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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 © 2001-2010 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>) |
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