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| 24 | <h1>Boost Library reuse: cost versus benefit trade-offs</h1> |
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| 25 | <p>A Boost library <b>should not</b> use libraries other than Boost or the C++ |
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| 26 | Standard Library.</p> |
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| 27 | <p>A Boost library <b>should</b> use other Boost Libraries or the C++ Standard |
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| 28 | Library, but only when the benefits outweigh the costs. </p> |
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| 29 | <p>The benefits of using components from other libraries may include clearer, |
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| 30 | more understandable code, reduced development and maintenance costs, and the |
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| 31 | assurance which comes from reusing well-known and trusted building blocks.</p> |
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| 32 | <p>The costs may include undesirable coupling between components, and added |
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| 33 | compilation and runtime costs. If the interface to the additional |
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| 34 | component is complex, using it may make code less readable, and thus actually |
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| 35 | increase development and maintenance costs.</p> |
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| 36 | <p>Negative effects of coupling become obvious when one library uses a second |
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| 37 | library which uses a third, and so on. The worst form of coupling requires the |
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| 38 | user understand each of the coupled libraries. Coupling may also reduce the |
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| 39 | portability of a library - even in case when all used libraries are |
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| 40 | self-sufficient (see example of questionable usage of <iostream> library |
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| 41 | below).</p> |
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| 42 | <p><b>Example where another boost component should certainly be used:</b> |
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| 43 | boost::noncopyable (in <a href="../boost/utility.hpp">boost/utility.hpp</a>) has |
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| 44 | considerable benefits; it simplifies code, improves readability, and signals |
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| 45 | intent. Costs are low as coupling is limited; noncopyable itself |
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| 46 | uses no other classes and its header includes only the lightweight headers |
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| 47 | <boost/config.hpp> and <cstddef>. There are no runtime costs |
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| 48 | at all. With costs so low and benefits so high, other boost libraries should use |
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| 49 | boost::noncopyable when the need arises except in exceptional circumstances.</p> |
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| 50 | <p><b>Example where a standard library component might possibly be used:</b> |
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| 51 | Providing diagnostic output as a debugging aid can be a nice feature for a |
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| 52 | library. Yet using Standard Library <iostream> can involves a lot of |
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| 53 | additional cost, particularly if <iostream> is unlikely to be use |
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| 54 | elsewhere in the application. In certain GUI or embedded applications, |
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| 55 | coupling to <iostream> would be a disqualification. |
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| 56 | Consider redesign of the boost library in question so that the user supplies the |
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| 57 | diagnostic output mechanism.</p> |
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| 58 | <p><b>Example where another boost component should not be used:</b> The |
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| 59 | boost dir_it library has considerable coupling and runtime costs, not to mention |
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| 60 | portability issues for unsupported operating systems. While completely |
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| 61 | appropriate when directory iteration is required, it would not be reasonable for |
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| 62 | another boost library to use dir_it just to check that a file is available |
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| 63 | before opening. C++ Standard Library file open functionality does this at |
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| 64 | lower cost. Don't use dir_it just for the sake of using a boost library.</p> |
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| 65 | <hr> |
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| 66 | <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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| 67 | <p>© Copyright Beman Dawes 2000</p> |
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| 68 | <p> Use, modification, and distribution are subject to the Boost Software |
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| 69 | License, Version 1.0. (See accompanying file <a href="../LICENSE_1_0.txt"> |
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| 70 | LICENSE_1_0.txt</a> or copy at <a href="http://www.boost.org/LICENSE_1_0.txt"> |
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| 71 | www.boost.org/LICENSE_1_0.txt</a>)</p> |
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| 72 | |
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