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 involve a lot of additional cost, particularly if <iostream> is unlikely to be used 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.