[Boost-users] [Units]

Subject: [Boost-users] [Units]
From: CD Clark (clifton.clark_at_[hidden])
Date: 2017-10-05 01:58:13

I am trying to create a set of units to use in a library, but I having
trouble getting IO to work for some of them. Here is a simple example that
demonstrates the problem.

============================

#include<iostream>

#include <boost/units/systems/si.hpp>
#include <boost/units/systems/si/io.hpp>

using namespace boost::units;

// short hand for meter
namespace t { typedef si::length m; }
namespace i { BOOST_UNITS_STATIC_CONSTANT( m, t::m ); }

// centimeter
namespace t { typedef make_scaled_unit< si::length, scale< 10,
static_rational<-2> > >::type cm; }
namespace i { BOOST_UNITS_STATIC_CONSTANT( cm, t::cm ); }

// meter squared
namespace t { typedef multiply_typeof_helper< m, m>::type m_m; }
namespace i { BOOST_UNITS_STATIC_CONSTANT( m_m, t::m_m ); }

// centimeter squared
namespace t { typedef multiply_typeof_helper< cm, cm>::type cm_cm; }
namespace i { BOOST_UNITS_STATIC_CONSTANT( cm_cm, t::cm_cm ); }

// meter centimeter
namespace t { typedef multiply_typeof_helper< m, cm>::type m_cm; }
namespace i { BOOST_UNITS_STATIC_CONSTANT( m_cm, t::m_cm ); }

// override default output, which prints "c(m^2)"
namespace boost { namespace units {
inline std::string name_string(const t::m_cm&)
{ return "meter centimeter"; }
inline std::string symbol_string(const t::m_cm&)
{ return "m cm"; }
} }

// trying to overload output for centimeter squared
namespace boost { namespace units {
inline std::string name_string(const t::cm_cm&)
{ return "centimeter squared"; }
inline std::string symbol_string(const t::cm_cm&)
{ return "cm^2"; }
} }

int main(int argc, char *argv[])
{
  std::cout << i::m << std::endl;
  std::cout << i::cm << std::endl;
  std::cout << i::m_m << std::endl;
  std::cout << i::m_cm << std::endl;
  //std::cout << i::cm_cm << std::endl; // this won't compile

  quantity<t::m_m> a1(100*i::cm_cm);
  std::cout << a1<< std::endl; // should be 0.01 m^2

  quantity<t::m_m> a2(100*i::m_cm);
  std::cout << a2<< std::endl; // should be 1 m^2

  return 0;
}

============================

Basically, I am just trying to create a convenient set of short hands. Unit
types are put in a 't' namespace and instances are put in a 'i' namespace
with the same name.

This works fine for doing calculations and conversions with the units, but
I am having trouble with getting output to work for some of them. In the
example above I create a centimeter unit by scaling the meter. Then I
create three different area units with the product_typeof_helper struct:
meter squared, meter centimeter, and centimeter squared. Conversions work
for all three of these, but output will not work for the centimeter squared
unit.

The program above outputs

m
cm
m^2
m cm
0.01 m^2
1 m^2

If I uncomment the line that tries to print i::cm_cm it won't compile. I
get an error that says

'symbol' is not a member of ...

and points at a line that says

str += Begin::item::symbol()

I can't understand why it doesn't work for a product of two scaled units.

cdclark