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<div class="section">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="fusion.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
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</h2></div></div></div>
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<p>
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An advantage other languages such as Python and Lisp/ Scheme, ML and Haskell,
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etc., over C++ is the ability to have heterogeneous containers that can hold
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arbitrary element types. All the containers in the standard library can only
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hold a specific type. A <code class="computeroutput"><span class="identifier">vector</span><span class="special"><</span><span class="keyword">int</span><span class="special">></span></code>
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can only hold <code class="computeroutput"><span class="keyword">int</span></code>s. A <code class="computeroutput"><span class="identifier">list</span><span class="special"><</span><span class="identifier">X</span><span class="special">></span></code> can
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only hold elements of type <code class="computeroutput"><span class="identifier">X</span></code>,
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and so on.
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</p>
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<p>
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True, you can use inheritance to make the containers hold different types,
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related through subclassing. However, you have to hold the objects through
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a pointer or smart reference of some sort. Doing this, you'll have to rely
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on virtual functions to provide polymorphic behavior since the actual type
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is erased as soon as you store a pointer to a derived class to a pointer to
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its base. The held objects must be related: you cannot hold objects of unrelated
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types such as <code class="computeroutput"><span class="keyword">char</span></code>, <code class="computeroutput"><span class="keyword">int</span></code>, <code class="computeroutput"><span class="keyword">class</span>
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<span class="identifier">X</span></code>, <code class="computeroutput"><span class="keyword">float</span></code>,
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etc. Oh sure you can use something like <a href="http://boost.org/doc/html/any.html" target="_top">Boost.Any</a>
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to hold arbitrary types, but then you pay more in terms of runtime costs and
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due to the fact that you practically erased all type information, you'll have
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to perform dangerous casts to get back the original type.
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</p>
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<p>
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The <a href="http://www.boost.org/libs/tuple/doc/tuple_users_guide.html" target="_top">Boost.Tuple</a>
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library written by <a href="http://www.boost.org/people/jaakko_jarvi.htm" target="_top">Jaakko
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Jarvi</a> provides heterogeneous containers in C++. The <code class="computeroutput"><span class="identifier">tuple</span></code>
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is a basic data structure that can hold heterogeneous types. It's a good first
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step, but it's not complete. What's missing are the algorithms. It's nice that
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we can store and retrieve data to and from tuples, pass them around as arguments
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and return types. As it is, the <a href="http://www.boost.org/libs/tuple/doc/tuple_users_guide.html" target="_top">Boost.Tuple</a>
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facility is already very useful. Yet, as soon as you use it more often, usage
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patterns emerge. Eventually, you collect these patterns into algorithm libraries.
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</p>
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<p>
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Hmmm, kinda reminds us of STL right? Right! Can you imagine how it would be
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like if you used STL without the algorithms? Everyone will have to reinvent
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their own <span class="emphasis"><em>algorithm</em></span> wheels.
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</p>
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<p>
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Fusion is a library and a framework similar to both <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
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and the boost <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>.
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The structure is modeled after <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>,
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which is modeled after <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
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It is named "fusion" because the library is reminiscent of the "fusion"
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of compile time meta-programming with runtime programming. The library inherently
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has some interesting flavors and characteristics of both <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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and <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
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It lives in the twilight zone between compile time meta-programming and run
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time programming. <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
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containers work on values. MPL containers work on types. Fusion containers
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work on both types and values.
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</p>
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<p>
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Unlike <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, Fusion
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algorithms are lazy and non sequence-type preserving. What does that mean?
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It means that when you operate on a sequence through a Fusion algorithm that
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returns a sequence, the sequence returned may not be of the same class as the
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original. This is by design. Runtime efficiency is given a high priority. Like
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<a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>, and unlike
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<a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>,
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fusion algorithms are functional in nature such that algorithms are non mutating
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(no side effects). However, due to the high cost of returning full sequences
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such as vectors and lists, <span class="emphasis"><em>Views</em></span> are returned from Fusion
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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
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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
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reference to the original sequence plus the transform function. Iteration over
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the <a class="link" href="view/transform_view.html" title="transform_view"><code class="computeroutput"><span class="identifier">transform_view</span></code></a>
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will apply the transform function over the sequence elements on demand. This
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<span class="emphasis"><em>lazy</em></span> evaluation scheme allows us to chain as many algorithms
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as we want without incurring a high runtime penalty.
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</p>
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<p>
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The <span class="emphasis"><em>lazy</em></span> evaluation scheme where algorithms return views
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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
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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
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that works on all sequences. Given an input sequence <code class="computeroutput"><span class="identifier">s</span></code>
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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
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a <a class="link" href="view/joint_view.html" title="joint_view"><code class="computeroutput"><span class="identifier">joint_view</span></code></a>:
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a view that holds a reference to the original sequence <code class="computeroutput"><span class="identifier">s</span></code>
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and the value <code class="computeroutput"><span class="identifier">x</span></code>. Functions
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that were once sequence specific and need to be implemented N times over N
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different sequences are now implemented only once.
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</p>
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<p>
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Fusion provides full round compatibility with <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>.
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Fusion sequences are fully conforming <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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sequences and <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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sequences are fully compatible with Fusion. You can work with Fusion sequences
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on <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> if you
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wish to work solely on types <sup>[<a name="id795466" href="#ftn.id795466" class="footnote">1</a>]</sup>. In <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>,
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Fusion sequences follow <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>'s
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sequence-type preserving semantics (i.e. algorithms preserve the original sequence
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type. e.g. transforming a vector returns a vector). You can also convert from
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an <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> sequence
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to a Fusion sequence. For example, there are times when it is convenient to
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work solely on <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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using pure <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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sequences, then, convert them to Fusion sequences as a final step before actual
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instantiation of real runtime objects with data. You have the best of both
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worlds.
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</p>
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<div class="footnotes">
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<br><hr width="100" align="left">
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<div class="footnote"><p><sup>[<a name="ftn.id795466" href="#id795466" class="para">1</a>] </sup>
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Choose <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
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over fusion when doing pure type calculations. Once the static type calculation
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is finished, you can instantiate a fusion sequence (see <a class="link" href="container/conversion/functions.html" title="Functions">Conversion</a>)
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for the runtime part.
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</p></div>
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</div>
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Joel de Guzman, Dan Marsden, Tobias
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Schwinger<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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</div></td>
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