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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.abstracts.attributes.compound_attributes"></a><a class="link" href="compound_attributes.html" title="Attributes of Compound Components">Attributes
of Compound Components</a>
</h4></div></div></div>
<p>
<span class="emphasis"><em>Spirit.Qi</em></span> and <span class="emphasis"><em>Spirit.Karma</em></span> implement
well defined attribute type propagation rules for all compound parsers
and generators, such as sequences, alternatives, Kleene star, etc. The
main attribute propagation rule for a sequences is for instance:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Library
</p>
</th>
<th>
<p>
Sequence attribute propagation rule
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Qi
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">:</span>
<span class="identifier">A</span><span class="special">,</span>
<span class="identifier">b</span><span class="special">:</span>
<span class="identifier">B</span> <span class="special">--&gt;</span>
<span class="special">(</span><span class="identifier">a</span>
<span class="special">&gt;&gt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Karma
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">a</span><span class="special">:</span>
<span class="identifier">A</span><span class="special">,</span>
<span class="identifier">b</span><span class="special">:</span>
<span class="identifier">B</span> <span class="special">--&gt;</span>
<span class="special">(</span><span class="identifier">a</span>
<span class="special">&lt;&lt;</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
which reads as:
</p>
<div class="blockquote"><blockquote class="blockquote"><p>
Given <code class="computeroutput"><span class="identifier">a</span></code> and <code class="computeroutput"><span class="identifier">b</span></code> are parsers (generators), and <code class="computeroutput"><span class="identifier">A</span></code> is the attribute type of <code class="computeroutput"><span class="identifier">a</span></code>, and <code class="computeroutput"><span class="identifier">B</span></code>
is the attribute type of <code class="computeroutput"><span class="identifier">b</span></code>,
then the attribute type of <code class="computeroutput"><span class="identifier">a</span>
<span class="special">&gt;&gt;</span> <span class="identifier">b</span></code>
(<code class="computeroutput"><span class="identifier">a</span> <span class="special">&lt;&lt;</span>
<span class="identifier">b</span></code>) will be <code class="computeroutput"><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code>.
</p></blockquote></div>
<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 notation <code class="computeroutput"><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code>
is used as a placeholder expression for any fusion sequence holding the
types A and B, such as <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span></code> (for more information see <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a>).
</p></td></tr>
</table></div>
<p>
As you can see, in order for a type to be compatible with the attribute
type of a compound expression it has to
</p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem">
either be convertible to the attribute type,
</li>
<li class="listitem">
or it has to expose certain functionalities, i.e. it needs to conform
to a concept compatible with the component.
</li>
</ul></div>
<p>
Each compound component implements its own set of attribute propagation
rules. For a full list of how the different compound generators consume
attributes see the sections <a class="link" href="../../qi/quick_reference/compound_attribute_rules.html" title="Compound Attribute Rules">Parser
Compound Attribute Rules</a> and <a class="link" href="../../karma/quick_reference/compound_attribute_rules.html" title="Compound Attribute Rules">Generator
Compound Attribute Rules</a>.
</p>
<a name="spirit.abstracts.attributes.compound_attributes.the_attribute_of_sequence_parsers_and_generators"></a><h6>
<a name="id793093"></a>
<a class="link" href="compound_attributes.html#spirit.abstracts.attributes.compound_attributes.the_attribute_of_sequence_parsers_and_generators">The
Attribute of Sequence Parsers and Generators</a>
</h6>
<p>
Sequences require an attribute type to expose the concept of a fusion sequence,
where all elements of that fusion sequence have to be compatible with the
corresponding element of the component sequence. For example, the expression:
</p>
<div class="informaltable"><table class="table">
<colgroup>
<col>
<col>
</colgroup>
<thead><tr>
<th>
<p>
Library
</p>
</th>
<th>
<p>
Sequence expression
</p>
</th>
</tr></thead>
<tbody>
<tr>
<td>
<p>
Qi
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">double_</span> <span class="special">&gt;&gt;</span>
<span class="identifier">double_</span></code>
</p>
</td>
</tr>
<tr>
<td>
<p>
Karma
</p>
</td>
<td>
<p>
<code class="computeroutput"><span class="identifier">double_</span> <span class="special">&lt;&lt;</span>
<span class="identifier">double_</span></code>
</p>
</td>
</tr>
</tbody>
</table></div>
<p>
is compatible with any fusion sequence holding two types, where both types
have to be compatible with <code class="computeroutput"><span class="keyword">double</span></code>.
The first element of the fusion sequence has to be compatible with the
attribute of the first <code class="computeroutput"><span class="identifier">double_</span></code>,
and the second element of the fusion sequence has to be compatible with
the attribute of the second <code class="computeroutput"><span class="identifier">double_</span></code>.
