Nathan Crookston wrote:
Hi Robert,
It's a little confusing
it refers to my test case or the units library?
but it's printing in the si units because consumption_rate_unit is generated from the si::system. You can create your own system with volume and time and create the consumption rate from it. The following shows how:
... thanks for this i appreciate you'r going to this effort. ...
typedef boost::units::make_system<us::gallon_base_unit, metric::hour_base_unit>::type consumption_system;
I concede that I don't get the "system" concept. It's hardly mentioned in the documentation. I sort of had the feeling that all the units had to be part of the same "system" in order to automatically converted.
typedef unit< consumption_rate_dimension, consumption_system
consumption_rate_unit;
typedef quantity<consumption_rate_unit, float> consumption_rate;
hmmm - so this doesn't use the header ? #include <boost/units/base_units/us/gallon.hpp>
typedef unit<volume_dimension, consumption_system> gallon_unit; BOOST_UNITS_STATIC_CONSTANT(gallon, gallon_unit); BOOST_UNITS_STATIC_CONSTANT(gallons, gallon_unit);
typedef unit<time_dimension, consumption_system> hour_unit; BOOST_UNITS_STATIC_CONSTANT(hours, hour_unit); BOOST_UNITS_STATIC_CONSTANT(hour, hour_unit);
int main() { quantity<consumption_rate_unit> C(1.0 * gallon / hour);
std::cout << 1.0 * gallons / hour << " - ok displays gal/hour" << std::endl; std::cout << C << " - ok displays gal/hour" << std::endl ;
return 0; }
OK - but now how would I output in meters^3/second ? It seems that the output is coupled to the way the unit is setup. I would have expected that the format of the output display could be delayed until the actual output operation is performed - just like i can on input. that is, Since I can do (using si) consumption_rate cr = static_cast<consumption_rate>( 24.0 * gallons / hour) I expect to be able to do some similar operation equivalent to static_cast<gallons/hour>(r) - OK I suppose that some sort of symetical operation is not implementable. But on the other hand, the following does work cout << 24.0 gallons / hour. Thanks for your help. Robert Ramey