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<title>Boost Library Reuse</title>
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<td bgcolor="#FFFFFF"><img src="../boost.png" alt="boost.png (6897 bytes)" width="277" height="86"></td>
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<td><a href="../index.htm"><font face="Arial" color="#FFFFFF"><big>Home</big></font></a></td>
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<td><a href="../libs/libraries.htm"><font face="Arial" color="#FFFFFF"><big>Libraries</big></font></a></td>
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<td><a href="faq.htm"><font face="Arial" color="#FFFFFF"><big>FAQ</big></font></a></td>
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<td><a href="index.htm"><font face="Arial" color="#FFFFFF"><big>More</big></font></a></td>
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<h1>Boost Library reuse: cost versus benefit trade-offs</h1>
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<p>A Boost library <b>should not</b> use libraries other than Boost or the C++
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Standard Library.</p>
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<p>A Boost library <b>should</b> use other Boost Libraries or the C++ Standard
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Library, but only when the benefits outweigh the costs. </p>
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<p>The benefits of using components from other libraries may include clearer,
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more understandable code, reduced development and maintenance costs, and the
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assurance which comes from reusing well-known and trusted building blocks.</p>
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<p>The costs may include undesirable coupling between components, and added
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compilation and runtime costs. If the interface to the additional
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component is complex, using it may make code less readable, and thus actually
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increase development and maintenance costs.</p>
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<p>Negative effects of coupling become obvious when one library uses a second
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library which uses a third, and so on. The worst form of coupling requires the
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user understand each of the coupled libraries. Coupling may also reduce the
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portability of a library - even in case when all used libraries are
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self-sufficient (see example of questionable usage of <iostream> library
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below).</p>
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<p><b>Example where another boost component should certainly be used:</b>
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boost::noncopyable (in <a href="../boost/utility.hpp">boost/utility.hpp</a>) has
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considerable benefits; it simplifies code, improves readability, and signals
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intent. Costs are low as coupling is limited; noncopyable itself
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uses no other classes and its header includes only the lightweight headers
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<boost/config.hpp> and <cstddef>. There are no runtime costs
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at all. With costs so low and benefits so high, other boost libraries should use
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boost::noncopyable when the need arises except in exceptional circumstances.</p>
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<p><b>Example where a standard library component might possibly be used:</b>
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Providing diagnostic output as a debugging aid can be a nice feature for a
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library. Yet using Standard Library <iostream> can involves a lot of
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additional cost, particularly if <iostream> is unlikely to be use
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elsewhere in the application. In certain GUI or embedded applications,
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coupling to <iostream> would be a disqualification.
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Consider redesign of the boost library in question so that the user supplies the
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diagnostic output mechanism.</p>
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<p><b>Example where another boost component should not be used:</b> The
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boost dir_it library has considerable coupling and runtime costs, not to mention
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portability issues for unsupported operating systems. While completely
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appropriate when directory iteration is required, it would not be reasonable for
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another boost library to use dir_it just to check that a file is available
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before opening. C++ Standard Library file open functionality does this at
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lower cost. Don't use dir_it just for the sake of using a boost library.</p>
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<hr>
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<p>Revised <!--webbot bot="Timestamp" s-type="EDITED" s-format="%d %B %Y" startspan -->02 October 2003<!--webbot bot="Timestamp" endspan i-checksum="32277" --></p>
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<p><EFBFBD> Copyright Beman Dawes 2000</p>
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<p> Use, modification, and distribution are subject to the Boost Software
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License, Version 1.0. (See accompanying file <a href="../LICENSE_1_0.txt">
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LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt">
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www.boost.org/LICENSE_1_0.txt</a>)</p>
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