already, so the only non-degenerate use case of quantity_cast is to take a raw value and return a quantity:
q = quantity_cast< quantity<SI::length,double> >(1.5);
Maybe I'm missing something here, but why shouldn't this last cast be done using a constructor instead:
q = quantity<SI::length,double>(1.5*meter);
This way, the set of library capabilities is no different, but the resulting code is more explicit, and quantity_cast cannot be used in a type-unsafe way. Further, it's name suggests that quantity_cast only casts between quantities, not between underlying values and quantites. Taking a raw value and constructing a quantity from it should be the sole purview of the quantity constructor.
Yes, that would be my preference. We intentionally made the constructors require that everything be explicitly specified or convertible, so direct construction from a bare value type is forbidden - I believe that you agree that this is important. We do need to be able to make end-runs somehow for reasons that have been elucidated already : 1) accessing a non-const reference to the quantity data member this is going to be done through quantity_reinterpret_cast<> 2) construction of a quantity from a bare value type this is done through the static member function quantity<>::from_value (val) 3) getting a const value type reference this is done through the non-mutating value() member function 4) safe conversion of units from one system to another or from one value type to another this is done through the explicit constructor unless implicit conversion has been specifically enabled, either through BOOST_UNITS_ALLOW_IMPLICIT_CONVERSIONS or by enabling specific implicit conversions. I don't think we need any of the quantity_cast variants anymore, but am happy to be corrected... Matthias