<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>Attributes of Compound Components</title> <link rel="stylesheet" href="../../../../../../../doc/src/boostbook.css" type="text/css"> <meta name="generator" content="DocBook XSL Stylesheets V1.75.0"> <link rel="home" href="../../../index.html" title="Spirit 2.4"> <link rel="up" href="../attributes.html" title="Attributes"> <link rel="prev" href="primitive_attributes.html" title="Attributes of Primitive Components"> <link rel="next" href="more_compound_attributes.html" title="More About Attributes of Compound Components"> </head> <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> <table cellpadding="2" width="100%"><tr> <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../../../boost.png"></td> <td align="center"><a href="../../../../../../../index.html">Home</a></td> <td align="center"><a 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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="id837098"></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="id839086"></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> <hr> <div class="spirit-nav"> <a accesskey="p" href="primitive_attributes.html"><img src="../../../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../attributes.html"><img src="../../../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../../../index.html"><img src="../../../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="more_compound_attributes.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>