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<div class="section">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="fusion.introduction"></a><a class="link" href="introduction.html" title="Introduction">Introduction</a>
</h2></div></div></div>
<p>
An advantage other languages such as Python and Lisp/ Scheme, ML and Haskell,
etc., over C++ is the ability to have heterogeneous containers that can hold
arbitrary element types. All the containers in the standard library can only
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>
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
only hold elements of type <code class="computeroutput"><span class="identifier">X</span></code>,
and so on.
</p>
<p>
True, you can use inheritance to make the containers hold different types,
related through subclassing. However, you have to hold the objects through
a pointer or smart reference of some sort. Doing this, you'll have to rely
on virtual functions to provide polymorphic behavior since the actual type
is erased as soon as you store a pointer to a derived class to a pointer to
its base. The held objects must be related: you cannot hold objects of unrelated
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>
<span class="identifier">X</span></code>, <code class="computeroutput"><span class="keyword">float</span></code>,
etc. Oh sure you can use something like <a href="http://boost.org/doc/html/any.html" target="_top">Boost.Any</a>
to hold arbitrary types, but then you pay more in terms of runtime costs and
due to the fact that you practically erased all type information, you'll have
to perform dangerous casts to get back the original type.
</p>
<p>
The <a href="http://www.boost.org/libs/tuple/doc/tuple_users_guide.html" target="_top">Boost.Tuple</a>
library written by <a href="http://www.boost.org/people/jaakko_jarvi.htm" target="_top">Jaakko
Jarvi</a> provides heterogeneous containers in C++. The <code class="computeroutput"><span class="identifier">tuple</span></code>
is a basic data structure that can hold heterogeneous types. It's a good first
step, but it's not complete. What's missing are the algorithms. It's nice that
we can store and retrieve data to and from tuples, pass them around as arguments
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>
facility is already very useful. Yet, as soon as you use it more often, usage
patterns emerge. Eventually, you collect these patterns into algorithm libraries.
</p>
<p>
Hmmm, kinda reminds us of STL right? Right! Can you imagine how it would be
like if you used STL without the algorithms? Everyone will have to reinvent
their own <span class="emphasis"><em>algorithm</em></span> wheels.
</p>
<p>
Fusion is a library and a framework similar to both <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
and the boost <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>.
The structure is modeled after <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>,
which is modeled after <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
It is named "fusion" because the library is reminiscent of the "fusion"
of compile time meta-programming with runtime programming. The library inherently
has some interesting flavors and characteristics of both <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
and <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>.
It lives in the twilight zone between compile time meta-programming and run
time programming. <a href="http://en.wikipedia.org/wiki/Standard_Template_Library" target="_top">STL</a>
containers work on values. MPL containers work on types. Fusion containers
work on both types and values.
</p>
<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>
<p>
The <span class="emphasis"><em>lazy</em></span> evaluation scheme where algorithms return views
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.
</p>
<p>
Fusion provides full round compatibility with <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>.
Fusion sequences are fully conforming <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences and <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences are fully compatible with Fusion. You can work with Fusion sequences
on <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> if you
wish to work solely on types <sup>[<a name="fusion.introduction.f0" href="#ftn.fusion.introduction.f0" class="footnote">1</a>]</sup>. In <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>,
Fusion sequences follow <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>'s
sequence-type preserving semantics (i.e. algorithms preserve the original sequence
type. e.g. transforming a vector returns a vector). You can also convert from
an <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a> sequence
to a Fusion sequence. For example, there are times when it is convenient to
work solely on <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
using pure <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
sequences, then, convert them to Fusion sequences as a final step before actual
instantiation of real runtime objects with data. You have the best of both
worlds.
</p>
<div class="footnotes">
<br><hr width="100" align="left">
<div class="footnote"><p><sup>[<a id="ftn.fusion.introduction.f0" href="#fusion.introduction.f0" class="para">1</a>] </sup>
Choose <a href="http://www.boost.org/libs/mpl/index.html" target="_top">MPL</a>
over fusion when doing pure type calculations. Once the static type calculation
is finished, you can instantiate a fusion sequence (see <a class="link" href="container/conversion/functions.html" title="Functions">Conversion</a>)
for the runtime part.
</p></div>
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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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