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| <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="id830904" href="#ftn.id830904" 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 name="ftn.id830904" href="#id830904" 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> |
| </div> |
| </div> |
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| <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>) |
| </p> |
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