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<html>
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<title>Determine the Type to be Stored in a Container (Qi)</title>
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</div>
<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.advanced.customize.store_value.container_value"></a><a class="link" href="container_value.html" title="Determine the Type to be Stored in a Container (Qi)">Determine
the Type to be Stored in a Container (Qi)</a>
</h5></div></div></div>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h0"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.container_value"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_value.container_value">container_value</a>
</h6>
<p>
The template <code class="computeroutput"><span class="identifier">container_value</span></code>
is a template meta function 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 Parser (*a)">Kleene</a>, <a class="link" href="../../../qi/reference/operator/plus.html" title="Plus Parser (+a)">Plus</a>,
<a class="link" href="../../../qi/reference/operator/list.html" title="List Parser (a % b)">List</a>, and <a class="link" href="../../../qi/reference/directive/repeat.html" title="Repetition Parser Directive (repeat[])">Repeat</a>) to determine
the type to store in a container.
</p>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h1"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.module_headers"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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">container</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>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h2"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.namespace"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h3"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.synopsis"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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">Container</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">container_value</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="special">};</span>
</pre>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h4"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.template_parameters"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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">Container</span></code>
</p>
</td>
<td>
<p>
The type <code class="computeroutput"><span class="identifier">Container</span></code>
is the type for which the type f the elements has to be deduced.
</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">container_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>
<div class="variablelist">
<p class="title"><b></b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">C</span></code></span></dt>
<dd><p>
A type to be tested whether it needs to be treated as a container.
</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>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h5"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.expression_semantics"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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>
<code class="computeroutput"><span class="identifier">container_value</span><span class="special"><</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Metafunction that evaluates to the type to be stored in a given
container type, <code class="computeroutput"><span class="identifier">C</span></code>.
</p>
</td>
</tr></tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h6"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.predefined_specializations"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_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>
Semantics
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">C</span></code>
</p>
</td>
<td>
<p>
The non-const <code class="computeroutput"><span class="identifier">value_type</span></code>
of the given container type, <code class="computeroutput"><span class="identifier">C</span></code>.
</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">C</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
Returns <code class="computeroutput"><span class="identifier">container_value</span><span class="special"><</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>
</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>
Returns <code class="computeroutput"><span class="identifier">container_value</span><span class="special"><</span><span class="identifier">TN</span><span class="special">>::</span><span class="identifier">value</span></code>
for the first <code class="computeroutput"><span class="identifier">TN</span></code>
(out of <code class="computeroutput"><span class="identifier">T1</span></code>,
<code class="computeroutput"><span class="identifier">T2</span></code>, ...) for
which <code class="computeroutput"><span class="identifier">is_container</span><span class="special"><</span><span class="identifier">TN</span><span class="special">>::</span><span class="identifier">type</span></code>
evaluates to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>.
Otherwise it will return <code class="computeroutput"><span class="identifier">unused_type</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">unused_type</span></code>
</p>
</td>
<td>
<p>
Returns <code class="computeroutput"><span class="identifier">unused_type</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h7"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.when_to_implement"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_value.when_to_implement">When
to implement</a>
</h6>
<p>
The customization point <code class="computeroutput"><span class="identifier">is_container</span></code>
needs to be implemented for a specific type whenever this type is to
be used as an attribute in place of a STL container. It is applicable
for parsers (<span class="emphasis"><em>Spirit.Qi</em></span>) only. As a rule of thumb:
it has to be implemented whenever a certain type is to be passed as an
attribute to a parser normally exposing a STL container and if the type
does not expose the interface of a STL container (i.e. no embedded typedef
for <code class="computeroutput"><span class="identifier">value_type</span></code>). These
components have an attribute propagation rule in the form:
</p>
<pre class="programlisting"><span class="identifier">a</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--></span> <span class="identifier">Op</span><span class="special">(</span><span class="identifier">a</span><span class="special">):</span> <span class="identifier">vector</span><span class="special"><</span><span class="identifier">A</span><span class="special">></span>
</pre>
<p>
where <code class="computeroutput"><span class="identifier">Op</span><span class="special">(</span><span class="identifier">a</span><span class="special">)</span></code>
stands for any meaningful operation on the component <code class="computeroutput"><span class="identifier">a</span></code>.
</p>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h8"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.related_attribute_customization_points"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_value.related_attribute_customization_points">Related
Attribute Customization Points</a>
</h6>
<p>
If this customization point is implemented, the following other customization
points might need to be implemented as well.
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Name
</p>
</th>
<th>
<p>
When to implement
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<a class="link" href="push_back.html" title="Store a Parsed Attribute Value into a Container (Qi)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">push_back_container</span></code></a>
</p>
</td>
<td>
<p>
Qi: <a class="link" href="../../../qi/reference/operator/list.html" title="List Parser (a % b)">List</a>,
<a class="link" href="../../../qi/reference/operator/kleene.html" title="Kleene Parser (*a)">Kleene</a>,
<a class="link" href="../../../qi/reference/operator/plus.html" title="Plus Parser (+a)">Plus</a>,
<a class="link" href="../../../qi/reference/directive/repeat.html" title="Repetition Parser Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="../clear_value.html" title="Re-Initialize an Attribute Value before Parsing (Qi)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">clear_value</span></code></a>
</p>
</td>
<td>
<p>
Qi: <a class="link" href="../../../qi/reference/operator/list.html" title="List Parser (a % b)">List</a>,
<a class="link" href="../../../qi/reference/operator/kleene.html" title="Kleene Parser (*a)">Kleene</a>,
<a class="link" href="../../../qi/reference/operator/plus.html" title="Plus Parser (+a)">Plus</a>,
<a class="link" href="../../../qi/reference/directive/repeat.html" title="Repetition Parser Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.advanced.customize.store_value.container_value.h9"></a>
<span><a name="spirit.advanced.customize.store_value.container_value.example"></a></span><a class="link" href="container_value.html#spirit.advanced.customize.store_value.container_value.example">Example</a>
</h6>
<p>
Here is an example showing the default implementation of the <a class="link" href="container_value.html" title="Determine the Type to be Stored in a Container (Qi)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">container_value</span></code></a> customization
point provided by the library:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Enable</span><span class="comment">/* = void*/</span><span class="special">></span>
<span class="keyword">struct</span> <span class="identifier">container_value</span>
<span class="special">:</span> <span class="identifier">detail</span><span class="special">::</span><span class="identifier">remove_value_const</span><span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Container</span><span class="special">::</span><span class="identifier">value_type</span><span class="special">></span>
<span class="special">{};</span>
</pre>
<p>
</p>
<p>
This template is instantiated by the library at the appropriate places
while using the supplied container type as the template argument. The
embedded <code class="computeroutput"><span class="identifier">type</span></code> is used
as the attribute type while parsing the elements to be store in that
container.
</p>
<p>
The following example shows the predefined specialization for <code class="computeroutput"><span class="identifier">unused_type</span></code>:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><></span>
<span class="keyword">struct</span> <span class="identifier">container_value</span><span class="special"><</span><span class="identifier">unused_type</span><span class="special">></span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="identifier">unused_type</span> <span class="identifier">type</span><span class="special">;</span>
<span class="special">};</span>
</pre>
<p>
</p>
<p>
which defines its embedded <code class="computeroutput"><span class="identifier">type</span></code>
to be <code class="computeroutput"><span class="identifier">unused_type</span></code> as
well, this way propagating the 'don't care' attribute status to the embedded
parser.
</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 © 2001-2011 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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