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| <div class="section"> |
| <div class="titlepage"><div><div><h4 class="title"> |
| <a name="spirit.karma.tutorials.karma_adapted_complex"></a><a class="link" href="karma_adapted_complex.html" title="Complex - Fully Integrated">Complex |
| - Fully Integrated</a> |
| </h4></div></div></div> |
| <p> |
| Until now, we have been working around the fact that <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><></span></code> is not a native <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a> |
| sequence. We have not been able to use it with the same simplicity and |
| natural grace of a <code class="computeroutput"><span class="identifier">fusion</span><span class="special">::</span><span class="identifier">tuple</span><span class="special"><></span></code> or a similar <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a> |
| data structure. Fortunately, starting with Boost V1.43 it is possible to |
| adapt any data structure (not only, as before, structures with publicly |
| accessible members) as a <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a> |
| sequence. All we have to do is to employ one of the new <code class="computeroutput"><span class="identifier">BOOST_FUSION_ADAPT_CLASS</span></code> |
| macros. |
| </p> |
| <a name="spirit.karma.tutorials.karma_adapted_complex.adapting_a_class_as_a_fusion_sequence"></a><h6> |
| <a name="id962129"></a> |
| <a class="link" href="karma_adapted_complex.html#spirit.karma.tutorials.karma_adapted_complex.adapting_a_class_as_a_fusion_sequence">Adapting |
| a Class As a Fusion Sequence</a> |
| </h6> |
| <p> |
| Let us start with the code again, following up with the explanations afterwards. |
| </p> |
| <p> |
| Wouldn't it be optimal if we could pass our instance of a <code class="computeroutput"><span class="identifier">std</span><span class="special">::</span><span class="identifier">complex</span><span class="special"><></span></code> |
| directly to <span class="emphasis"><em>Karma's</em></span> <code class="computeroutput"><span class="identifier">generate</span><span class="special">()</span></code> function: |
| </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">bool_</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">true_</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">true_</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">bool_</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="comment">// Data to output |
| </span> <span class="special">);</span> |
| <span class="special">}</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Indeed, this is possible! All we have to supply to make this work is a |
| magic incantation (somewhere in the global namespace): |
| </p> |
| <p> |
| |
| </p> |
| <pre class="programlisting"><span class="comment">// We can leave off the setters as Karma does not need them. |
| </span><span class="identifier">BOOST_FUSION_ADAPT_ADT</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="special">(</span><span class="keyword">bool</span><span class="special">,</span> <span class="keyword">bool</span><span class="special">,</span> <span class="identifier">obj</span><span class="special">.</span><span class="identifier">imag</span><span class="special">()</span> <span class="special">!=</span> <span class="number">0</span><span class="special">,</span> <span class="comment">/**/</span><span class="special">)</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">obj</span><span class="special">.</span><span class="identifier">real</span><span class="special">(),</span> <span class="comment">/**/</span><span class="special">)</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">obj</span><span class="special">.</span><span class="identifier">imag</span><span class="special">(),</span> <span class="comment">/**/</span><span class="special">)</span> |
| <span class="special">)</span> |
| </pre> |
| <p> |
| </p> |
| <p> |
| Most of the formatting grammar itself has not changed from the last section. |
| We still utilize a very similar scheme. We have an alternative providing |
| the formatting rules for our both use cases: one for the full complex format |
| and one for complex numbers with a zero imaginary part. But instead of |
| selecting the required alternative by comparing the imaginary part to zero |
| in the grammar we assume to receive a boolean attribute carrying this information: |
| </p> |
| <pre class="programlisting"><span class="special">&</span><span class="identifier">true_</span> <span class="special"><<</span> <span class="string">"("</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="string">")"</span> |
| </pre> |
| <p> |
| This reads as: 'if the first (boolean) element of the supplied fusion sequence |
| is <code class="computeroutput"><span class="keyword">true</span></code>, proceed as specified, |
| else select the next alternative'. The next alternative now accounts for |
| the boolean element as well, but is otherwise (almost) unchanged from the |
| last section's example. |
| </p> |
| <p> |
| Now it should be clear why our adapt construct above exposes a three element |
| <a href="../../../../../../../libs/fusion/doc/html/index.html" target="_top">Boost.Fusion</a> |
| sequence: a boolean and two double values (the real and the imaginary part |
| of the complex number). We want it to match the requirements of our formatting |
| grammar, which expects those exact values. The <code class="computeroutput"><span class="identifier">BOOST_FUSION_ADAPT_CLASS</span></code> |
| macro allows us to specify an arbitrary accessor construct, not necessarily |
| limited to just calling a member function of the object instance (represented |
| by <code class="computeroutput"><span class="identifier">obj</span></code> in the context of |
| this macro). This allows us to nicely encapsulate the decision logic into |
| the class adaptation. |
| </p> |
| <p> |
| Here is the last new bit of information. If you look closely you realize |
| the second alternative to be 'shorter' than the first one. It consumes |
| only two elements of the supplied fusion sequence: it ignores the boolean |
| and uses the real part of the complex number to generate its output. If |
| there are more elements in our attribute than needed, we now can safely |
| omit them from the grammar (which is a new 'feature' added to <a href="http://boost-spirit.com" target="_top">Spirit</a> |
| in V1.43 as well). Note, we could have written the alternative as |
| </p> |
| <pre class="programlisting"><span class="special">&</span><span class="identifier">false_</span> <span class="special"><<</span> <span class="identifier">double_</span> |
| </pre> |
| <p> |
| but this would have been a bit less efficient as we needed to compare the |
| boolean value again, while the final solution provided will just ignore |
| it. |
| </p> |
| </div> |
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| <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> |
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