Hi, Regarding using Typeof with compile time libraries such as mpl. mpl doesnt actually deal in types so when I operate on a mpl vector I might get back a mpl::vector7 or something else. Similarly with mpl::math::rational I might get back a mpl::math::rational or a mpl::math::simplified_rational. That makes for a very flakey quantities library unfortunately, because in compliant mode Typeof is totally dependent on implementation changes in these other libraries, and when the compiler crashes because of unregistered types it does so in a huge fashion. Not sure what if anything can be done about it but I'm wondering whether it is worth asking library authors now to provide a mechanism so that user can register their libraries types (that would be eg a mpl::vector +++ anything it might turn into... is that possible or impractical? )without hassle. That would solve my problem I think, but maybe would result in a type registration explosion? Any mechanism that allows use of Typeof without dependencies on implementation details of other libraries would be appreciated! regards Andy Little