I have been playing with Boost.Units as part of a work project recently, and I am not sure what the best approach to "demarshalling" values in a specific unit. For example, I would like to do something like:

using namespace boost::units;

struct A {
   enum { feet, kilometers };

   quantity<si::length> some_length;

   double foo(int type) {
      switch(type) {
      case feet:
          return quantity<feet>(some_length).value();
      case kilometers:
          return quantity<kilometers>(some_length).value();
      }
   }
};

This approach is necessary for passing values to external libraries which do not (and probably will never) use Boost.Units. Is conversion_factor the best approach, or is there a way to achieve this implicitly using the type system?

Thanks
Justin