Re: [Boost-users] [Units] conversion between double and millimeter

On 14/01/2014 22:46, Quoth Richel Bilderbeek:

When I convert a double to a unit (e.g. meter) and back, this works as expected. When I convert a double to a prefixed unit (e.g. millimeter) and back, this fails to compile. The code below shows how I'd expect it to work. What do I overlook? Thanks, Richel Bilderbeek

const double x_in_mm = 1.0; const Length x(x_in_mm * milli * meter); //This works as expected: const double x_again_in_m = x / meter; //Why doesn't this: const double x_again_in_mm = x / (milli * meter); //Fails

I don't know why that doesn't work (it seems logical to me). But (until someone chimes in with a better one) a somewhat ugly workaround would be: const double x_again_in_mm = x / meter * conversion_factor(meter, milli * meter); If you're doing this a lot, you could define a to_mm(x) helper that does the above, or you could define a new system with a millimeters base unit: typedef scaled_base_unit

...

millimeter_base_unit; // I don't know if there's a way to extract the scale from "milli"

typedef make_system::type mm_system; typedef unit millimeter_unit; typedef quantity LengthMM; BOOST_UNITS_STATIC_CONSTANT(millimeters, millimeter_unit); // this works const double x_again_in_mm = LengthMM(x) / millimeters; (There's probably a better way to do this.) FWIW (for any Boost.Units developers watching), the error (in MSVC) with the original form of the last line is: error C2440: 'initializing' : cannot convert from 'quantity' to 'const double' with [ Unit=unit>>, dimensionless_type>>>>, Y=double ] No user-defined-conversion operator available that can perform this conversion, or the operator cannot be called ie. it looks like it correctly gets down to a dimensionless unit with a scaling factor, but the conversion to double is only defined for a "real" dimensionless unit, not one with a scaling factor attached. You might want to file an issue in Trac to see if support can be added for the original syntax you tried.