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Runtime Conversion Factor Example

(runtime_conversion_factor.cpp)

The Boost.Units library does not require that the conversion factors be compile time constants, as is demonstrated in this example:

using boost::units::base_dimension;
using boost::units::base_unit;

static const long currency_base = 1;

struct currency_base_dimension : base_dimension<currency_base_dimension, 1> {};

typedef currency_base_dimension::dimension_type currency_type;

template<long N>
struct currency_base_unit :
    base_unit<currency_base_unit<N>, currency_type, currency_base + N> {};

typedef currency_base_unit<0> us_dollar_base_unit;
typedef currency_base_unit<1> euro_base_unit;

typedef us_dollar_base_unit::unit_type us_dollar;
typedef euro_base_unit::unit_type euro;

// an array of all possible conversions
double conversion_factors[2][2] = {
    {1.0, 1.0},
    {1.0, 1.0}
};

double get_conversion_factor(long from, long to) {
    return(conversion_factors[from][to]);
}

void set_conversion_factor(long from, long to, double value) {
    conversion_factors[from][to] = value;
    conversion_factors[to][from] = 1.0 / value;
}

BOOST_UNITS_DEFINE_CONVERSION_FACTOR_TEMPLATE((long N1)(long N2),
    currency_base_unit<N1>,
    currency_base_unit<N2>,
    double, get_conversion_factor(N1, N2));


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