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POUsing Boost.Units and Boost.Multiprecision
 

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  1. POUsing Boost.Units and Boost.Multiprecision
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    <p>I am attempting to write a molecular dynamics program, and I thought that Boost.Units was a logical choice for the variables, and I also decided that Boost.Multiprecision offered a better option than <code>double</code> or <code>long double</code> with respect to round off errors. A combination of the two seems fairly straight forward until I attempt to use a constant, then it breaks down.</p> <pre><code>#include &lt;boost/multiprecision/gmp.hpp&gt; #include &lt;boost/units/io.hpp&gt; #include &lt;boost/units/pow.hpp&gt; #include &lt;boost/units/quantity.hpp&gt; #include &lt;boost/units/systems/si.hpp&gt; #include &lt;boost/units/systems/si/codata/physico-chemical_constants.hpp&gt; namespace units = boost::units; namespace si = boost::si; namespace mp = boost::multiprecision; units::quantity&lt;si::mass, mp::mpf_float_50&gt; mass = 1.0 * si::kilogram; units::quantity&lt;si::temperature, mp::mpf_float_50&gt; temperature = 300. * si::kelvin; auto k_B = si::constants::codata::k_B; // Boltzmann constant units::quantity&lt;si::velocity, mp::mpf_float_50&gt; velocity = units::root&lt;2&gt;(temperature * k_B / mass); std::cout &lt;&lt; velocity &lt;&lt; std::endl; </code></pre> <p>The output will be <code>1 M S^-1</code>. If I use <code>long double</code> in lieu of <code>mp::mpf_float_50</code>, then the result is <code>2.87818e-11 m s^-1</code>. I know that the problem likes within the conversion between the constant and the other data because the constant defaults to a <code>double</code>. I have thought about creating my own Boltzmann constant, but I prefer to use the predefined value if possible.</p> <p>My question, therefore, is how do I go about using Boost.Multiprecision when I have predefined constants from Boost.Units? If I must concede to using <code>double</code> or <code>long double</code>, then I will, but I suspect that a way exists to convert or utilize the other on the constants.</p> <p>I am working with Mac OS X 10.7, Xcode 4.6.2, Clang 3.2, Boost 1.53.0 and the C++11 extensions.</p> <p>I appreciate any help that can be offered.</p>
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