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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.karma.tutorials.karma_easier_complex"></a><a class="link" href="karma_easier_complex.html" title="Complex - Made easier">Complex |
| - Made easier</a> |
| </h4></div></div></div> |
| <p> |
| In the previous section we showed how to format a complex number (i.e. |
| a pair of doubles). In this section we will build on this example with |
| the goal to avoid using semantic actions in the format specification. Let's |
| have a look at the resulting code first, trying to understand it afterwards |
| (the full source file for this example can be found here: <a href="../../../../../example/karma/complex_number_easier.cpp" target="_top">complex_number_easier.cpp</a>): |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">OutputIterator</span><span class="special">></span> |
| <span class="keyword">bool</span> <span class="identifier">generate_complex</span><span class="special">(</span><span class="identifier">OutputIterator</span> <span class="identifier">sink</span><span class="special">,</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><</span><span class="keyword">double</span><span class="special">></span> <span class="keyword">const</span><span class="special">&</span> <span class="identifier">c</span><span class="special">)</span> |
| <span class="special">{</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</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="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</span><span class="special">::</span><span class="identifier">karma</span><span class="special">::</span><span class="identifier">omit</span><span class="special">;</span> |
| <span class="keyword">using</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">spirit</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="keyword">return</span> <span class="identifier">generate</span><span class="special">(</span><span class="identifier">sink</span><span class="special">,</span> |
| |
| <span class="comment">// Begin grammar |
| </span> <span class="special">(</span> |
| <span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span> <span class="special"><<</span> <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> |
| <span class="special">|</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">double_</span> |
| <span class="special">),</span> |
| <span class="comment">// End grammar |
| </span> |
| <span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">(),</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">imag</span><span class="special">()</span> <span class="comment">// Data to output |
| </span> <span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Let's cover some basic library features first. |
| </p> |
| <a name="spirit.karma.tutorials.karma_easier_complex.making_numeric_generators_fail"></a><h6> |
| <a name="id959849"></a> |
| <a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.making_numeric_generators_fail">Making |
| Numeric Generators Fail</a> |
| </h6> |
| <p> |
| All <a class="link" href="../reference/numeric.html" title="Numeric">Numeric Generators</a> |
| (such as <code class="computeroutput"><span class="identifier">double_</span></code>, et.al.) |
| take the value to emit from an attached attribute. |
| </p> |
| <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.5</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">double_</span><span class="special">,</span> <span class="identifier">d</span><span class="special">);</span> <span class="comment">// will emit '1.5' (without the quotes) |
| </span></pre> |
| <p> |
| Alternatively, they may be initialized from a literal value. For instance, |
| to emit a constant <code class="computeroutput"><span class="number">1.5</span></code> you |
| may write: |
| </p> |
| <pre class="programlisting"><span class="identifier">generate</span><span class="special">(</span><span class="identifier">out</span><span class="special">,</span> <span class="identifier">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">));</span> <span class="comment">// will emit '1.5' as well (without the quotes) |
| </span></pre> |
| <p> |
| The difference to a simple <code class="computeroutput"><span class="number">1.5</span></code> |
| or <code class="computeroutput"><span class="identifier">lit</span><span class="special">(</span><span class="number">1.5</span><span class="special">)</span></code> is that |
| the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">)</span></code> consumes |
| an attribute if one is available. Additionally, it compares its immediate |
| value to the value of the supplied attribute, and fails if those are not |
| equal. |
| </p> |
| <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.5</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">double_</span><span class="special">(</span><span class="number">1.5</span><span class="special">),</span> <span class="identifier">d</span><span class="special">);</span> <span class="comment">// will emit '1.5' as long as d == 1.5 |
| </span></pre> |
| <p> |
| This feature, namely to succeed generating only if the attribute matches |
| the immediate value, enables numeric generators to be used to dynamically |
| control the way output is generated. |
| </p> |
| <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> |
| Quite a few generators will fail if their immediate value is not equal |
| to the supplied attribute. Among those are all <a class="link" href="../reference/char.html" title="Char">Character |
