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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 class="toc"> |
| <dt><span class="section"><a href="algorithm/auxiliary.html">Auxiliary</a></span></dt> |
| <dd><dl> |
| <dt><span class="section"><a href="algorithm/auxiliary/functions.html">Functions</a></span></dt> |
| <dt><span class="section"><a href="algorithm/auxiliary/metafunctions.html">Metafunctions</a></span></dt> |
| </dl></dd> |
| <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> |
| </dl></dd> |
| <dt><span class="section"><a href="algorithm/query.html">Query</a></span></dt> |
| <dd><dl> |
| <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> |
| <h4> |
| <a name="fusion.algorithm.h0"></a> |
| <span class="phrase"><a name="fusion.algorithm.lazy_evaluation"></a></span><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<a href="#ftn.fusion.algorithm.f0" class="footnote" name="fusion.algorithm.f0"><sup class="footnote">[12]</sup></a> and non sequence-type preserving <a href="#ftn.fusion.algorithm.f1" class="footnote" name="fusion.algorithm.f1"><sup class="footnote">[13]</sup></a>. 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 mostly 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> |
| <h4> |
| <a name="fusion.algorithm.h1"></a> |
| <span class="phrase"><a name="fusion.algorithm.sequence_extension"></a></span><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. |
| </p> |
| <p> |
| 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> |
| <h4> |
| <a name="fusion.algorithm.h2"></a> |
| <span class="phrase"><a name="fusion.algorithm.header"></a></span><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 class="footnotes"> |
| <br><hr style="width:100; text-align:left;margin-left: 0"> |
| <div id="ftn.fusion.algorithm.f0" class="footnote"><p><a href="#fusion.algorithm.f0" class="para"><sup class="para">[12] </sup></a> |
| Except for some special cases such as <a class="link" href="algorithm/iteration/functions/for_each.html" title="for_each"><code class="computeroutput"><span class="identifier">for_each</span></code></a> and <a class="link" href="algorithm/auxiliary/functions/copy.html" title="copy"><code class="computeroutput"><span class="identifier">copy</span></code></a> which are inherently imperative |
| algorithms. |
| </p></div> |
| <div id="ftn.fusion.algorithm.f1" class="footnote"><p><a href="#fusion.algorithm.f1" class="para"><sup class="para">[13] </sup></a> |
| 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 |
| </p></div> |
| </div> |
| </div> |
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| 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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