The following code example is taken from the book
C++ Templates - The Complete Guide, 2nd Edition
by David Vandevoorde, Nicolai M. Josuttis, and Douglas Gregor,
Addison-Wesley, 2017
© Copyright David Vandevoorde, Nicolai M. Josuttis, Douglas Gregor 2017
// adding two durations where unit type might differ:
template<typename T1, typename U1, typename T2, typename U2>
auto constexpr operator+(Duration<T1, U1> const& lhs,
Duration<T2, U2> const& rhs)
{
// resulting type is a unit with 1 a nominator and
// the resulting denominator of adding both unit type fractions
using VT = Ratio<1,RatioAdd<U1,U2>::den>;
// resulting value is the sum of both values
// converted to the resulting unit type:
auto val = lhs.value() * VT::den / U1::den * U1::num +
rhs.value() * VT::den / U2::den * U2::num;
return Duration<decltype(val), VT>(val);
}