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<title>Attributes of Primitive Components</title>
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<div class="titlepage"><div><div><h4 class="title">
<a name="spirit.abstracts.attributes.primitive_attributes"></a><a class="link" href="primitive_attributes.html" title="Attributes of Primitive Components">Attributes
of Primitive Components</a>
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<p>
Parsers and generators in <a href="http://boost-spirit.com" target="_top">Spirit</a>
are fully attributed. <span class="emphasis"><em>Spirit.Qi</em></span> parsers always <span class="emphasis"><em>expose</em></span>
an attribute specific to their type. This is called <span class="emphasis"><em>synthesized
attribute</em></span> as it is returned from a successful match representing
the matched input sequence. For instance, numeric parsers, such as <code class="computeroutput"><span class="identifier">int_</span></code> or <code class="computeroutput"><span class="identifier">double_</span></code>,
return the <code class="computeroutput"><span class="keyword">int</span></code> or <code class="computeroutput"><span class="keyword">double</span></code> value converted from the matched
input sequence. Other primitive parser components have other intuitive
attribute types, such as for instance <code class="computeroutput"><span class="identifier">int_</span></code>
which has <code class="computeroutput"><span class="keyword">int</span></code>, or <code class="computeroutput"><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span></code> which has <code class="computeroutput"><span class="keyword">char</span></code>.
For primitive parsers apply the normal C++ convertibility rules: you can
use any C++ type to receive the parsed value as long as the attribute type
of the parser is convertible to the type provided. The following example
shows how a synthesized parser attribute (the <code class="computeroutput"><span class="keyword">int</span></code>
value) is extracted by calling the API function <code class="computeroutput"><span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span></code>:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">value</span> <span class="special">=</span> <span class="number">0</span><span class="special">;</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="string">"123"</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">str</span><span class="special">.</span><span class="identifier">begin</span><span class="special">();</span>
<span class="identifier">qi</span><span class="special">::</span><span class="identifier">parse</span><span class="special">(</span><span class="identifier">strbegin</span><span class="special">,</span> <span class="identifier">str</span><span class="special">.</span><span class="identifier">end</span><span class="special">(),</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">value</span><span class="special">);</span> <span class="comment">// value == 123</span>
</pre>
<p>
The attribute type of a generator defines what data types this generator
is able to consume in order to produce its output. <span class="emphasis"><em>Spirit.Karma</em></span>
generators always <span class="emphasis"><em>expect</em></span> an attribute specific to
their type. This is called <span class="emphasis"><em>consumed attribute</em></span> and
is expected to be passed to the generator. The consumed attribute is most
of the time the value the generator is designed to emit output for. For
primitive generators the normal C++ convertibility rules apply. Any data
type convertible to the attribute type of a primitive generator can be
used to provide the data to generate. We present a similar example as above,
this time the consumed attribute of the <code class="computeroutput"><span class="identifier">int_</span></code>
generator (the <code class="computeroutput"><span class="keyword">int</span></code> value)
is passed to the API function <code class="computeroutput"><span class="identifier">karma</span><span class="special">::</span><span class="identifier">generate</span></code>:
</p>
<pre class="programlisting"><span class="keyword">int</span> <span class="identifier">value</span> <span class="special">=</span> <span class="number">123</span><span class="special">;</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"><</span><span class="identifier">std</span><span class="special">::</span><span class="identifier">string</span><span class="special">></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</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">int_</span><span class="special">,</span> <span class="identifier">value</span><span class="special">);</span> <span class="comment">// str == "123"</span>
</pre>
<p>
Other primitive generator components have other intuitive attribute types,
very similar to the corresponding parser components. For instance, the
<code class="computeroutput"><span class="identifier">ascii</span><span class="special">::</span><span class="identifier">char_</span></code> generator has <code class="computeroutput"><span class="keyword">char</span></code>
as consumed attribute. For a full list of available parser and generator
primitives and their attribute types please see the sections <a class="link" href="../../qi/quick_reference/qi_parsers.html" title="Qi Parsers">Qi
Parsers</a> and <a class="link" href="../../karma/quick_reference/primitive_generators.html" title="Karma Generators">Karma
Generators</a>.
</p>
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<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>)
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