<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/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="../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="../../../../../../../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="clear_value.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="iterate.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
<div class="section">
<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="id1220009"></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>
<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 interface of this customization point has been changed with Boost
V1.44. We added the <code class="computeroutput"><span class="identifier">Exposed</span></code>
template parameter to allow for more fine grained specializations of
the required <span class="emphasis"><em>Spirit.Karma</em></span> attribute transformations.
</p></td></tr>
</table></div>
<a name="spirit.advanced.customize.extract_from.module_headers"></a><h6>
<a name="id1220051"></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"><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="id1220143"></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="id1220216"></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">Exposed</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="id1220405"></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">Exposed</span></code>
</p>
</td>
<td>
<p>
The type, <code class="computeroutput"><span class="identifier">Exposed</span></code>
of the attribute natively exposed by the component the <code class="computeroutput"><span class="identifier">extract_from</span></code> is invoked from.
</p>
</td>
<td>
<p>
none
</p>
</td>
</tr>
<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="id1220650"></a>
<a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.notation">Notation</a>
</h6>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Exposed</span></code></span></dt>
<dd><p>
A type exposed as the native attribute of a component.
</p></dd>
<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="id1220737"></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">Exposed</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="id1220855"></a>
<a class="link" href="extract_from.html#spirit.advanced.customize.extract_from.predefined_specializations">Predefined
Specializations</a>
</h6>
<p>
<a href="http://boost-spirit.com" 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="id1221206"></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="id1221240"></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/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="iterate.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a>
</div>
</body>
</html>
