I've recently been discussing the guideline recently added to the exceptions policy page with Dave Abrahams and he has asked me to post my views here.
There is a seductive form of arguement that I've seen repeatedly lead projects into trouble which has made me very suspicious of the form: "provide X because someone might want to do Y". One common example is "provide public access to member data because someone might need to do something with the members that isn't supported directly". (I hope that this is a forum that would reject this argument.) Another arguement I've encountered recently is that "all classes should have a virtual destructor because someone may want to specialise them". The arguement is especially seductive when the cost of "providing X" is either small or hidden. But many a project has failed through creeping featuritis driven by such concerns. The cost of virtual inheritance from std::exception may well appear small - indeed, most of it is the intellectual cost of understanding the intent. Arguments of this form need to be challenged on the basis of whether "to do Y" is a reasonable expectation. If not, then the solution isn't "provide X" it is "discourage Y". Now, unlike the examples cited above, I've *never* encountered anyone arguing that exception hierarchies should be designed to support MI in a real design scenario. Nor, when challenged on this point, has Dave. The hypothetial example cited seems very implausible: when developing a subsystem to meet two interfaces I invariably develop a pair of adaption wrappers over the core functionality. (And, in general, repackaging of exceptions is a very small part of that wrapper.) The obvious conclusion from the example cited is "don't multiply inherit from classes that are not designed to support it". I'm always willing to learn, but in the absence of a compelling example that MI is useful in exception heirarchies, I am unwilling to support the new guideline. PS Sorry for starting a new thread, but I cannot connect to gmane from work and home is 130 miles away. ------------------------------------------------------------------------ For more information about Barclays Capital, please visit our web site at http://www.barcap.com. Internet communications are not secure and therefore the Barclays Group does not accept legal responsibility for the contents of this message. Although the Barclays Group operates anti-virus programmes, it does not accept responsibility for any damage whatsoever that is caused by viruses being passed. Any views or opinions presented are solely those of the author and do not necessarily represent those of the Barclays Group. Replies to this email may be monitored by the Barclays Group for operational or business reasons. ------------------------------------------------------------------------ _______________________________________________ Unsubscribe & other changes: http://lists.boost.org/mailman/listinfo.cgi/boost
