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+<title>Rationale</title>
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+</div>
+<div class="section">
+<div class="titlepage"><div><div><h2 class="title" style="clear: both">
+<a name="boost_config.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
+</h2></div></div></div>
+<div class="toc"><dl>
+<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_problem">The problem</a></span></dt>
+<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_solution">The solution</a></span></dt>
+</dl></div>
+<p>
+ The problem with many traditional "textbook" implementations of configuration
+ headers (where all the configuration options are in a single "monolithic"
+ header) is that they violate certain fundamental software engineering principles
+ which would have the effect of making boost more fragile, more difficult to
+ maintain and more difficult to use safely. You can find a description of the
+ principles from the <a href="http://www.objectmentor.com/resources/articles/Principles_and_Patterns.pdf" target="_top">following
+ article</a>.
+ </p>
+<div class="section">
+<div class="titlepage"><div><div><h3 class="title">
+<a name="boost_config.rationale.the_problem"></a><a class="link" href="rationale.html#boost_config.rationale.the_problem" title="The problem">The problem</a>
+</h3></div></div></div>
+<p>
+ Consider a situation in which you are concurrently developing on multiple
+ platforms. Then consider adding a new platform or changing the platform definitions
+ of an existing platform. What happens? Everything, and this does literally
+ mean everything, recompiles. Isn't it quite absurd that adding a new platform,
+ which has absolutely nothing to do with previously existing platforms, means
+ that all code on all existing platforms needs to be recompiled?
+ </p>
+<p>
+ Effectively, there is an imposed physical dependency between platforms that
+ have nothing to do with each other. Essentially, the traditional solution
+ employed by configuration headers does not conform to the Open-Closed Principle:
+ </p>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <span class="bold"><strong>"A module should be open for extension but closed
+ for modification."</strong></span>
+ </p></blockquote></div>
+<p>
+ Extending a traditional configuration header implies modifying existing code.
+ </p>
+<p>
+ Furthermore, consider the complexity and fragility of the platform detection
+ code. What if a simple change breaks the detection on some minor platform?
+ What if someone accidentally or on purpose (as a workaround for some other
+ problem) defines some platform dependent macros that are used by the detection
+ code? A traditional configuration header is one of the most volatile headers
+ of the entire library, and more stable elements of Boost would depend on
+ it. This violates the Stable Dependencies Principle:
+ </p>
+<div class="blockquote"><blockquote class="blockquote"><p>
+ <span class="bold"><strong>"Depend in the direction of stability."</strong></span>
+ </p></blockquote></div>
+<p>
+ After even a minor change to a traditional configuration header on one minor
+ platform, almost everything on every platform should be tested if we follow
+ sound software engineering practice.
+ </p>
+<p>
+ Another important issue is that it is not always possible to submit changes
+ to <code class="computeroutput"><span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">config</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>.
+ Some boost users are currently working on platforms using tools and libraries
+ that are under strict Non-Disclosure Agreements. In this situation it is
+ impossible to submit changes to a traditional monolithic configuration header,
+ instead some method by which the user can insert their own configuration
+ code must be provided.
+ </p>
+</div>
+<div class="section">
+<div class="titlepage"><div><div><h3 class="title">
+<a name="boost_config.rationale.the_solution"></a><a class="link" href="rationale.html#boost_config.rationale.the_solution" title="The solution">The solution</a>
+</h3></div></div></div>
+<p>
+ The approach taken by boost's configuration headers is to separate configuration
+ into three orthogonal parts: the compiler, the standard library and the platform.
+ Each compiler/standard library/platform gets its own mini-configuration header,
+ so that changes to one compiler's configuration (for example) does not affect
+ other compilers. In addition there are measures that can be taken both to
+ omit the compiler/standard library/platform detection code (so that adding
+ support to a new platform does not break dependencies), or to freeze the
+ configuration completely; providing almost complete protection against dependency
+ changes.
+ </p>
+</div>
+</div>
+<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
+<td align="left"></td>
+<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Beman Dawes, Vesa Karvonen, John
+ Maddock<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>
+</div></td>
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