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+<meta name="GENERATOR" content="Microsoft FrontPage 4.0">
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+<meta name="ProgId" content="FrontPage.Editor.Document">
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+<title>Library Reuse</title>
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+</head>
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+<body bgcolor="#FFFFFF" text="#000000">
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+<table border="1" bgcolor="#007F7F" cellpadding="2">
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+ <tr>
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+ <td bgcolor="#FFFFFF"><img src="../c++boost.gif" alt="c++boost.gif (8819 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="../libraries.htm"><font face="Arial" color="#FFFFFF"><big>Libraries</big></font></a></td>
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+ <td><a href="../people.htm"><font face="Arial" color="#FFFFFF"><big>People</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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+ </tr>
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+</table>
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+
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+<h1>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 -->17 August 2000<!--webbot bot="Timestamp" endspan i-checksum="31251" --></p>
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+<p> </p>
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+<p> </p>
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+
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+</body>
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+
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+</html>
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Beman Dawes