutput"><span class="keyword">void</span></code> 167 </p> 168 </td> 169</tr> 170</tbody> 171</table></div> 172<div class="variablelist"> 173<p class="title"><b>Notation</b></p> 174<dl class="variablelist"> 175<dt><span class="term"><code class="computeroutput"><span class="identifier">attr</span></code></span></dt> 176<dd><p> 177 An instance of type <code class="computeroutput"><span class="identifier">Attrib</span></code>. 178 </p></dd> 179</dl> 180</div> 181<h6> 182<a name="spirit.advanced.customize.attribute_as.h5"></a> 183 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.expression_semantics"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.expression_semantics">Expression 184 Semantics</a> 185 </h6> 186<div class="informaltable"><table class="table"> 187<colgroup> 188<col> 189<col> 190</colgroup> 191<thead><tr> 192<th> 193 <p> 194 Expression 195 </p> 196 </th> 197<th> 198 <p> 199 Semantics 200 </p> 201 </th> 202</tr></thead> 203<tbody> 204<tr> 205<td> 206 <p> 207 <code class="computeroutput"><span class="identifier">attribute_as</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Attribute</span><span class="special">>::</span><span class="identifier">type</span></code> 208 </p> 209 </td> 210<td> 211 <p> 212 The result type of the extraction. 213 </p> 214 </td> 215</tr> 216<tr> 217<td> 218 <p> 219 <code class="computeroutput"><span class="identifier">attribute_as</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Attribute</span><span class="special">>::</span><span class="identifier">call</span><span class="special">(</span><span class="identifier">attr</span><span class="special">)</span></code> 220 </p> 221 </td> 222<td> 223 <p> 224 Extract and return an instance of <code class="computeroutput"><span class="identifier">type</span></code>. 225 </p> 226 </td> 227</tr> 228<tr> 229<td> 230 <p> 231 <code class="computeroutput"><span class="identifier">attribute_as</span><span class="special"><</span><span class="identifier">T</span><span class="special">,</span> <span class="identifier">Attribute</span><span class="special">>::</span><span class="identifier">valid_as</span><span class="special">(</span><span class="identifier">attr</span><span class="special">)</span></code> 232 </p> 233 </td> 234<td> 235 <p> 236 Determine, at runtime, if the extraction of an instance of <code class="computeroutput"><span class="identifier">type</span></code> from <code class="computeroutput"><span class="identifier">attr</span></code> 237 would cause an error. 238 </p> 239 </td> 240</tr> 241</tbody> 242</table></div> 243<h6> 244<a name="spirit.advanced.customize.attribute_as.h6"></a> 245 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.predefined_specializations"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.predefined_specializations">Predefined 246 Specializations</a> 247 </h6> 248<p> 249 <a href="http://boost-spirit.com" target="_top">Spirit</a> predefines specializations 250 of this customization point for several types. The following table lists 251 those types together with the types exposed and the corresponding semantics: 252 </p> 253<div class="informaltable"><table class="table"> 254<colgroup> 255<col> 256<col> 257</colgroup> 258<thead><tr> 259<th> 260 <p> 261 Template Parameters 262 </p> 263 </th> 264<th> 265 <p> 266 Semantics 267 </p> 268 </th> 269</tr></thead> 270<tbody><tr> 271<td> 272 <p> 273 <code class="computeroutput"><span class="identifier">unused_type</span></code> 274 </p> 275 </td> 276<td> 277 <p> 278 The exposed typedef <code class="computeroutput"><span class="identifier">type</span></code> 279 is defined to <code class="computeroutput"><span class="identifier">unused_type</span></code>. 280 The function <code class="computeroutput"><span class="identifier">call</span><span class="special">()</span></code> returns an instance of <code class="computeroutput"><span class="identifier">unused_type</span></code>. 281 </p> 282 </td> 283</tr></tbody> 284</table></div> 285<h6> 286<a name="spirit.advanced.customize.attribute_as.h7"></a> 287 <span class="phrase"><a name="spirit.advanced.customize.attribute_as.when_to_implement"></a></span><a class="link" href="attribute_as.html#spirit.advanced.customize.attribute_as.when_to_implement">When 288 to implement</a> 289 </h6> 290<p> 291 This customization point may need to be implemented when using the <a class="link" href="../../karma/reference/directive/as.html" title="Generator Directives Forcing Atomic Extraction (as<T>, as_string[], as_wstring[])"><code class="computeroutput"><span class="identifier">as</span><span class="special"><</span><span class="identifier">T</span><span class="special">>[]</span></code></a> directive. 292 </p> 293</div> 294<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr> 295<td align="left"></td> 296<td align="right"><div class="copyright-footer">Copyright © 2001-2011 Joel de Guzman, Hartmut Kaiser<p> 297 Distributed under the Boost Software License, Version 1.0. (See accompanying 298 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>) 299 </p> 300</div></td> 301</tr></table> 302<hr> 303<div class="spirit-nav"> 304<a accesskey="p" href="string_traits/extract_c_string.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="auto.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> 305</div> 306</body> 307</html> 308
