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| <div class="section"> |
| <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">--></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">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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">--></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">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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">>></span> <span class="identifier">b</span></code> |
| (<code class="computeroutput"><span class="identifier">a</span> <span class="special"><<</span> |
| <span class="identifier">b</span></code>) will be <code class="computeroutput"><span class="identifier">tuple</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></span></code> or <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">pair</span><span class="special"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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">>></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"><<</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"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></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"><</span><span class="keyword">double</span><span class="special">,</span> <span class="keyword">double</span><span class="special">></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">>></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"><</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_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"><<</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">>></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"><<</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">--></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"><</span><span class="identifier">A</span><span class="special">,</span> <span class="identifier">B</span><span class="special">></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">--></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> |
| <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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