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<div class="section" title="Transform an Attribute to a Different Type (Qi and Karma)">
<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.advanced.customize.transform"></a><a class="link" href="transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"> Transform an
Attribute to a Different Type (Qi and Karma)</a>
</h4></div></div></div>
<a name="spirit.advanced.customize.transform.transform_attribute"></a><h6>
<a name="id977978"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.transform_attribute">transform_attribute</a>
</h6>
<p>
The template <code class="computeroutput"><span class="identifier">transform_attribute</span></code>
is a type used as an attribute customization point. It is invoked by <span class="emphasis"><em>Qi</em></span>
<code class="computeroutput"><span class="identifier">rule</span></code> and <code class="computeroutput"><span class="identifier">attr_cast</span></code>, and <span class="emphasis"><em>Karma</em></span>
<code class="computeroutput"><span class="identifier">rule</span></code> and <a class="link" href="../../karma/reference/auxiliary/attr_cast.html" title="Attribute Transformation Pseudo Generator (attr_cast)"><code class="computeroutput"><span class="identifier">attr_cast</span></code></a>. It is used to automatically
transform the user provided attribute to the attribute type expected by
the right hand side component (for <code class="computeroutput"><span class="identifier">rule</span></code>)
or the embedded component (for <code class="computeroutput"><span class="identifier">attr_cast</span></code>).
</p>
<a name="spirit.advanced.customize.transform.module_headers"></a><h6>
<a name="id978055"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.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" 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.transform.namespace"></a><h6>
<a name="id978140"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.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.transform.synopsis"></a><h6>
<a name="id978213"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.synopsis">Synopsis</a>
</h6>
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">Exposed</span><span class="special">,</span> <span class="keyword">typename</span> <span class="identifier">Transformed</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">transform_attribute</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="keyword">static</span> <span class="identifier">type</span> <span class="identifier">pre</span><span class="special">(</span><span class="identifier">Exposed</span><span class="special">&</span> <span class="identifier">val</span><span class="special">);</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">post</span><span class="special">(</span><span class="identifier">Exposed</span><span class="special">&</span> <span class="identifier">val</span><span class="special">,</span> <span class="identifier">type</span> <span class="identifier">attr</span><span class="special">);</span> <span class="comment">// Qi only
</span><span class="special">};</span>
</pre>
<a name="spirit.advanced.customize.transform.template_parameters"></a><h6>
<a name="id978406"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.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">Exposed</span></code>
</p>
</td>
<td>
<p>
The attribute type supplied to the component which needs to be
transformed.
</p>
</td>
<td>
<p>
none
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Transformed</span></code>
</p>
</td>
<td>
<p>
The attribute type expected by the component to be provided as
the result of the transformation.
</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">transform_attribute</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.transform.notation"></a><h6>
<a name="id978592"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.notation">Notation</a>
</h6>
<div class="variablelist" title="Notation">
<p class="title"><b>Notation</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Exposed</span></code></span></dt>
<dd><p>
The type, <code class="computeroutput"><span class="identifier">Exposed</span></code> is
the type of the attribute as passed in by the user.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">Transformed</span></code></span></dt>
<dd><p>
The type, <code class="computeroutput"><span class="identifier">Transformed</span></code>
is the type of the attribute as passed along to the right hand side
of the <code class="computeroutput"><span class="identifier">rule</span></code> (embedded
component of <code class="computeroutput"><span class="identifier">attr_cast</span></code>).
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">exposed</span></code></span></dt>
<dd><p>
An instance of type <code class="computeroutput"><span class="identifier">Exposed</span></code>.
</p></dd>
<dt><span class="term"><code class="computeroutput"><span class="identifier">transformed</span></code></span></dt>
<dd><p>
An instance of type <code class="computeroutput"><span class="identifier">Transformed</span></code>.
</p></dd>
</dl>
</div>
<a name="spirit.advanced.customize.transform.expression_semantics"></a><h6>
<a name="id978730"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.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">transform_attribute</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">,</span> <span class="identifier">Transformed</span><span class="special">>::</span><span class="identifier">type</span></code>
</p>
</td>
<td>
<p>
Evaluates to the type to be used as the result of the transformation
(to be passed to the right hand side of the <code class="computeroutput"><span class="identifier">rule</span></code>
or to the embedded component of the <code class="computeroutput"><span class="identifier">attr_cast</span></code>.
Most of the time this is equal to <code class="computeroutput"><span class="identifier">Transformed</span></code>,
but in other cases this might evaluate to <code class="computeroutput"><span class="identifier">Transformed</span><span class="special">&</span></code> instead avoiding superfluous
object creation.
</p>
</td>
</tr>
<tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="identifier">type</span> <span class="identifier">transform_attribute</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">,</span> <span class="identifier">Transformed</span><span class="special">>::</span><span class="identifier">pre</span><span class="special">(</span><span class="identifier">exposed</span><span class="special">)</span></pre>
<p>
</p>
</td>
<td>
<p>
Do <code class="computeroutput"><span class="identifier">pre</span></code>-transformation
before invoking the right hand side component for <code class="computeroutput"><span class="identifier">rule</span></code> (or the embedded component
for <code class="computeroutput"><span class="identifier">attr_cast</span></code>).
This takes the attribute supplied as by the user (of type <code class="computeroutput"><span class="identifier">Exposed</span></code>) and returns the attribute
to be passed down the component hierarchy (of the type as exposed
by the metafunction <code class="computeroutput"><span class="identifier">type</span></code>).