| Generators</a> and all <a class="link" href="../reference/string.html" title="String">String |
| Generators</a>. Generally, all generators having a sibling created |
| by a variant of <code class="computeroutput"><span class="identifier">lit</span><span class="special">()</span></code> |
| belong into this category. |
| </p></td></tr> |
| </table></div> |
| <a name="spirit.karma.tutorials.karma_easier_complex.predicates___the_conditionals_for_output_generators"></a><h6> |
| <a name="id960137"></a> |
| <a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.predicates___the_conditionals_for_output_generators">Predicates |
| - The Conditionals for Output Generators</a> |
| </h6> |
| <p> |
| In addition to the <a class="link" href="../reference/auxiliary/eps.html" title="Epsilon (eps)"><code class="computeroutput"><span class="identifier">eps</span></code></a> generator mentioned earlier |
| <span class="emphasis"><em>Spirit.Karma</em></span> provides two special operators enabling |
| dynamic flow control: the <a class="link" href="../reference/operator/and_predicate.html" title="And-Predicate (&a)">And |
| predicate (unary <code class="computeroutput"><span class="special">&</span></code>)</a> |
| and the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate (!a)">Not |
| predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a>. |
| The main property of both predicates is to discard all output emitted by |
| the attached generator. This is equivalent to the behavior of predicates |
| used for parsing. There the predicates do not consume any input allowing |
| to look ahead in the input stream. In Karma, the and predicate succeeds |
| as long as its associated generator succeeds, while the not predicate succeeds |
| only if its associated generator fails. |
| </p> |
| <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 generator predicates in <span class="emphasis"><em>Spirit.Karma</em></span> consume |
| an attribute, if available. This makes them behave differently from predicates |
| in <span class="emphasis"><em>Spirit.Qi</em></span>, where they do not expose any attribute. |
| This is because predicates allow to make decisions based on data available |
| only at runtime. While in <span class="emphasis"><em>Spirit.Qi</em></span> during parsing |
| the decision is made based on looking ahead a few more input tokens, |
| in <span class="emphasis"><em>Spirit.Karma</em></span> the criteria has to be supplied |
| by the user. The simplest way to do this is by providing an attribute. |
| </p></td></tr> |
| </table></div> |
| <p> |
| As an example, the following generator succeeds generating |
| </p> |
| <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span> |
| <span class="identifier">BOOST_ASSERT</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="special">&</span><span class="identifier">double_</span><span class="special">(</span><span class="number">1.0</span><span class="special">),</span> <span class="identifier">d</span><span class="special">));</span> <span class="comment">// succeeds as d == 1.0 |
| </span></pre> |
| <p> |
| while this one will fail: |
| </p> |
| <pre class="programlisting"><span class="keyword">double</span> <span class="identifier">d</span> <span class="special">=</span> <span class="number">1.0</span><span class="special">;</span> |
| <span class="identifier">BOOST_ASSERT</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="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">1.0</span><span class="special">),</span> <span class="identifier">d</span><span class="special">));</span> <span class="comment">// fails as d == 1.0 |
| </span></pre> |
| <p> |
| Neither of these will emit any output. The predicates discard everything |
| emitted by the generators to which they are applied. |
| </p> |
| <a name="spirit.karma.tutorials.karma_easier_complex.ignoring_supplied_attributes"></a><h6> |
| <a name="id961534"></a> |
| <a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.ignoring_supplied_attributes">Ignoring |
| Supplied Attributes</a> |
| </h6> |
| <p> |
| Sometimes it is desirable to 'skip' (i.e. ignore) a provided attribute. |
| This happens for instance in alternative generators, where some of the |
| alternatives need to extract only part of the overall attribute passed |
| to the alternative generator. <span class="emphasis"><em>Spirit.Karma</em></span> has a special |
| pseudo generator for that: the directive <a class="link" href="../reference/directive/omit.html" title="Consume Attribute (omit[] and skip[])"><code class="computeroutput"><span class="identifier">omit</span></code></a><code class="computeroutput"><span class="special">[]</span></code>. |
| This directive consumes an attribute of the type defined by its embedded |
| generator but it does not emit any output. |
| </p> |
| <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 <span class="emphasis"><em>Spirit.Karma</em></span> <a class="link" href="../reference/directive/omit.html" title="Consume Attribute (omit[] and skip[])"><code class="computeroutput"><span class="identifier">omit</span></code></a> directive does the 'opposite' |
| of the directive of the same name in <span class="emphasis"><em>Spirit.Qi</em></span>. |
| While the <a class="link" href="../../qi/reference/directive/omit.html" title="Ignoring Attribute (omit[])"><code class="computeroutput"><span class="identifier">omit</span></code></a> in <span class="emphasis"><em>Spirit.Qi</em></span> |
| consumes input without exposing an attribute, its <span class="emphasis"><em>Spirit.Karma</em></span> |
| counterpart consumes an attribute without emitting any output. |
