Re: [Boost-users] [units] conversion factor for non-base

Subject: Re: [Boost-users] [units] conversion factor for non-base
From: Matthias Schabel (boost_at_[hidden])
Date: 2010-04-23 18:26:59

> "The flow coefficient is Q/sqrt(dP), a volumetric flow/sqrt(pressure)."
> 
> The units, specifically, are:
> 
> Kv = (m3/h)/sqrt(bar)
> Cv = (usgal/min)/sqrt(psi)
> 

#include <boost/units/io.hpp>
#include <boost/units/pow.hpp>

#include <boost/units/base_units/metric/bar.hpp>
#include <boost/units/base_units/metric/hour.hpp>
#include <boost/units/base_units/metric/minute.hpp>

#include <boost/units/base_units/us/gallon.hpp>
#include <boost/units/base_units/us/inch.hpp>
#include <boost/units/base_units/us/pound_force.hpp>

#include <boost/units/systems/si/length.hpp>
#include <boost/units/systems/si/pressure.hpp>
#include <boost/units/systems/si/time.hpp>
#include <boost/units/systems/si/volume.hpp>

#include <iostream>

namespace boost {
        
        namespace units {
                
                namespace us {
                        
                        typedef pound_force_base_unit::unit_type	pound_force;
                        typedef inch_base_unit::unit_type			inch;
                        
                } // namespace us
                
        } // namespace units
        
} // namespace boost

using namespace boost::units;

typedef typeof(us::pound_force()/pow<2>(us::inch()))	psi;

typedef typeof(si::cubic_meter/metric::hour_base_unit::unit_type()/root<2>(metric::bar_base_unit::unit_type()))	Kv;
typedef typeof(us::gallon_base_unit::unit_type()/metric::minute_base_unit::unit_type()/root<2>(us::pound_force()/pow<2>(us::inch()))) Cv;

void f1(const quantity<Kv>& arg)
{
        std::cout << arg << std::endl;
}

int main(void)
{
        std::cout << 1.0*Kv() << std::endl;
        std::cout << 1.0*Cv() << std::endl;
        
        f1(1.0*Kv());
        f1(quantity<Kv>(1.0*Cv()));
}

gives

1 m^3 h^-1 bar^(-1/2)
1 lbf^(-1/2) gal in min^-1
1 m^3 h^-1 bar^(-1/2)
0.864978 m^3 h^-1 bar^(-1/2)

You need a compiler that supports typeof for this to work... If you want a function that will take any quantity that is dimensionally-equivalent to (volumetric flow/sqrt(pressure)) then you will have to template it something like this :

template<class Y,class System>
void f(const quantity<unit<Dim,System>,Y>& arg);

where you use derived_dimension to define the dimension list Dim for your units.

Matthias

If