Won't respond to all the details in this thread, but a few thoughts. There is already something in the date-time implementation that does much of this called constrained_value. http://cvs.sourceforge.net/viewcvs.py/boost/boost/boost/date_time/constrained_value.hpp?rev=1.4&view=auto The constraints are specified at build time to reduce the footprint of the runtime object. Boost date-time uses this to range check things like the month (1..12). The date-time version is policy based so you can specify how to handle the out of range condition: exception, log, etc. For example, here's how the month is setup to throw a bad_month exception: struct bad_month : public std::out_of_range {...} //! Build a policy class for the greg_month_rep typedef CV::simple_exception_policy<unsigned short, 1, 12, bad_month> greg_month_policies; //! A constrained range that implements the gregorian_month rules typedef CV::constrained_value<greg_month_policies> greg_month_rep; greg_month_rep m(13); //bad_month exception Christopher Diggins has actually extended this idea and published it in CUJ. For more you can see his weblog. http://www.artima.com/weblogs/viewpost.jsp?thread=79470 We also discussed this on the developer list, so you'll want to search the archives. Long story short is I'd like to see this as a standalone library -- so I'll help however I can. Jeff