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| <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="id1174802"></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="id1174848"></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="id1174934"></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="id1175007"></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="id1175114"></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="id1175286"></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="id1175382"></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="id1175500"></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="id1175858"></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="id1175892"></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> |
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