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26<div class="titlepage"><div><div><h2 class="title" style="clear: both">
27<a name="boost_config.rationale"></a><a class="link" href="rationale.html" title="Rationale">Rationale</a>
28</h2></div></div></div>
29<div class="toc"><dl>
30<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_problem">The problem</a></span></dt>
31<dt><span class="section"><a href="rationale.html#boost_config.rationale.the_solution">The solution</a></span></dt>
32</dl></div>
33<p>
34 The problem with many traditional "textbook" implementations of configuration
35 headers (where all the configuration options are in a single "monolithic"
36 header) is that they violate certain fundamental software engineering principles
37 which would have the effect of making boost more fragile, more difficult to
38 maintain and more difficult to use safely. You can find a description of the
39 principles from the <a href="http://www.objectmentor.com/resources/articles/Principles_and_Patterns.pdf" target="_top">following
40 article</a>.
41 </p>
42<div class="section">
43<div class="titlepage"><div><div><h3 class="title">
44<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>
45</h3></div></div></div>
46<p>
47 Consider a situation in which you are concurrently developing on multiple
48 platforms. Then consider adding a new platform or changing the platform definitions
49 of an existing platform. What happens? Everything, and this does literally
50 mean everything, recompiles. Isn't it quite absurd that adding a new platform,
51 which has absolutely nothing to do with previously existing platforms, means
52 that all code on all existing platforms needs to be recompiled?
53 </p>
54<p>
55 Effectively, there is an imposed physical dependency between platforms that
56 have nothing to do with each other. Essentially, the traditional solution
57 employed by configuration headers does not conform to the Open-Closed Principle:
58 </p>
59<div class="blockquote"><blockquote class="blockquote"><p>
60 <span class="bold"><strong>"A module should be open for extension but closed
61 for modification."</strong></span>
62 </p></blockquote></div>
63<p>
64 Extending a traditional configuration header implies modifying existing code.
65 </p>
66<p>
67 Furthermore, consider the complexity and fragility of the platform detection
68 code. What if a simple change breaks the detection on some minor platform?
69 What if someone accidentally or on purpose (as a workaround for some other
70 problem) defines some platform dependent macros that are used by the detection
71 code? A traditional configuration header is one of the most volatile headers
72 of the entire library, and more stable elements of Boost would depend on
73 it. This violates the Stable Dependencies Principle:
74 </p>
75<div class="blockquote"><blockquote class="blockquote"><p>
76 <span class="bold"><strong>"Depend in the direction of stability."</strong></span>
77 </p></blockquote></div>
78<p>
79 After even a minor change to a traditional configuration header on one minor
80 platform, almost everything on every platform should be tested if we follow
81 sound software engineering practice.
82 </p>
83<p>
84 Another important issue is that it is not always possible to submit changes
85 to <code class="computeroutput"><span class="special">&lt;</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">&gt;</span></code>.
86 Some boost users are currently working on platforms using tools and libraries
87 that are under strict Non-Disclosure Agreements. In this situation it is
88 impossible to submit changes to a traditional monolithic configuration header,
89 instead some method by which the user can insert their own configuration
90 code must be provided.
91 </p>
92</div>
93<div class="section">
94<div class="titlepage"><div><div><h3 class="title">
95<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>
96</h3></div></div></div>
97<p>
98 The approach taken by boost's configuration headers is to separate configuration
99 into three orthogonal parts: the compiler, the standard library and the platform.
100 Each compiler/standard library/platform gets its own mini-configuration header,
101 so that changes to one compiler's configuration (for example) does not affect
102 other compilers. In addition there are measures that can be taken both to
103 omit the compiler/standard library/platform detection code (so that adding
104 support to a new platform does not break dependencies), or to freeze the
105 configuration completely; providing almost complete protection against dependency
106 changes.
107 </p>
108</div>
109</div>
110<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
111<td align="left"></td>
112<td align="right"><div class="copyright-footer">Copyright &#169; 2001-2007 Beman Dawes, Vesa Karvonen, John
113 Maddock<p>
114 Distributed under the Boost Software License, Version 1.0. (See accompanying
115 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>)
116 </p>
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