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A Boost library should not use libraries other than Boost or the C++ +Standard Library.
+A Boost library should use other Boost Libraries or the C++ Standard +Library, but only when the benefits outweigh the costs.
+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.
+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.
+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).
+Example where another boost component should certainly be used: +boost::noncopyable (in boost/utility.hpp) 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.
+Example where a standard library component might possibly be used: +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.
+Example where another boost component should not be used: 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.
+Revised 17 August 2000
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