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<div class="section" title="Determine if a Type Should be Treated as a Container (Qi and Karma)">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.advanced.customize.is_container"></a><a class="link" href="is_container.html" title="Determine if a Type Should be Treated as a Container (Qi and Karma)"> Determine
if a Type Should be Treated as a Container (Qi and Karma)</a>
</h4></div></div></div>
<a name="spirit.advanced.customize.is_container.is_container"></a><h6>
<a name="id974887"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.is_container">is_container</a>
</h6>
<p>
The template <code class="computeroutput"><span class="identifier">is_container</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> <a class="link" href="../../qi/reference/operator/sequence.html" title="Sequence (a >> b)">Sequence</a>
(<code class="computeroutput"><span class="special">>></span></code>) and <span class="emphasis"><em>Karma</em></span>
<a class="link" href="../../karma/reference/operator/sequence.html" title="Sequences (a << b)">Sequence (<code class="computeroutput"><span class="special"><<</span></code>)</a> operators in order to determine
whether a supplied attribute can potentially be treated as a container.
</p>
<a name="spirit.advanced.customize.is_container.header"></a><h6>
<a name="id974942"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.header">Header</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" title="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.is_container.namespace"></a><h6>
<a name="id975027"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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.is_container.synopsis"></a><h6>
<a name="id975100"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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">is_container</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>
<a name="spirit.advanced.customize.is_container.template_parameters"></a><h6>
<a name="id975199"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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>
which needs to be tested whether it has to be treated as a container
</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">is_container</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.is_container.notation"></a><h6>
<a name="id975364"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.notation">Notation</a>
</h6>
<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>
<a name="spirit.advanced.customize.is_container.expression_semantics"></a><h6>
<a name="id975430"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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">is_container</span><span class="special"><</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Result of the metafunction that evaluates to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
if a given type, <code class="computeroutput"><span class="identifier">C</span></code>,
is to be treated as a container, <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>
otherwise (See <a href="../../../../../../../libs/mpl/doc/refmanual/integral-constant.html" target="_top">MPL
Boolean Constant</a>).
</p>
</td>
</tr></tbody>
</table></div>
<a name="spirit.advanced.customize.is_container.predefined_specializations"></a><h6>
<a name="id975560"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.predefined_specializations">Predefined
Specializations</a>
</h6>
<p>
<a href="http://spirit.sourceforge.net" target="_top">Spirit</a> predefines specializations
of this customization point for several types. The following table lists
those types together with the conditions for which the corresponding specializations
will evaluate to <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> (see <a href="../../../../../../../libs/mpl/doc/refmanual/integral-constant.html" target="_top">MPL
Boolean Constant</a>):
</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">T</span></code>
</p>
</td>
<td>
<p>
Returns <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> if <code class="computeroutput"><span class="identifier">T</span></code>
has the following embedded types defined: <code class="computeroutput"><span class="identifier">value_type</span></code>,
<code class="computeroutput"><span class="identifier">iterator</span></code>, <code class="computeroutput"><span class="identifier">size_type</span></code>, and<code class="computeroutput"><span class="identifier">reference</span></code>.
Otherwise it will return <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</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">T</span><span class="special">></span></code>
</p>
</td>
<td>
<p>
Returns <code class="computeroutput"><span class="identifier">is_container</span><span class="special"><</span><span class="identifier">T</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">mpl</span><span class="special">::</span><span class="identifier">true_</span></code> if at least one of the
<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> returns <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">true_</span></code>
(where <code class="computeroutput"><span class="identifier">TN</span></code> is <code class="computeroutput"><span class="identifier">T1</span></code>, <code class="computeroutput"><span class="identifier">T2</span></code>,
...). Otherwise it will return <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</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">mpl</span><span class="special">::</span><span class="identifier">false_</span></code>.
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.advanced.customize.is_container.when_to_implement"></a><h6>
<a name="id975972"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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>) and generators (<span class="emphasis"><em>Spirit.Karma</em></span>).
As a rule of thumb: it has to be implemented whenever a certain type is
to be passed as an attribute to a parser or a generator normally exposing
a STL container, <code class="computeroutput"><span class="identifier">C</span></code> and
if the type does not expose the interface of a STL container (i.e. <code class="computeroutput"><span class="identifier">is_container</span><span class="special"><</span><span class="identifier">C</span><span class="special">>::</span><span class="identifier">type</span></code> would normally return <code class="computeroutput"><span class="identifier">mpl</span><span class="special">::</span><span class="identifier">false_</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>
<a name="spirit.advanced.customize.is_container.related_attribute_customization_points"></a><h6>
<a name="id977390"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.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="store_value/container_value.html" title="Determine the Type to be Stored in a Container (Qi)"><code class="computeroutput"><span class="identifier">container_value</span></code></a>
</p>
</td>
<td>
<p>
Needs to be implemented whenever <code class="computeroutput"><span class="identifier">is_container</span></code>
is implemented.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="store_value/push_back.html" title="Store a Parsed Attribute Value into a Container (Qi)"><code class="computeroutput"><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 (a % b)">List</a>,
<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/directive/repeat.html" title="Repetition (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/container_iterator.html" title="Determine the Type of the Iterator of a Container (Karma)"><code class="computeroutput"><span class="identifier">container_iterator</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/begin_container.html" title="Get the Iterator pointing to the Begin of a Container Attribute"><code class="computeroutput"><span class="identifier">begin_container</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/end_container.html" title="Get the Iterator pointing to the End of a Container Attribute"><code class="computeroutput"><span class="identifier">end_container</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/deref_iterator.html" title="Dereference the Iterator pointing into a Container Attribute"><code class="computeroutput"><span class="identifier">deref_iterator</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/next_iterator.html" title="Increment the Iterator pointing into a Container Attribute"><code class="computeroutput"><span class="identifier">next_iterator</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<a class="link" href="iterate/compare_iterators.html" title="Compare two Iterator pointing into a Container Attribute for Equality"><code class="computeroutput"><span class="identifier">compare_iterators</span></code></a>
</p>
</td>
<td>
<p>
Karma: <a class="link" href="../../karma/reference/operator/list.html" title="Lists (a % b)">List
(<code class="computeroutput"><span class="special">%</span></code>)</a>, <a class="link" href="../../karma/reference/operator/kleene.html" title="Kleene Star (*a)">Kleene (unary
<code class="computeroutput"><span class="special">*</span></code>)</a>, <a class="link" href="../../karma/reference/operator/plus.html" title="Plus (+a)">Plus
(unary <code class="computeroutput"><span class="special">+</span></code>)</a>,
<a class="link" href="../../karma/reference/directive/repeat.html" title="Repetition Directive (repeat[])">Repeat</a>.
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.advanced.customize.is_container.example"></a><h6>
<a name="id977923"></a>
<a class="link" href="is_container.html#spirit.advanced.customize.is_container.example">Example</a>
</h6>
<p>
For examples of how to use the customization point <code class="computeroutput"><span class="identifier">is_container</span></code>
please see here: <a class="link" href="iterate/container_iterator.html#spirit.advanced.customize.iterate.container_iterator.example">embedded_container_example</a>,
<a class="link" href="iterate/next_iterator.html#spirit.advanced.customize.iterate.next_iterator.example">use_as_container</a>,
and <a class="link" href="iterate/deref_iterator.html#spirit.advanced.customize.iterate.deref_iterator.example">counter_example</a>.
</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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