Creating user-defined conversions

Question

I'm trying to create units for meter and kilometer. I want then to sum and convert them accordingly. I know that the boost::units library has already SI system, but I want create all from scratch, because then I need to create my own systems for my projects (so I'm doing this to learn). My purpose is to declare a "Length" variable that can be modified using units: for example I want to write

Length xLength1 = 5350 * Meters + 2 Kilometers;

For this purpose, I've created the length.h file, containing definitions of meter and kilometer, and at the end I declare the conversion between these two units:

#ifndef LENGTH_H_
#define LENGTH_H_

#include <boost/units/base_dimension.hpp>
#include <boost/units/base_unit.hpp>
#include <boost/units/scaled_base_unit.hpp>
#include <boost/units/quantity.hpp>
#include <boost/units/conversion.hpp>

struct LengthBaseDimension : boost::units::base_dimension<LengthBaseDimension,1>{};

typedef LengthBaseDimension::dimension_type LengthDimension;

struct MeterBaseUnit : boost::units::base_unit<MeterBaseUnit, LengthDimension, 1>{};
template <> struct boost::units::base_unit_info<MeterBaseUnit>
{
  static std::string name()   { return "meter"; }
  static std::string symbol() { return "m";     }
};

struct KilometerBaseUnit : boost::units::base_unit<KilometerBaseUnit, LengthDimension, 2>{};
template <> struct boost::units::base_unit_info<KilometerBaseUnit>
{
  static std::string name()   { return "kilometer"; }
  static std::string symbol() { return "km";        }
};

// I don't want to use scaled_unit because I need this for my real purpose
BOOST_UNITS_DEFINE_CONVERSION_FACTOR(KilometerBaseUnit, MeterBaseUnit, double, 1000.0);

#endif

Then I create the file units.h in which I define my own unit system

#ifndef LIB_UNITS_H_
#define LIB_UNITS_H_

#include "length.h"
#include <boost/units/unit.hpp>
#include <boost/units/static_constant.hpp>
#include <boost/units/make_system.hpp>
#include <boost/units/io.hpp>

typedef boost::units::make_system<MeterBaseUnit>::type UnitsSystem;

typedef boost::units::unit<boost::units::dimensionless_type, UnitsSystem> Dimensionless;
typedef boost::units::unit<LengthDimension                 , UnitsSystem> SystemLength;


BOOST_UNITS_STATIC_CONSTANT(Kilometer  , SystemLength);
BOOST_UNITS_STATIC_CONSTANT(Kilometers , SystemLength);
BOOST_UNITS_STATIC_CONSTANT(Meter      , SystemLength);
BOOST_UNITS_STATIC_CONSTANT(Meters     , SystemLength);

// Typedefs of dimensions
typedef boost::units::quantity<SystemLength> Length;

#endif

At least, I use this header in my main function

#include "units.h"
#include <iostream>

int main(void)
{
  Length xLength1 ( 300.0 * Meters);
  Length xLength2 (1500.0 * Kilometer );
  Length xLength3;
  std::cout << xLength2 << std::endl;
  xLength3 = xLength1 + xLength2;

  return 0;
}

This projects compiles, but It does not do what I want. When I print the variable xLength2, I obtain 1500 m instead of 1500 km or 1500000 m. The sum is also wrong, because I obrain 1800 m. It's like I consider kilometers as meters and conversion does not work.

What I'm doing wrong?

Defiant

You define Kilometer to be the same as Meter. I don't recall offhand what is the preferred way to define Kilometer (perhaps there is a different macro), but you can try something like const Length Kilometer = 1000 * Meters; (shuffling the order of definitions). – Commonplace

But in this case I can't write and take value in kilometers, but only in meters. I want the possibility to work with both units, and use both values where needed. – Defiant

Just a heads up, if you're using MSVC++ or the gcc, you can replace your #IFDEF LENGTH_H_ etc guards with #pragma once at the top of each header. – Jollification

Apparently what you want is a "variant" type of boost quantity. There are some clues here: boost.2283326.n4.nabble.com/… – Dandy

Do you "have" to use boost? There are simpler ways in C++ to do this. :) – Rewrite

Answer 1

Unless you're project is to replicate the Boost library, this seems like an insane way to code a unit system. Template hackery is rarely the best option.

Instead, use a class! Not real code below:

class temperature{
   double temp; //always in kelvin
   temperature(double temperature, char unit){
       if(unit=='k')
                temp=temperature
       if(unit=='c')
                temp=temperature+273
    }
 };

And so on to implement conversion back to whatever unit you want. Basically you pick a base unit that everything is stored as, and convert to and from that. If you're doing SI, perhaps grams, meters, seconds, kelvin, etc.

Ranite

This is not Boost.Units way, thus doesn't answer the question. – Leshia