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<td><a href="../index.htm"><font face="Arial" color="#FFFFFF"><big>Home</big></font></a></td>
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&nbsp;
<h1>Library reuse: cost versus benefit trade-offs</h1>
<p>A Boost library <b>should not</b> use libraries other than Boost or the C++
Standard Library.</p>
<p>A Boost library <b>should</b> use other Boost Libraries or the C++ Standard
Library, but only when the benefits outweigh the costs.&nbsp;</p>
<p>The benefits of using components from other libraries may include clearer,
more understandable code, reduced development and maintenance costs, and the
assurance which comes from reusing well-known and trusted building blocks.</p>
<p>The costs may include undesirable coupling between components, and added
compilation and runtime costs.&nbsp; If the interface to the additional
component is complex, using it may make code less readable, and thus actually
increase development and maintenance costs.</p>
<p>Negative effects of coupling become obvious when one library uses a second
library which uses a third, and so on. The worst form of coupling requires the
user understand each of the coupled libraries. Coupling may also reduce the
portability of a library - even in case when all used libraries are
self-sufficient (see example of questionable usage of &lt;iostream&gt; library
below).</p>
<p><b>Example where another boost component should certainly be used:</b>&nbsp;
boost::noncopyable (in <a href="../boost/utility.hpp">boost/utility.hpp</a>) has
considerable benefits; it simplifies code, improves readability, and signals
intent.&nbsp; Costs are low as coupling is limited;&nbsp; noncopyable itself
uses no other classes and its header includes only the lightweight headers
&lt;boost/config.hpp&gt; and &lt;cstddef&gt;.&nbsp; There are no runtime costs
at all. With costs so low and benefits so high, other boost libraries should use
boost::noncopyable when the need arises except in exceptional circumstances.</p>
<p><b>Example where a standard library component might possibly be used:</b>
Providing diagnostic output as a debugging aid can be a nice feature for a
library. Yet using Standard Library &lt;iostream&gt; can involves a lot of
additional cost, particularly if &lt;iostream&gt; is unlikely to be use
elsewhere in the application.&nbsp; In certain GUI or embedded applications,
coupling to &lt;iostream&gt; would be a disqualification.&nbsp;&nbsp;&nbsp;
Consider redesign of the boost library in question so that the user supplies the
diagnostic output mechanism.</p>
<p><b>Example where another boost component should not be used:</b>&nbsp; The
boost dir_it library has considerable coupling and runtime costs, not to mention
portability issues for unsupported operating systems.&nbsp; While completely
appropriate when directory iteration is required, it would not be reasonable for
another boost library to use dir_it just to check that a file is available
before opening.&nbsp; C++ Standard Library file open functionality does this at
lower cost.&nbsp; Don't use dir_it just for the sake of using a boost library.</p>
<hr>
<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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