std::ratio_subtract

template <typename R1, typename R2>
using ratio_subtract = std::ratio < R1::num*R2::den-R2::num*R1::den, R1::den*R2::den >;

// ratio_subtract example
#include <iostream>
#include <ratio>

int main ()
{
  typedef std::ratio<2,3> two_thirds;
  typedef std::ratio<1,2> one_half;

  typedef std::ratio_subtract<two_thirds,one_half> diff;

  std::cout << "diff = " << diff::num << "/" << diff::den;
  std::cout << " (which is: " << ( double(diff::num) / diff::den ) << ")" << std::endl;

  return 0;
}