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<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. </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. 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 <iostream> library
below).</p>
<p><b>Example where another boost component should certainly be used:</b>
boost::noncopyable (in <a href="../boost/utility.hpp">boost/utility.hpp</a>) has
considerable benefits; it simplifies code, improves readability, and signals
intent. Costs are low as coupling is limited; noncopyable itself
uses no other classes and its header includes only the lightweight headers
<boost/config.hpp> and <cstddef>. 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 <iostream> can involves a lot of
additional cost, particularly if <iostream> is unlikely to be use
elsewhere in the application. In certain GUI or embedded applications,
coupling to <iostream> would be a disqualification.
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> The
boost dir_it library has considerable coupling and runtime costs, not to mention
portability issues for unsupported operating systems. 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. C++ Standard Library file open functionality does this at
lower cost. 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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