<html> <head> <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> <title>More About 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="compound_attributes.html" title="Attributes of Compound Components"> <link rel="next" href="nonterminal_attributes.html" title="Attributes of Rules and Grammars"> </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 href="../../../../../../../libs/libraries.htm">Libraries</a></td> <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> <td align="center"><a href="../../../../../../../more/index.htm">More</a></td> </tr></table> <hr> <div class="spirit-nav"> <a accesskey="p" href="compound_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="nonterminal_attributes.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> <div class="section"> <div class="titlepage"><div><div><h4 class="title"> <a name="spirit.abstracts.attributes.more_compound_attributes"></a><a class="link" href="more_compound_attributes.html" title="More About Attributes of Compound Components"> More About Attributes of Compound Components</a> </h4></div></div></div> <p> While parsing input or generating output it is often desirable to combine some constant elements with variable parts. For instance, let us look at the example of parsing or formatting a complex number, which is written as <code class="computeroutput"><span class="special">(</span><span class="identifier">real</span><span class="special">,</span> <span class="identifier">imag</span><span class="special">)</span></code>, where <code class="computeroutput"><span class="identifier">real</span></code> and <code class="computeroutput"><span class="identifier">imag</span> </code> are the variables representing the real and imaginary parts of our complex number. This can be achieved by writing: </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="char">'('</span> <span class="special">>></span> <span class="identifier">double_</span> <span class="special">>></span> <span class="string">", "</span> <span class="special">>></span> <span class="identifier">double_</span> <span class="special">>></span> <span class="char">')'</span></code> </p> </td> </tr> <tr> <td> <p> Karma </p> </td> <td> <p> <code class="computeroutput"><span class="char">'('</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="string">", "</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="char">')'</span></code> </p> </td> </tr> </tbody> </table></div> <p> Fortunately, literals (such as <code class="computeroutput"><span class="char">'('</span></code> and <code class="computeroutput"><span class="string">", "</span></code>) do <span class="emphasis"><em>not</em></span> expose any attribute (well actually, they do expose the special type <code class="computeroutput"><span class="identifier">unused_type</span></code>, but in this context <code class="computeroutput"><span class="identifier">unused_type</span></code> is interpreted as if the component does not expose any attribute at all). It is very important to understand that the literals don't consume any of the elements of a fusion sequence passed to this component sequence. As said, they just don't expose any attribute and don't produce (consume) any data. The following example shows this: </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">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="char">'('</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="string">", "</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="char">')'</span><span class="special">,</span> <span class="comment">// parser grammar </span> <span class="identifier">p</span><span class="special">);</span> <span class="comment">// attribute to fill while parsing </span></pre> <p> and here is the equivalent <span class="emphasis"><em>Spirit.Karma</em></span> code snippet: </p> <pre class="programlisting"><span class="comment">// the following generates: (1.0, 2.0) </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">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="char">'('</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="string">", "</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="char">')'</span><span class="special">,</span> <span class="comment">// generator grammar (format description) </span> <span class="identifier">p</span><span class="special">);</span> <span class="comment">// data to use as the attribute </span></pre> <p> where the first element of the pair passed in as the data to generate is still associated with the first <code class="computeroutput"><span class="identifier">double_</span></code>, and the second element is associated with the second <code class="computeroutput"><span class="identifier">double_</span></code> generator. </p> <p> This behavior should be familiar as it conforms to the way other input and output formatting libraries such as <code class="computeroutput"><span class="identifier">scanf</span></code>, <code class="computeroutput"><span class="identifier">printf</span></code> or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">format</span></code> are handling their variable parts. In this context you can think about <span class="emphasis"><em>Spirit.Qi</em></span>'s and <span class="emphasis"><em>Spirit.Karma</em></span>'s primitive components (such as the <code class="computeroutput"><span class="identifier">double_</span></code> above) as of being typesafe placeholders for the attribute values. </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> Similarly to the tip provided above, this example could be rewritten using <span class="emphasis"><em>Spirit's</em></span> multi-attribute API function: </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">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="char">'('</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="string">", "</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="char">')'</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</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="char">'('</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="string">", "</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="char">')'</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> which provides a clear and comfortable syntax, more similar to the placeholder based syntax as exposed by <code class="computeroutput"><span class="identifier">printf</span></code> or <code class="computeroutput"><span class="identifier">boost</span><span class="special">::</span><span class="identifier">format</span></code>. </p> </td></tr> </table></div> <p> Let's take a look at this from a more formal perspective. The sequence attribute propagation rules define a special behavior if generators exposing <code class="computeroutput"><span class="identifier">unused_type</span></code> as their attribute are involved (see <a class="link" href="../../karma/quick_reference/compound_attribute_rules.html" title="Compound Attribute Rules">Generator Compound Attribute Rules</a>): </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">Unused</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></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">Unused</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></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">unused_type</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">A</span></code> as well. This rule applies regardless of the position the element exposing the <code class="computeroutput"><span class="identifier">unused_type</span></code> is at. </p></blockquote></div> <p> This rule is the key to the understanding of the attribute handling in sequences as soon as literals are involved. It is as if elements with <code class="computeroutput"><span class="identifier">unused_type</span></code> attributes 'disappeared' during attribute propagation. Notably, this is not only true for sequences but for any compound components. For instance, for alternative componets the corresponding rule 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">Unused</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> again, allowing to simplify the overall attribute type of an expression. </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="compound_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="nonterminal_attributes.html"><img src="../../../../../../../doc/src/images/next.png" alt="Next"></a> </div> </body> </html>