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| <div class="titlepage"><div><div><h2 class="title" style="clear: both"> |
| <a name="fusion.algorithm"></a><a class="link" href="algorithm.html" title="Algorithm">Algorithm</a> |
| </h2></div></div></div> |
| <div class="toc"><dl> |
| <dt><span class="section"><a href="algorithm/iteration.html">Iteration</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="algorithm/iteration/functions.html">Functions</a></span></dt> |
| <dt><span class="section"><a href="algorithm/iteration/metafunctions.html">Metafunctions</a></span></dt> |
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| <dt><span class="section"><a href="algorithm/query.html">Query</a></span></dt> |
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| <dt><span class="section"><a href="algorithm/query/functions.html">Functions</a></span></dt> |
| <dt><span class="section"><a href="algorithm/query/metafunctions.html">Metafunctions</a></span></dt> |
| </dl></dd> |
| <dt><span class="section"><a href="algorithm/transformation.html">Transformation</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="algorithm/transformation/functions.html">Functions</a></span></dt> |
| <dt><span class="section"><a href="algorithm/transformation/metafunctions.html">Metafunctions</a></span></dt> |
| </dl></dd> |
| </dl></div> |
| <a name="fusion.algorithm.lazy_evaluation"></a><h4> |
| <a name="id986992"></a> |
| <a class="link" href="algorithm.html#fusion.algorithm.lazy_evaluation">Lazy Evaluation</a> |
| </h4> |
| <p> |
| Unlike <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, Fusion |
| algorithms are lazy and non sequence-type preserving. What does that mean? |
| It means that when you operate on a sequence through a Fusion algorithm that |
| returns a sequence, the sequence returned may not be of the same class as the |
| original. This is by design. Runtime efficiency is given a high priority. Like |
| <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, and unlike |
| <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>, |
| fusion algorithms are functional in nature such that algorithms are non mutating |
| (no side effects). However, due to the high cost of returning full sequences |
| such as vectors and lists, <span class="emphasis"><em>Views</em></span> are returned from Fusion |
| algorithms instead. For example, the <a class="link" href="algorithm/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> algorithm does not actually |
| return a transformed version of the original sequence. <a class="link" href="algorithm/transformation/functions/transform.html" title="transform"><code class="computeroutput"><span class="identifier">transform</span></code></a> returns a <a class="link" href="view/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>. This view holds a |
| reference to the original sequence plus the transform function. Iteration over |
| the <a class="link" href="view/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a> |
| will apply the transform function over the sequence elements on demand. This |
| <span class="emphasis"><em>lazy</em></span> evaluation scheme allows us to chain as many algorithms |
| as we want without incurring a high runtime penalty. |
| </p> |
| <a name="fusion.algorithm.sequence_extension"></a><h4> |
| <a name="id987069"></a> |
| <a class="link" href="algorithm.html#fusion.algorithm.sequence_extension">Sequence Extension</a> |
| </h4> |
| <p> |
| The <span class="emphasis"><em>lazy</em></span> evaluation scheme where <a class="link" href="algorithm.html" title="Algorithm">Algorithms</a> |
| return <a class="link" href="view.html" title="View">Views</a> also allows operations such |
| as <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> to be totally generic. In |
| Fusion, <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> is actually a generic algorithm |
| that works on all sequences. Given an input sequence <code class="computeroutput"><span class="identifier">s</span></code> |
| and a value <code class="computeroutput"><span class="identifier">x</span></code>, Fusion's <a class="link" href="algorithm/transformation/functions/push_back.html" title="push_back"><code class="computeroutput"><span class="identifier">push_back</span></code></a> algorithm simply returns |
| a <a class="link" href="view/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>: |
| a view that holds a reference to the original sequence <code class="computeroutput"><span class="identifier">s</span></code> |
| and the value <code class="computeroutput"><span class="identifier">x</span></code>. Functions |
| that were once sequence specific and need to be implemented N times over N |
| different sequences are now implemented only once. That is to say that Fusion |
| sequences are cheaply extensible. To regain the original sequence, <a class="link" href="container/conversion/functions.html" title="Functions">Conversion</a> |
| functions are provided. You may use one of the <a class="link" href="container/conversion/functions.html" title="Functions">Conversion</a> |
| functions to convert back to the original sequence type. |
| </p> |
| <a name="fusion.algorithm.header"></a><h4> |
| <a name="id987177"></a> |
| <a class="link" href="algorithm.html#fusion.algorithm.header">Header</a> |
| </h4> |
| <pre class="programlisting"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| <span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">fusion</span><span class="special">/</span><span class="identifier">include</span><span class="special">/</span><span class="identifier">algorithm</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span> |
| </pre> |
| </div> |
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| <td align="left"></td> |
| <td align="right"><div class="copyright-footer">Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias |
| Schwinger<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>) |
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