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<title>Rationale</title>
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<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>
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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="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>)
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