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<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
<title>Attribute Transformation Pseudo Parser (attr_cast)</title>
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<div class="section">
<div class="titlepage"><div><div><h5 class="title">
<a name="spirit.qi.reference.auxiliary.attr_cast"></a><a class="link" href="attr_cast.html" title="Attribute Transformation Pseudo Parser (attr_cast)">Attribute
Transformation Pseudo Parser (<code class="computeroutput"><span class="identifier">attr_cast</span></code>)</a>
</h5></div></div></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h0"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.description"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.description">Description</a>
</h6>
<p>
The <code class="computeroutput"><span class="identifier">attr_cast</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">,</span>
<span class="identifier">Transformed</span><span class="special">>()</span></code>
component invokes the embedded parser while supplying an attribute of
type <code class="computeroutput"><span class="identifier">Transformed</span></code>. The
supplied attribute gets created from the original attribute (of type
<code class="computeroutput"><span class="identifier">Exposed</span></code>) passed to this
component using the customization point <a class="link" href="../../../advanced/customize/transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">transform_attribute</span></code></a>.
</p>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h1"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.header"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.header">Header</a>
</h6>
<pre class="programlisting"><span class="comment">// forwards to <boost/spirit/home/qi/auxiliary/attr_cast.hpp></span>
<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">include</span><span class="special">/</span><span class="identifier">qi_attr_cast</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>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h2"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.namespace"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.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">attr_cast</span> <span class="comment">//
alias: boost::spirit::qi::attr_cast</span></code>
</p>
</td></tr></tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h3"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.synopsis"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.synopsis">Synopsis</a>
</h6>
<pre class="programlisting"><span class="keyword">template</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="special"><</span><span class="identifier">unspecified</span><span class="special">></span> <span class="identifier">attr_cast</span><span class="special">(<</span><span class="identifier">unspecified</span><span class="special">>);</span>
</pre>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h4"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.template_parameters"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.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 type of the attribute supplied to the <code class="computeroutput"><span class="identifier">attr_cast</span></code>.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">unused_type</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">Transformed</span></code>
</p>
</td>
<td>
<p>
The type of the attribute expected by the embedded parser
<code class="computeroutput"><span class="identifier">p</span></code>.
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">unused_type</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
The <code class="computeroutput"><span class="identifier">attr_cast</span></code> is a function
template. It is possible to invoke it using the following schemes:
</p>
<pre class="programlisting"><span class="identifier">attr_cast</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span>
<span class="identifier">attr_cast</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">>(</span><span class="identifier">p</span><span class="special">)</span>
<span class="identifier">attr_cast</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">p</span><span class="special">)</span>
</pre>
<p>
depending on which of the attribute types can be deduced properly if
not explicitly specified.
</p>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h5"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.model_of"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.model_of">Model of</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
<a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a>
</p></blockquote></div>
<div class="variablelist">
<p class="title"><b>Notation</b></p>
<dl>
<dt><span class="term"><code class="computeroutput"><span class="identifier">p</span></code></span></dt>
<dd><p>
A parser object.
</p></dd>
</dl>
</div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h6"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.expression_semantics"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.expression_semantics">Expression
Semantics</a>
</h6>
<p>
Semantics of an expression is defined only where it differs from, or
is not defined in <a class="link" href="../parser_concepts/unaryparser.html" title="UnaryParser"><code class="computeroutput"><span class="identifier">UnaryParser</span></code></a>.
</p>
<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">attr_cast</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Create a component invoking the parser <code class="computeroutput"><span class="identifier">p</span></code>
while passing an attribute of the type as normally expected
by <code class="computeroutput"><span class="identifier">p</span></code>. The type
of the supplied attribute will be transformed to the type
<code class="computeroutput"><span class="identifier">p</span></code> exposes as
its attribute type (by using the attribute customization point
<a class="link" href="../../../advanced/customize/transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">transform_attribute</span></code></a>).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">attr_cast</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">>(</span><span class="identifier">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Create a component invoking the parser <code class="computeroutput"><span class="identifier">p</span></code>
while passing an attribute of the type as normally expected
by <code class="computeroutput"><span class="identifier">p</span></code>. The supplied
attribute is expected to be of the type <code class="computeroutput"><span class="identifier">Exposed</span></code>,
it will be transformed to the type <code class="computeroutput"><span class="identifier">p</span></code>
exposes as its attribute type (using the attribute customization
point <a class="link" href="../../../advanced/customize/transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">transform_attribute</span></code></a>).
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">attr_cast</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">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
Create a component invoking the parser <code class="computeroutput"><span class="identifier">p</span></code>
while passing an attribute of type <code class="computeroutput"><span class="identifier">Transformed</span></code>.
The supplied attribute is expected to be of the type <code class="computeroutput"><span class="identifier">Exposed</span></code>, it will be transformed
to the type <code class="computeroutput"><span class="identifier">Transformed</span></code>
(using the attribute customization point <a class="link" href="../../../advanced/customize/transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">transform_attribute</span></code></a>).
