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<p><img border="0" src="../boost.png" alt="Boost logo" width="277" height="86"></p>
<h1>Feature Model Diagrams in text and HTML</h1>
<p>By <a href="../people/beman_dawes.html">Beman Dawes</a></p>
<h2>Introduction</h2>
<p>In their seminal book, Generative Programming, Czarnecki and Eisenecker (<a href="#Generative%20Programming">C&amp;E</a>))
describe how to build feature models [C&amp;E 4.4] consisting of a feature
diagram plus semantic, rationale, and other attributes.&nbsp; Feature models are
then used to drive design cycles which eventually lead to manual or automatic
assembly of configurations.</p>
<p>Feature models provide a language to describe the library variability that is
often such an issue in boost.org discussions. The Whorf hypothesis that
&quot;Language shapes the way we think, and determines what we can think
about&quot; seems to apply.&nbsp; In discussion of library variability issues,
we have been crippled by lack of a good language. With feature models we now
have a language to carry on the dialog.</p>
<p>The graphical feature diagrams presented by C&amp;E are not in a suitable
form for the email discussions boost.org depends upon. The hierarchical nature
of feature diagrams can be represented by a simple text-based feature diagram
language.&nbsp; A feature model can also take advantage of the hyperlinks
inherent in HTML.</p>
<h2><a name="Grammar">Grammar</a></h2>
<p>The grammar for the feature diagram language is expressed in Extended
Bakus-Naur Form; ::= represents productions, [...] represents options, {...}
represents zero or more instances, and represents | alternatives.</p>
<blockquote>
  <pre>feature-model       ::= concept-name details { feature }</pre>
  <pre>feature             ::= feature-name [details]</pre>
  <pre>details             ::= &quot;(&quot; feature-list &quot;)&quot;      // required features
                      | &quot;[&quot; feature-list &quot;]&quot;      // optional features</pre>
  <pre>feature-list        ::= element { &quot;|&quot; element }   // one only
                      | element { &quot;+&quot; element }   // one or more
                      | element { &quot;,&quot; element }   // all
                                                  // [a+b] equivalent to [a,b]</pre>
  <pre>element             ::= feature
                      | details</pre>
  <pre>concept-name        ::= name</pre>
  <pre>feature-name        ::= name</pre>
</blockquote>
<p>The usual lexical conventions apply. Names are case-insensitive and consist
of a leading letter, followed by letters, digits, underscores or hyphens, with
no spaces allowed.</p>
<p>At least one instance of each name should be hyperlinked to the corresponding
<a href="#FeatureDescriptions">Feature Description</a>.</p>
<p>While the grammar is intended for written communication between people, it
may also be trivially machine parsed for use by automatic tools.</p>
<h2><a id="FeatureDescriptions" name="FeatureDescriptions"></a></h2>
<p>Descriptive information is associated with each concept or feature. According
to [C&amp;E 4.4.2] this includes:</p>
<ul>
  <li>Semantic descriptions.</li>
  <li>Rationale.</li>
  <li>Stakeholders and client programs.</li>
  <li>Exemplar systems.</li>
  <li>Constraints and default dependency rules.</li>
  <li>Availability sites, binding sites, and binding mode.</li>
  <li>Open/Closed attribute.</li>
</ul>
<h2>What is a Feature?</h2>
<p>A feature [C&amp;E 4.9.1] is &quot;anything users or client programs might
want to control about a concept.&nbsp; Thus, during feature modeling, we
document no only functional features ... but also implementation features, ...,
various optimizations, alternative implementation techniques, and so on.&quot;</p>
<h2>Example</h2>
<blockquote>
  <pre>special-container ( organization,
                    performance,
                    interface  )         // all required</pre>
  <pre>organization [ ordered + indexed ]       // zero or more (4 configurations)</pre>
  <pre>indexed [ hash-function ]                // zero or one  (2 configurations)</pre>
  <pre>performance ( fast | small | balanced )  // exactly one  (3 configurations)</pre>
  <pre>interface ( STL-style + cursor-style )   // one or more  (3 configurations)</pre>
</blockquote>
<p>There should be feature descriptions for <code>some-container, organization,
ordered, indexed, hash-function, performance, fast, small, balanced, interface,
STL-style, and cursor-style</code>.</p>
<p>The number of possible configurations is&nbsp; (2 + 2*2) * 3 * 3 = 54,
assuming no constraints.</p>
<p>There are equivalent representations. For example:</p>
<blockquote>
  <pre>special-container ( organization[ ordered+indexed[ hash-function ]],
                    performance( fast|small|balanced ),
                    interface( STL-style+cursor-style ) )</pre>
</blockquote>
<h2>References</h2>
<p>Krzysztof Czarnecki and Ulrich W. Eisenecker, <a href="http://www.generative-programming.org">Generative
Programming</a>, Addison-Wesley, 2000, ISBN 0-201-30977-7</p>
<hr>
<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan -->26 August 2004<!--webbot bot="Timestamp" endspan i-checksum="32277" --></p>
<p>© Copyright Beman Dawes, 2000</p>

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