Unable to identify solution to compiler error using Boost Units

Question

Boost Units version: 1.45

Compiler: Visual Studio 2008

Platform: Windows 7

Error: When compiling the following code I get an error C2440 "initializing' : cannot convert from 'boost::units::quantity' to 'boost::units::quantity' with

[ Unit=boost::units::unit>,boost::units::detail::static_power_impl<0>::apply>::type,boost::units::hogogeneous_system>>>>, Y = double ]

and

[ Unit=myproject::types::myproject_length ]

Constructor for class 'boost::units::quantity' is declared 'explicit' with

[ Unit=myproject::types::myproject_length ]

I am not sure what is wrong here. If I remove the "radii_t result =" from the following code and comment out the IO statement I get the same error as previously quoted above. Below is the code I used. I appreciate your patience as I learn Boost Units.

Q1: What is the reason for this error? What in the error output helped you to figure that out.

Q2: Even if the above error did not exist you mentioned that it would fail the dimensional analysis. I think we both agree the type would be radii^-1. How do you declare a type to be raised to the correct power?

Stephen

#include <boost/units/base_unit.hpp>
#include <boost/units/base_units/angle/radian.hpp>
#include <boost/units/io.hpp>
#include <boost/units/make_system.hpp>
#include <boost/units/physical_dimensions/length.hpp>
#include <boost/units/physical_dimensions/time.hpp>
#include <boost/units/static_constant.hpp>

#include <iostream>

namespace myproject {
 namespace types {

   //-------------------------------------
   //          Base dimensions
   //-------------------------------------
   struct length_base_dimension : public boost::units::base_dimension<length_base_dimension,1> {};

   //-------------------------------------
   //          Dimensions
   //-------------------------------------

   struct radii_base_unit : public boost::units::base_unit<radii_base_unit, boost::units::length_dimension, 1>
   {
     static std::string name()   { return("radii"); }
     static std::string symbol() { return("r"); }
   };

   struct minute_base_unit : public boost::units::base_unit<minute_base_unit,boost::units::time_dimension,3>
   {
     static std::string name() { return ("minute"); }
     static std::string symbol() { return ("min"); }
   };

typedef boost::units::make_system<radii_base_unit,
                                 minute_base_unit,
                                 boost::units::angle::radian_base_unit >::type myproject_system_t;

   typedef boost::units::unit<boost::units::length_dimension,myproject_system_t> myproject_length;

   typedef boost::units::quantity<myproject_length, double> radii_t;

   BOOST_UNITS_STATIC_CONSTANT(radii,myproject_length);
 }
}

int main ( int, char** )
{
 using namespace myproject::types;

 radii_t val1 ( 5 * radii );
 radii_t val2 ( 3 * radii );

 radii_t result =  1.0 / ( val1 - val2 );

 std::cout << result << std::endl;

 return 0;
}

Answer 1

1. The core problem is that your compiler message is truncated. In gcc, it reads:

54:40: error: conversion from ‘boost::units::divide_typeof_helper<double, boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<myproject::types::radii_base_unit, boost::units::list<myproject::types::minute_base_unit, boost::units::dimensionless_type> > > > >, double> >::type {aka boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension, boost::units::static_rational<-0x00000000000000001l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<myproject::types::radii_base_unit, boost::units::list<myproject::types::minute_base_unit, boost::units::dimensionless_type> > > >, void>, double>}’ to non-scalar type ‘myproject::types::radii_t {aka boost::units::quantity<boost::units::unit<boost::units::list<boost::units::dim<boost::units::length_base_dimension, boost::units::static_rational<1l> >, boost::units::dimensionless_type>, boost::units::homogeneous_system<boost::units::list<boost::units::angle::radian_base_unit, boost::units::list<myproject::types::radii_base_unit, boost::units::list<myproject::types::minute_base_unit, boost::units::dimensionless_type> > > > >, double>}’ requested

However, in general boost-units compiler errors are a little ... verbose. Normally, I don't try to understand them, but rather use boost.units to verify that my units are correct, and if they aren't, have a long hard look at the code to determine what's wrong. In your case, you could compare the aka types and see that the signs for the length base dimensions differ.

For Q2, boost::units::power_typeof_helper is your friend, as well as the other type helpers.