This function will be called in <span class="emphasis"><em>Qi</em></span> and for
<span class="emphasis"><em>Karma</em></span>.
</p>
</td>
</tr>
<tr>
<td>
<p>
</p>
<pre xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" class="table-programlisting"><span class="keyword">void</span> <span class="identifier">transform_attribute</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">,</span> <span class="identifier">Transformed</span><span class="special">>::</span><span class="identifier">post</span><span class="special">(</span><span class="identifier">exposed</span><span class="special">,</span> <span class="identifier">transformed</span><span class="special">)</span></pre>
<p>
</p>
</td>
<td>
<p>
Do <code class="computeroutput"><span class="identifier">post</span></code>-transformation
after the invocation of the right hand side component for <code class="computeroutput"><span class="identifier">rule</span></code> (or the embedded component
for <code class="computeroutput"><span class="identifier">attr_cast</span></code>).
This takes the original attribute as supplied by the user and the
attribute as returned from the right hand side (embedded) component
and is expected to propagate the result back into the supplied
attribute instance. This function will be called in <span class="emphasis"><em>Qi</em></span>
only.
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.advanced.customize.transform.predefined_specializations"></a><h6>
<a name="id979064"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.predefined_specializations">Predefined
Specializations</a>
</h6>
<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">Exposed</span></code>, <code class="computeroutput"><span class="identifier">Transformed</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">type</span></code> evaluates
to <code class="computeroutput"><span class="identifier">Transformed</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns a new instance of <code class="computeroutput"><span class="identifier">Transformed</span></code>
constructed from the argument of type <code class="computeroutput"><span class="identifier">Exposed</span></code>,
<code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
assigns <code class="computeroutput"><span class="identifier">transformed</span></code>
to <code class="computeroutput"><span class="identifier">exposed</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Exposed</span><span class="special">&</span></code>,
<code class="computeroutput"><span class="identifier">Transformed</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">type</span></code> evaluates
to <code class="computeroutput"><span class="identifier">Transformed</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns a new instance of <code class="computeroutput"><span class="identifier">Transformed</span></code>
constructed from the argument of type <code class="computeroutput"><span class="identifier">Exposed</span></code>,
<code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
assigns <code class="computeroutput"><span class="identifier">transformed</span></code>
to <code class="computeroutput"><span class="identifier">exposed</span></code>.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Attrib</span><span class="special">&</span></code>,
<code class="computeroutput"><span class="identifier">Attrib</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">type</span></code> evaluates
to <code class="computeroutput"><span class="identifier">Attrib</span><span class="special">&</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns it's argument, <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code> does nothing.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Exposed</span> <span class="keyword">const</span></code>,
<code class="computeroutput"><span class="identifier">Transformed</span></code>
</p>
</td>
<td>
<p>
(usind in <span class="emphasis"><em>Karma</em></span> only) <code class="computeroutput"><span class="identifier">type</span></code>
evaluates to <code class="computeroutput"><span class="identifier">Transformed</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns it's argument, <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code> is not implemented.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>, <code class="computeroutput"><span class="identifier">Attrib</span></code>
</p>
</td>
<td>
<p>
(usind in <span class="emphasis"><em>Karma</em></span> only) <code class="computeroutput"><span class="identifier">type</span></code>
evaluates to <code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns it's argument, <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code> is not implemented.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span></code>,
<code class="computeroutput"><span class="identifier">Attrib</span></code>
</p>
</td>
<td>
<p>
(usind in <span class="emphasis"><em>Karma</em></span> only) <code class="computeroutput"><span class="identifier">type</span></code>
evaluates to <code class="computeroutput"><span class="identifier">Attrib</span> <span class="keyword">const</span><span class="special">&</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
returns it's argument, <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code> is not implemented.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">unused_type</span></code>, <code class="computeroutput"><span class="identifier">Attrib</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">type</span></code> evaluates
to <code class="computeroutput"><span class="identifier">unused_type</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
and <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
do nothing.
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Attrib</span></code>, <code class="computeroutput"><span class="identifier">unused_type</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">type</span></code> evaluates
to <code class="computeroutput"><span class="identifier">unused_type</span></code>,
<code class="computeroutput"><span class="identifier">pre</span><span class="special">()</span></code>
and <code class="computeroutput"><span class="identifier">post</span><span class="special">()</span></code>
do nothing.
</p>
</td>
</tr>
</tbody>
</table></div>
<a name="spirit.advanced.customize.transform.when_to_implement"></a><h6>
<a name="id979779"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.when_to_implement">When
to implement</a>
</h6>
<p>
The customization point <code class="computeroutput"><span class="identifier">transform_attribute</span></code>
needs to be implemented for a specific pair of types whenever the attribute
type supplied to a <code class="computeroutput"><span class="identifier">rule</span></code>
or <code class="computeroutput"><span class="identifier">attr_cast</span></code> cannot automatically
transformed to the attribute type expected by the right hand side of the
<code class="computeroutput"><span class="identifier">rule</span></code> (embedded component
of the <code class="computeroutput"><span class="identifier">attr_cast</span></code>) because
the default implementation as shown above is not applicable. Examples for
this could be that the type <code class="computeroutput"><span class="identifier">Transformed</span></code>
is not constructible from the type <code class="computeroutput"><span class="identifier">Exposed</span></code>.
</p>
<a name="spirit.advanced.customize.transform.example"></a><h6>
<a name="id979846"></a>
<a class="link" href="transform.html#spirit.advanced.customize.transform.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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