| </p></td></tr> |
| </table></div> |
| <a name="spirit.karma.tutorials.karma_easier_complex.putting_everything_together"></a><h6> |
| <a name="id961612"></a> |
| <a class="link" href="karma_easier_complex.html#spirit.karma.tutorials.karma_easier_complex.putting_everything_together">Putting |
| everything together</a> |
| </h6> |
| <p> |
| Very similar to our first example earlier we use two alternatives to allow |
| for the two different output formats depending on whether the imaginary |
| part of the complex number is equal to zero or not. The first alternative |
| is executed if the imaginary part is not zero, the second alternative otherwise. |
| This time we make the decision during runtime using the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate (!a)">Not |
| predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a> |
| combined with the feature of many Karma primitive generators to <span class="emphasis"><em>fail</em></span> |
| under certain conditions. Here is the first alternative again for your |
| reference: |
| </p> |
| <pre class="programlisting"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span> <span class="special"><<</span> <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> |
| </pre> |
| <p> |
| The generator <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code> |
| does several things. First, because of the <a class="link" href="../reference/operator/not_predicate.html" title="Not-Predicate (!a)">Not |
| predicate (unary <code class="computeroutput"><span class="special">!</span></code>)</a>, |
| it succeeds only if the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code> |
| generator <span class="emphasis"><em>fails</em></span>, making the whole first alternative |
| fail otherwise. Second, the <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code> |
| generator succeeds only if the value of its attribute is equal to its immediate |
| parameter (i.e. in this case <code class="computeroutput"><span class="number">0.0</span></code>). |
| And third, the not predicate does not emit any output (regardless whether |
| it succeeds or fails), discarding any possibly emitted output from the |
| <code class="computeroutput"><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>. |
| </p> |
| <p> |
| As we pass the imaginary part of the complex number as the attribute value |
| for the <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code>, |
| the overall first alternative will be chosen only if it is not equal to |
| zero (the <code class="computeroutput"><span class="special">!</span><span class="identifier">double_</span><span class="special">(</span><span class="number">0.0</span><span class="special">)</span></code> |
| does not fail). That is exactly what we need! |
| </p> |
| <p> |
| Now, the second alternative has to emit the real part of the complex number |
| only. In order to simplify the overall grammar we strive to unify the attribute |
| types of all alternatives. As the attribute type exposed by the first alternative |
| is <code class="computeroutput"><span class="identifier">tuple</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="keyword">double</span><span class="special">></span></code>, |
| we need to skip the first and last element of the attribute (remember, |
| we pass the real part as the second attribute element). We achieve this |
| by using the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> |
| directive: |
| </p> |
| <pre class="programlisting"><span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> <span class="special"><<</span> <span class="identifier">double_</span> <span class="special"><<</span> <span class="identifier">omit</span><span class="special">[</span><span class="identifier">double_</span><span class="special">]</span> |
| </pre> |
| <p> |
| The overall attribute of this expression is <code class="computeroutput"><span class="identifier">tuple</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="keyword">double</span><span class="special">></span></code>, but the <code class="computeroutput"><span class="identifier">omit</span><span class="special">[]</span></code> 'eats up' the first and the last element. |
| The output emitted by this expression consist of a single generated double |
| representing the second element of the tuple, i.e. the real part of our |
| complex number. |
| </p> |
| <div class="important"><table border="0" summary="Important"> |
| <tr> |
| <td rowspan="2" align="center" valign="top" width="25"><img alt="[Important]" src="../../../images/important.png"></td> |
| <th align="left">Important</th> |
| </tr> |
| <tr><td align="left" valign="top"><p> |
| Generally, it is preferable to use generator constructs not requiring |
| semantic actions. The reason is that semantic actions often use constructs |
| like: <code class="computeroutput"><span class="identifier">double_</span><span class="special">[</span><span class="identifier">_1</span> <span class="special">=</span> <span class="identifier">c</span><span class="special">.</span><span class="identifier">real</span><span class="special">()]</span></code>. |
| But this assignment is a real one! The data is in fact <span class="emphasis"><em>copied</em></span> |
| to the attribute value of the generator attached to the action. On the |
| other hand, grammars without any semantic actions usually don't have |
| to copy the attributes, making them more efficient. |
| </p></td></tr> |
| </table></div> |
| </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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