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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="boost_config.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
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</h2></div></div></div>
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<div class="toc"><dl class="toc">
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<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_problem">The problem</a></span></dt>
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<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_solution">The solution</a></span></dt>
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</dl></div>
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<p>
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The problem with many traditional "textbook" implementations of configuration
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headers (where all the configuration options are in a single "monolithic"
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header) is that they violate certain fundamental software engineering principles
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which would have the effect of making boost more fragile, more difficult to
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maintain and more difficult to use safely. You can find a description of the
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principles from the <a href="http://www.objectmentor.com/resources/articles/Principles_and_Patterns.pdf" target="_top">following
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article</a>.
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</p>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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>
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</h3></div></div></div>
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<p>
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Consider a situation in which you are concurrently developing on multiple
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platforms. Then consider adding a new platform or changing the platform definitions
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of an existing platform. What happens? Everything, and this does literally
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mean everything, recompiles. Isn't it quite absurd that adding a new platform,
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which has absolutely nothing to do with previously existing platforms, means
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that all code on all existing platforms needs to be recompiled?
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</p>
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<p>
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Effectively, there is an imposed physical dependency between platforms that
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have nothing to do with each other. Essentially, the traditional solution
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employed by configuration headers does not conform to the Open-Closed Principle:
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</p>
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<div class="blockquote"><blockquote class="blockquote"><p>
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<span class="bold"><strong>"A module should be open for extension but closed
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for modification."</strong></span>
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</p></blockquote></div>
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<p>
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Extending a traditional configuration header implies modifying existing code.
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</p>
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<p>
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Furthermore, consider the complexity and fragility of the platform detection
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code. What if a simple change breaks the detection on some minor platform?
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What if someone accidentally or on purpose (as a workaround for some other
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problem) defines some platform dependent macros that are used by the detection
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code? A traditional configuration header is one of the most volatile headers
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of the entire library, and more stable elements of Boost would depend on
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it. This violates the Stable Dependencies Principle:
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</p>
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<div class="blockquote"><blockquote class="blockquote"><p>
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<span class="bold"><strong>"Depend in the direction of stability."</strong></span>
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</p></blockquote></div>
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<p>
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After even a minor change to a traditional configuration header on one minor
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platform, almost everything on every platform should be tested if we follow
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sound software engineering practice.
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</p>
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<p>
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Another important issue is that it is not always possible to submit changes
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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>.
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Some boost users are currently working on platforms using tools and libraries
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that are under strict Non-Disclosure Agreements. In this situation it is
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impossible to submit changes to a traditional monolithic configuration header,
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instead some method by which the user can insert their own configuration
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code must be provided.
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</p>
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</div>
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<div class="section">
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<div class="titlepage"><div><div><h3 class="title">
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<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>
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</h3></div></div></div>
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<p>
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The approach taken by boost's configuration headers is to separate configuration
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into three orthogonal parts: the compiler, the standard library and the platform.
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Each compiler/standard library/platform gets its own mini-configuration header,
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so that changes to one compiler's configuration (for example) does not affect
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other compilers. In addition there are measures that can be taken both to
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omit the compiler/standard library/platform detection code (so that adding
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support to a new platform does not break dependencies), or to freeze the
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configuration completely; providing almost complete protection against dependency
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changes.
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</p>
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</div>
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</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="left"></td>
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<td align="right"><div class="copyright-footer">Copyright © 2001-2007 Beman Dawes, Vesa Karvonen, John
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Maddock<p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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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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</p>
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