| 3 | | = Boost Library reuse: cost versus benefit trade-offs = #intro |
| 4 | | |
| 5 | | A Boost library '''should not''' use libraries |
| 6 | | other than Boost or the C++ Standard Library. |
| 7 | | |
| 8 | | A Boost library '''should''' use other Boost |
| 9 | | Libraries or the C++ Standard Library, but only when the |
| 10 | | benefits outweigh the costs. |
| 11 | | |
| 12 | | The benefits of using components from other libraries may |
| 13 | | include clearer, more understandable code, reduced development |
| 14 | | and maintenance costs, and the assurance which comes from |
| 15 | | reusing well-known and trusted building blocks. |
| 16 | | |
| 17 | | The costs may include undesirable coupling between |
| 18 | | components, and added compilation and runtime costs. If the |
| 19 | | interface to the additional component is complex, using it may |
| 20 | | make code less readable, and thus actually increase development |
| 21 | | and maintenance costs. |
| 22 | | |
| 23 | | Negative effects of coupling become obvious when one library |
| 24 | | uses a second library which uses a third, and so on. The worst |
| 25 | | form of coupling requires the user understand each of the |
| 26 | | coupled libraries. Coupling may also reduce the portability of |
| 27 | | a library - even in case when all used libraries are |
| 28 | | self-sufficient (see example of questionable usage of |
| 29 | | <iostream> library below). |
| 30 | | |
| 31 | | '''Example where another boost component should |
| 32 | | certainly be used:''' boost::noncopyable (in |
| 33 | | [http://www.boost.org/doc/libs/release/boost/utility.hpp boost/utility.hpp]) |
| 34 | | has considerable benefits; it simplifies code, improves |
| 35 | | readability, and signals intent. Costs are low as coupling is |
| 36 | | limited; noncopyable itself uses no other classes and its |
| 37 | | header includes only the lightweight headers |
| 38 | | <boost/config.hpp> and <cstddef>. There are no |
| 39 | | runtime costs at all. With costs so low and benefits so high, |
| 40 | | other boost libraries should use boost::noncopyable when the |
| 41 | | need arises except in exceptional circumstances. |
| 42 | | |
| 43 | | '''Example where a standard library component might |
| 44 | | possibly be used:''' Providing diagnostic output as a |
| 45 | | debugging aid can be a nice feature for a library. Yet using |
| 46 | | Standard Library <iostream> can involves a lot of |
| 47 | | additional cost, particularly if <iostream> is unlikely |
| 48 | | to be use elsewhere in the application. In certain GUI or |
| 49 | | embedded applications, coupling to <iostream> would be a |
| 50 | | disqualification. Consider redesign of the boost library in |
| 51 | | question so that the user supplies the diagnostic output |
| 52 | | mechanism. |
| 53 | | |
| 54 | | '''Example where another boost component should not be |
| 55 | | used:''' The boost dir_it library has considerable |
| 56 | | coupling and runtime costs, not to mention portability issues |
| 57 | | for unsupported operating systems. While completely appropriate |
| 58 | | when directory iteration is required, it would not be |
| 59 | | reasonable for another boost library to use dir_it just to |
| 60 | | check that a file is available before opening. C++ Standard |
| 61 | | Library file open functionality does this at lower cost. Don't |
| 62 | | use dir_it just for the sake of using a boost library. |
| 63 | | |
| 64 | | Copyright Beman Dawes 2000. |
| | 3 | This has been move back to the [http://www.boost.org/development/reuse.html main site] |
