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| <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="id1175921"></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="id1175963"></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="id1177309"></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="id1177382"></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="id1177570"></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="id1177815"></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="id1177903"></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="id1178020"></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="id1178372"></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="id1178405"></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> |
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