| <html> |
| <head> |
| <meta http-equiv="Content-Type" content="text/html; charset=US-ASCII"> |
| <title>Rationale</title> |
| <link rel="stylesheet" href="../../../../../doc/src/boostbook.css" type="text/css"> |
| <meta name="generator" content="DocBook XSL Stylesheets V1.74.0"> |
| <link rel="home" href="../index.html" title="Boost.Config"> |
| <link rel="up" href="../index.html" title="Boost.Config"> |
| <link rel="prev" href="guidelines_for_boost_authors.html" title="Guidelines for Boost Authors"> |
| <link rel="next" href="acknowledgements.html" title="Acknowledgements"> |
| </head> |
| <body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF"> |
| <table cellpadding="2" width="100%"><tr> |
| <td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../../boost.png"></td> |
| <td align="center"><a href="../../../../../index.html">Home</a></td> |
| <td align="center"><a href="../../../../../libs/libraries.htm">Libraries</a></td> |
| <td align="center"><a href="http://www.boost.org/users/people.html">People</a></td> |
| <td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td> |
| <td align="center"><a href="../../../../../more/index.htm">More</a></td> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="guidelines_for_boost_authors.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| <div class="section" lang="en"> |
| <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" lang="en"> |
| <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" lang="en"> |
| <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> |
| </tr></table> |
| <hr> |
| <div class="spirit-nav"> |
| <a accesskey="p" href="guidelines_for_boost_authors.html"><img src="../../../../../doc/src/images/prev.png" alt="Prev"></a><a accesskey="u" href="../index.html"><img src="../../../../../doc/src/images/up.png" alt="Up"></a><a accesskey="h" href="../index.html"><img src="../../../../../doc/src/images/home.png" alt="Home"></a><a accesskey="n" href="acknowledgements.html"><img src="../../../../../doc/src/images/next.png" alt="Next"></a> |
| </div> |
| </body> |
| </html> |
