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| <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 |
| type safe 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 |
| components 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> |
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