<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Extract an Attribute Value to Generate Output (Karma)</title> <link rel="stylesheet" href="../../../../../../../doc/html/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../index.html" title="Spirit 2.2"> <link rel="up" href="../customize.html" title="Customization of Spirit's Attribute Handling"> <link rel="prev" href="clear_value.html" title="Re-Initialize an Attribute Value before Parsing (Qi)"> <link rel="next" href="iterate.html" title="Extract Attribute Values to Generate Output from a Container (Karma)"> </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="../../../../../../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="clear_value.html"><img src="../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../customize.html"><img src="../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="iterate.html"><img src="../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> <div class="section" title="Extract an Attribute Value to Generate Output (Karma)"> <div class="titlepage"><div><div><h4 class="title"> <a name="spirit.advanced.customize.extract_from"></a><a class="link" href="extract_from.html" title="Extract an Attribute Value to Generate Output (Karma)"> Extract an Attribute Value to Generate Output (Karma)</a> </h4></div></div></div> <a name="spirit.advanced.customize.extract_from.extract_from"></a><h6> <a name="id990612"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.extract_from">extract_from</a> </h6> <p> Before generating output for a value this value needs to extracted from the attribute instance provided by the user. The customization point <code class="computeroutput"><span class="identifier">extract_from</span></code> is utilized to adapt this extraction for any data type possibly used to store the values to output. </p> <a name="spirit.advanced.customize.extract_from.module_headers"></a><h6> <a name="id990636"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.module_headers">Module Headers</a> </h6> <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">karma</span><span class="special">/</span><span class="identifier">detail</span><span class="special">/</span><span class="identifier">extract_from</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> <div class="note" title="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> This header file does not need to be included directly by any user program as it is normally included by other Spirit header files relying on its content. </p></td></tr> </table></div> <a name="spirit.advanced.customize.extract_from.namespace"></a><h6> <a name="id990729"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.namespace">Namespace</a> </h6> <div class="informaltable"><table class="table"> <colgroup><col></colgroup> <thead><tr><th> <p> Name </p> </th></tr></thead> <tbody><tr><td> <p> <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> </p> </td></tr></tbody> </table></div> <a name="spirit.advanced.customize.extract_from.synopsis"></a><h6> <a name="id990802"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.synopsis">Synopsis</a> </h6> <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> <span class="keyword">struct</span> <span class="identifier">extract_from_attribute</span> <span class="special">{</span> <span class="keyword">typedef</span> <span class="special"><</span><span class="identifier">unspecified</span><span class="special">></span> <span class="identifier">type</span><span class="special">;</span> <span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Context</span><span class="special">></span> <span class="keyword">static</span> <span class="identifier">type</span> <span class="identifier">call</span><span class="special">(</span><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">,</span> <span class="identifier">Context</span><span class="special">&</span> <span class="identifier">context</span><span class="special">);</span> <span class="special">};</span> </pre> <a name="spirit.advanced.customize.extract_from.template_parameters"></a><h6> <a name="id990977"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.template_parameters">Template parameters</a> </h6> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Parameter </p> </th> <th> <p> Description </p> </th> <th> <p> Default </p> </th> </tr></thead> <tbody> <tr> <td> <p> <code class="computeroutput"><span class="identifier">Attrib</span></code> </p> </td> <td> <p> The type, <code class="computeroutput"><span class="identifier">Attrib</span></code> of the attribute to be used to generate output from. </p> </td> <td> <p> none </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">Enable</span></code> </p> </td> <td> <p> Helper template parameter usable to selectively enable or disable certain specializations of <code class="computeroutput"><span class="identifier">clear_value</span></code> utilizing SFINAE (i.e. <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">enable_if</span></code> or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">disable_if</span></code>). </p> </td> <td> <p> <code class="computeroutput"><span class="keyword">void</span></code> </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">Context</span></code> </p> </td> <td> <p> This is the type of the current generator execution context. </p> </td> <td