For * and / I need it for a different reason, the library *can* multiply quantities from different systems but them it becomes very difficult to convert that quantity to a third system of units. It seems that explicit conversion can't handle such conversion even though all the conversion factors are defined. So by adding factors in an expression one can inadvertedly introduce many different systems of units that later can not be converted.
AFAIK, if conversions are correctly defined there should not be a problem converting to a third system of units. Can you provide a small, self-contained example of the problem?
I can share the code of these auto_conversion_operators if you are interested.
Steven, any thoughts on implementing an expression-local way of allowing implicit conversions? It actually seems like it might be a great GSOC project to re-implement Boost.Units using Boost.Proto...
As far as internal implementation, what are the concerns?
In the same way that I implemented some additions I would have liked to implement other features like the explicit conversion in cases were three (or more) systems or units are involved. That part seemed to involve a fair amount of research in the library, in particular in its compile-time linear algebra, static_rational and a lot of MPL. My question about implementation is to know if the library is planned to change in some particular way.
The above comment aside, I'm not aware of any major planned changes in the Boost.Units implementation. Matthias