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- <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 class="toc">
- <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>
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