<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Re-Initialize an Attribute Value before Parsing (Qi)</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="store_value/push_back.html" title="Store a Parsed Attribute Value into a Container (Qi)"> <link rel="next" href="extract_from.html" title="Extract an Attribute Value to Generate Output (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="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> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <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 an Attribute Value before Parsing (Qi)</a> </h4></div></div></div> <a name="spirit.advanced.customize.clear_value.clear_value"></a><h6> <a name="id1218872"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.clear_value">clear_value</a> </h6> <p> The template <code class="computeroutput"><span class="identifier">clear_value</span></code> is a type used as an attribute customization point. It is invoked by the <span class="emphasis"><em>Qi</em></span> repetitive parsers (<a class="link" href="../../qi/reference/operator/kleene.html" title="Kleene (*a)">Kleene</a>, <a class="link" href="../../qi/reference/operator/plus.html" title="Plus (+a)">Plus</a>, <a class="link" href="../../qi/reference/operator/list.html" title="List (a % b)">List</a>, and <a class="link" href="../../qi/reference/directive/repeat.html" title="Repetition (repeat[])">Repeat</a>) in order to re-initialize the attribute instance passed to the embedded parser after it has been stored in the provided container. This re-initialized attribute instance is reused during the next iteration of the repetitive parser. </p> <a name="spirit.advanced.customize.clear_value.module_headers"></a><h6> <a name="id1218918"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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">support</span><span class="special">/</span><span class="identifier">attributes</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.clear_value.namespace"></a><h6> <a name="id1219004"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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.clear_value.synopsis"></a><h6> <a name="id1219077"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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">clear_value</span> <span class="special">{</span> <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> <span class="special">};</span> </pre> <a name="spirit.advanced.customize.clear_value.template_parameters"></a><h6> <a name="id1219184"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.template_parameters">Template parameters</a> </h6> <div class="informaltable"><table class="table"> <colgroup> <col> <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 re-initialized. </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> </tbody> </table></div> <a name="spirit.advanced.customize.clear_value.notation"></a><h6> <a name="id1219356"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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">Attrib</span></code></span></dt> <dd><p> A type to be used as a container to store attribute values in. </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> <dt><span class="term"><code class="computeroutput"><span class="identifier">T1</span></code>, <code class="computeroutput"><span class="identifier">T2</span></code>, ...</span></dt> <dd><p> Arbitrary types </p></dd> </dl> </div> <a name="spirit.advanced.customize.clear_value.expression_semantics"></a><h6> <a name="id1219452"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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">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> <p> </p> </td> <td> <p> Re-initialize the instance referred to by <code class="computeroutput"><span class="identifier">attr</span></code> in the most efficient way. </p> </td> </tr></tbody> </table></div> <a name="spirit.advanced.customize.clear_value.predefined_specializations"></a><h6> <a name="id1219570"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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> Re-initialize using assignment of default constructed value. </p> </td> </tr> <tr> <td> <p> Any type <code class="computeroutput"><span class="identifier">T</span></code> for which <code class="computeroutput"><span class="identifier">is_container</span><span class="special"><>::</span><span class="identifier">type</span></code> is <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> </p> </td> <td> <p> 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. </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> Clear the <code class="computeroutput"><span class="identifier">optional</span></code> instance and leave it uninitialized. </p> </td> </tr> <tr> <td> <p> <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> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...></span></code> </p> </td> <td> <p> Invoke the <code class="computeroutput"><span class="identifier">clear_value</span></code> customization point for the currently held value. </p> </td> </tr> <tr> <td> <p> <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> <span class="identifier">T2</span><span class="special">,</span> <span class="special">...></span></code> </p> </td> <td> <p> Invoke the <code class="computeroutput"><span class="identifier">clear_value</span></code> customization point for all elements of the tuple. </p> </td> </tr> <tr> <td> <p> <code class="computeroutput"><span class="identifier">unused_type</span></code> </p> </td> <td> <p> Do nothing. </p> </td> </tr> </tbody> </table></div> <a name="spirit.advanced.customize.clear_value.when_to_implement"></a><h6> <a name="id1219945"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.when_to_implement">When to Implement</a> </h6> <p> The customization point <code class="computeroutput"><span class="identifier">clear_value</span></code> needs to be implemented for a specific type whenever this type is to be used as an attribute to be stored into a STL container and if the type cannot be re-initialized using one of the specializations listed above. Examples for this might be types not being default constructible or container types not exposing a member function <code class="computeroutput"><span class="identifier">clear</span><span class="special">()</span></code>. </p> <a name="spirit.advanced.customize.clear_value.example"></a><h6> <a name="id1219980"></a> <a class="link" href="clear_value.html#spirit.advanced.customize.clear_value.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="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> </div> </body> </html>