If we assume to have an instance of a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span></code>, we can directly use the expressions
above to do both, parse input to fill the attribute:
</p>
<pre class="programlisting"><span class="comment">// the following parses "1.0 2.0" into a pair of double
</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">input</span><span class="special">(</span><span class="string">"1.0 2.0"</span><span class="special">);</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">::</span><span class="identifier">iterator</span> <span class="identifier">strbegin</span> <span class="special">=</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special">&lt;</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">&gt;</span> <span class="identifier">p</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">strbegin</span><span class="special">,</span> <span class="identifier">input</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">double_</span><span class="special">,</span> <span class="comment">// parser grammar
</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">space</span><span class="special">,</span> <span class="comment">// delimiter grammar
</span> <span class="identifier">p</span><span class="special">);</span> <span class="comment">// attribute to fill while parsing
</span></pre>
<p>
and generate output for it:
</p>
<pre class="programlisting"><span class="comment">// the following generates: "1.0 2.0" from the pair filled above
</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span> <span class="identifier">str</span><span class="special">;</span>
<span class="identifier">std</span><span class="special">::</span><span class="identifier">back_insert_iterator</span><span class="special">&lt;</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">&gt;</span> <span class="identifier">out</span><span class="special">(</span><span class="identifier">str</span><span class="special">);</span>
<span class="identifier">karma</span><span class="special">::</span><span class="identifier">generate_delimited</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span>
<span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span><span class="special">,</span> <span class="comment">// generator grammar (format description)
</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">space</span><span class="special">,</span> <span class="comment">// delimiter grammar
</span> <span class="identifier">p</span><span class="special">);</span> <span class="comment">// data to use as the attribute
</span></pre>
<p>
(where the <code class="computeroutput"><span class="identifier">karma</span><span class="special">::</span><span class="identifier">space</span></code> generator is used as the delimiter,
allowing to automatically skip/insert delimiting spaces in between all
primitives).
</p>
<div class="tip"><table border="0" summary="Tip">
<tr>
<td rowspan="2" align="center" valign="top" width="25"><img alt="[Tip]" src="../../../images/tip.png"></td>
<th align="left">Tip</th>
</tr>
<tr><td align="left" valign="top">
<p>
<span class="bold"><strong>For sequences only:</strong></span> <span class="emphasis"><em>Spirit.Qi</em></span>
and <span class="emphasis"><em>Spirit.Karma</em></span> expose a set of API functions usable
mainly with sequences. Very much like the functions of the <code class="computeroutput"><span class="identifier">scanf</span></code> and <code class="computeroutput"><span class="identifier">printf</span></code>
families these functions allow to pass the attributes for each of the
elements of the sequence separately. Using the corresponding overload
of <span class="emphasis"><em>Qi's</em></span> parse or <span class="emphasis"><em>Karma's</em></span> <code class="computeroutput"><span class="identifier">generate</span><span class="special">()</span></code>
the expression above could be rewritten as:
</p>
<pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d1</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">,</span> <span class="identifier">d2</span> <span class="special">=</span> <span class="number">0.0</span><span class="special">;</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">phrase_parse</span><span class="special">(</span><span class="identifier">begin</span><span class="special">,</span> <span class="identifier">end</span><span class="special">,</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&gt;&gt;</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">double_</span><span class="special">,</span> <span class="identifier">qi</span><span class="special">::</span><span class="identifier">space</span><span class="special">,</span> <span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
<span class="identifier">karma</span><span class="special">::</span><span class="identifier">generate_delimited</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span> <span class="special">&lt;&lt;</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">double_</span><span class="special">,</span> <span class="identifier">karma</span><span class="special">::</span><span class="identifier">space</span><span class="special">,</span> <span class="identifier">d1</span><span class="special">,</span> <span class="identifier">d2</span><span class="special">);</span>
</pre>
<p>
where the first attribute is used for the first <code class="computeroutput"><span class="identifier">double_</span></code>,
and the second attribute is used for the second <code class="computeroutput"><span class="identifier">double_</span></code>.
</p>
</td></tr>
</table></div>
<a name="spirit.abstracts.attributes.compound_attributes.the_attribute_of_alternative_parsers_and_generators"></a><h6>
<a name="id793989"></a>
<a class="link" href="compound_attributes.html#spirit.abstracts.attributes.compound_attributes.the_attribute_of_alternative_parsers_and_generators">The
Attribute of Alternative Parsers and Generators</a>
</h6>
<p>
Alternative parsers and generators are all about - well - alternatives.
In order to store possibly different result (attribute) types from the
different alternatives we use the data type <a href="http://www.boost.org/doc/html/variant.html" target="_top">Boost.Variant</a>.
The main attribute propagation rule of these components is:
</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">b</span><span class="special">:</span> <span class="identifier">B</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">variant</span><span class="special">&lt;</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">&gt;</span>
</pre>
<p>
Alternatives have a second very important attribute propagation rule:
</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">b</span><span class="special">:</span> <span class="identifier">A</span> <span class="special">--&gt;</span> <span class="special">(</span><span class="identifier">a</span> <span class="special">|</span> <span class="identifier">b</span><span class="special">):</span> <span class="identifier">A</span>
</pre>
<p>
often allowing to simplify things significantly. If all sub expressions
of an alternative expose the same attribute type, the overall alternative
will expose exactly the same attribute type as well.
</p>
</div>
<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<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>
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