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h7"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.attributes"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.attributes">Attributes</a>
</h6>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Expression
</p>
</th>
<th>
<p>
Attribute
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">attr_cast</span><span class="special">(</span><span class="identifier">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">p</span><span class="special">:</span>
<span class="identifier">A</span> <span class="special">--></span>
<span class="identifier">attr_cast</span><span class="special">(</span><span class="identifier">p</span><span class="special">):</span>
<span class="identifier">A</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">attr_cast</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">>(</span><span class="identifier">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">p</span><span class="special">:</span>
<span class="identifier">A</span> <span class="special">--></span>
<span class="identifier">attr_cast</span><span class="special"><</span><span class="identifier">Exposed</span><span class="special">>(</span><span class="identifier">p</span><span class="special">):</span>
<span class="identifier">Exposed</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
<code class="computeroutput"><span class="identifier">attr_cast</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">p</span><span class="special">)</span></code>
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">p</span><span class="special">:</span>
<span class="identifier">A</span> <span class="special">--></span>
<span class="identifier">attr_cast</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">p</span><span class="special">):</span>
<span class="identifier">Exposed</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h8"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.complexity"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.complexity">Complexity</a>
</h6>
<div class="blockquote"><blockquote class="blockquote"><p>
The complexity of this component is fully defined by the complexity
of the embedded parser <code class="computeroutput"><span class="identifier">p</span></code>.
</p></blockquote></div>
<h6>
<a name="spirit.qi.reference.auxiliary.attr_cast.h9"></a>
<span><a name="spirit.qi.reference.auxiliary.attr_cast.example"></a></span><a class="link" href="attr_cast.html#spirit.qi.reference.auxiliary.attr_cast.example">Example</a>
</h6>
<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>
The test harness for the example(s) below is presented in the <a class="link" href="../basics.html#spirit.qi.reference.basics.examples">Basics Examples</a>
section.
</p></td></tr>
</table></div>
<p>
Some using declarations:
</p>
<p>
</p>
<pre class="programlisting"><span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">qi</span><span class="special">::</span><span class="identifier">int_</span><span class="special">;</span>
</pre>
<p>
</p>
<p>
The example references data structure <code class="computeroutput"><span class="identifier">int_data</span></code>
which needs a specialization of the customization point <a class="link" href="../../../advanced/customize/transform.html" title="Transform an Attribute to a Different Type (Qi and Karma)"><code class="computeroutput"><span class="identifier">traits</span><span class="special">::</span><span class="identifier">transform_attribute</span></code></a>:
</p>
<p>
</p>
<pre class="programlisting"><span class="comment">// this is just a test structure we want to use in place of an int</span>
<span class="keyword">struct</span> <span class="identifier">int_data</span>
<span class="special">{</span>
<span class="keyword">int</span> <span class="identifier">i</span><span class="special">;</span>
<span class="special">};</span>
<span class="comment">// we provide a custom attribute transformation to allow its use as an int</span>
<span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">spirit</span> <span class="special">{</span> <span class="keyword">namespace</span> <span class="identifier">traits</span>
<span class="special">{</span>
<span class="comment">// in this case we just expose the embedded 'int' as the attribute instance</span>
<span class="comment">// to use, allowing to leave the function 'post()' empty</span>
<span class="keyword">template</span> <span class="special"><></span>
<span class="keyword">struct</span> <span class="identifier">transform_attribute</span><span class="special"><</span><span class="identifier">int_data</span><span class="special">,</span> <span class="keyword">int</span><span class="special">,</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">domain</span><span class="special">></span>
<span class="special">{</span>
<span class="keyword">typedef</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">type</span><span class="special">;</span>
<span class="keyword">static</span> <span class="keyword">int</span><span class="special">&</span> <span class="identifier">pre</span><span class="special">(</span><span class="identifier">int_data</span><span class="special">&</span> <span class="identifier">d</span><span class="special">)</span> <span class="special">{</span> <span class="keyword">return</span> <span class="identifier">d</span><span class="special">.</span><span class="identifier">i</span><span class="special">;</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">int_data</span><span class="special">&</span> <span class="identifier">val</span><span class="special">,</span> <span class="keyword">int</span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">attr</span><span class="special">)</span> <span class="special">{}</span>
<span class="keyword">static</span> <span class="keyword">void</span> <span class="identifier">fail</span><span class="special">(</span><span class="identifier">int_data</span><span class="special">&)</span> <span class="special">{}</span>
<span class="special">};</span>
<span class="special">}}}</span>
</pre>
<p>
</p>
<p>
Now we use the <code class="computeroutput"><span class="identifier">attr_cast</span></code>
pseudo parser to invoke the attribute transformation:
</p>
<p>
</p>
<pre class="programlisting"><span class="identifier">int_data</span> <span class="identifier">d</span> <span class="special">=</span> <span class="special">{</span> <span class="number">0</span> <span class="special">};</span>
<span class="identifier">test_parser_attr</span><span class="special">(</span><span class="string">"1"</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">qi</span><span class="special">::</span><span class="identifier">attr_cast</span><span class="special">(</span><span class="identifier">int_</span><span class="special">),</span> <span class="identifier">d</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special"><<</span> <span class="identifier">d</span><span class="special">.</span><span class="identifier">i</span> <span class="special"><<</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
</pre>
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