In C++ the const type mod­i­fiers when applied to meth­ods has the nice prop­erty of mak­ing it impos­si­ble by default (e.g. with­out a const_cast) to call non-const meth­ods. A sim­i­lar fea­ture is pro­vided by C++0x key­word con­s­t­expr.

It would be very nice if this prop­erty could be applied to cus­tom classes of func­tions. The idea would be to tag method def­i­n­i­tions with a cus­tom key­word, or a type. Tagged meth­ods would then only be able to call method tagged with the same type.

A nice usage I could find for this would be to define a non-blocking class of meth­ods. Non block­ing meth­ods are such if they never blocks for a poten­tially unbounded amount of time wait­ing for an event. Using the tags the com­piler will be able to detect if a method sup­posed to be non-blocking is try­ing to call some poten­tially block­ing code and this would be very use­ful to detect pos­si­ble dead­locks at com­pile time. Another usage would be to tag all the meth­ods that are sup­posed to be used by a spe­cific thread with the same tag. With such setup meth­ods that access shared data would be untagged and using them would require a cast or some spe­cial syn­tax, hope­fully help­ing the pro­gram­mer remem­ber that some syn­chro­niza­tion is needed.

A pos­si­ble approach to (ab)use the volatile key­word to detect race con­di­tions was described here in 2001. But maybe this approach could be more gen­eral. Any com­ment?