Boost Units

One of the BoostLibraries

A brilliant example of the power of Static Typing. An extensible framework for compile-time dimensional analysis in C++ with no runtime overhead.

http://boost.org/libs/units

Example:

 /// the ideal gas law in si units
 template<class Y>
 quantity<si::amount,Y> 
 idealGasLaw(const quantity<si::pressure,Y>& P,
             const quantity<si::volume,Y>& V,
             const quantity<si::temperature,Y>& T)
 {
     using namespace boost::units::si;

     #if BOOST_UNITS_HAS_TYPEOF 
     using namespace constants::codata;
     return (P*V/(R*T));
     #else
     return P*V/(8.314472*(joules/(kelvin*mole))*T);
     #endif // BOOST_UNITS_HAS_TYPEOF
 }

Which is used as follows:

 /// test ideal gas law
 quantity<temperature>   T = (273.+37.)*kelvin;
 quantity<pressure>      P = 1.01325e5*pascals;
 quantity<length>        r = 0.5e-6*meters;
 quantity<volume>        V = (4.0/3.0)*3.141592*pow<3>(r);
 quantity<amount>        n(idealGasLaw(P,V,T));

Note that defining the base units is sufficient to use any plausible derived unit.