class="auto-generated"> </td> </tr> </tbody> </table></div> <a name="spirit.advanced.customize.extract_from.notation"></a><h6> <a name="id991167"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.notation">Notation</a> </h6> <div class="variablelist" title="Notation"> <p class="title"><b>Notation</b></p> <dl> <dt><span class="term"><code class="computeroutput"><span class="identifier">Attrib</span></code></span></dt> <dd><p> A type to be used to generate output from. </p></dd> <dt><span class="term"><code class="computeroutput"><span class="identifier">attr</span></code></span></dt> <dd><p> A attribute instance of type <code class="computeroutput"><span class="identifier">Attrib</span></code>. </p></dd> </dl> </div> <a name="spirit.advanced.customize.extract_from.expression_semantics"></a><h6> <a name="id991233"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.expression_semantics">Expression Semantics</a> </h6> <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> </p> <pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">extract_from_attribute</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">attr</span><span class="special">,</span> <span class="identifier">ctx</span><span class="special">)</span></pre> <p> </p> </td> <td> <p> Extract the value to generate output from and return it to the caller. </p> </td> </tr></tbody> </table></div> <a name="spirit.advanced.customize.extract_from.predefined_specializations"></a><h6> <a name="id991340"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.predefined_specializations">Predefined Specializations</a> </h6> <p> <a href="http://spirit.sourceforge.net" target="_top">Spirit</a> predefines specializations of this customization point for several types. The following table lists those types together with the types exposed and the corresponding semantics: </p> <div class="informaltable"><table class="table"> <colgroup> <col> <col> </colgroup> <thead><tr> <th> <p> Template Parameters </p> </th> <th> <p> Value </p> </th> </tr></thead> <tbody> <tr> <td> <p> <code class="computeroutput"><span class="identifier">Attrib</span></code> </p> </td> <td> <p> The exposed typedef <code class="computeroutput"><span class="identifier">type</span></code> is defined to <code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>. The function <code class="computeroutput"><span class="identifier">call</span><span class="special">()</span></code> returns the argument by reference without change. </p> </td> </tr> <tr> <td> <p> <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> </p> </td> <td> <p> The exposed typedef <code class="computeroutput"><span class="identifier">type</span></code> is defined to <code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>. The function <code class="computeroutput"><span class="identifier">call</span><span class="special">()</span></code> returns the value held by the <code class="computeroutput"><span class="identifier">optional</span><span class="special"><></span></code> argument by reference without change. </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">reference_wrapper</span><span class="special"><</span><span class="identifier">Attrib</span><span class="special">></span></code> </p> </td> <td> <p> The exposed typedef <code class="computeroutput"><span class="identifier">type</span></code> is defined to <code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>. The function <code class="computeroutput"><span class="identifier">call</span><span class="special">()</span></code> returns the value held by the <code class="computeroutput"><span class="identifier">reference_wrapper</span><span class="special"><></span></code> argument by reference without change. </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">unused_type</span></code> </p> </td> <td> <p> The exposed typedef <code class="computeroutput"><span class="identifier">type</span></code> is defined to <code class="computeroutput"><span class="identifier">unused_type</span></code>. 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>. </p> </td> </tr> </tbody> </table></div> <a name="spirit.advanced.customize.extract_from.when_to_implement"></a><h6> <a name="id991684"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.when_to_implement">When to implement</a> </h6> <p> The customization point <code class="computeroutput"><span class="identifier">extract_from_attribute</span></code> needs to be implemented for a specific type whenever the default implementation as shown above is not applicable. Examples for this could be that the type to be extracted is different from <code class="computeroutput"><span class="identifier">Attrib</span></code> and is not copy constructible. </p> <a name="spirit.advanced.customize.extract_from.example"></a><h6> <a name="id991718"></a> <a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.example">Example</a> </h6> <p> TBD </p> </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="clear_value.html"><img src="../../../../../../../doc/html/images/prev.png" alt="Prev"></a><a accesskey="u" href="../customize.html"><img src="../../../../../../../doc/html/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/html/images/home.png" alt="Home"></a><a accesskey="n" href="iterate.html"><img src="../../../../../../../doc/html/images/next.png" alt="Next"></a> </div> </body> </html>