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<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="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">&lt;</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">&gt;</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">&lt;</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">&gt;</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">&amp;</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">&lt;</span><span class="identifier">Attrib</span><span class="special">&gt;::</span><span class="identifier">call</span><span class="special">(</span><span class="identifier">Attrib</span><span class="special">&amp;</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">&lt;&gt;::</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">&lt;</span><span class="identifier">Attrib</span><span class="special">&gt;</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">&lt;</span><span class="identifier">T1</span><span class="special">,</span>
<span class="identifier">T2</span><span class="special">,</span>
<span class="special">...&gt;</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">&lt;</span><span class="identifier">T1</span><span class="special">,</span>
<span class="identifier">T2</span><span class="special">,</span>
<span class="special">...&gt;</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 &#169; 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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