Converts a double to an SI string. Converted to C++ from https://stackoverflow.com/a/12181661/719528.

to_si_string.cpp

// Unicode `μ` for micro (10e-6)
std::wstring to_si_wstring(double number, int precision)
{
  std::wostringstream ss;
  ss << std::setprecision(precision);

  if (number != 0.0)
  {
    static const std::array<wchar_t, 17> prefixes = { 'y', 'z', 'a', 'f', 'p', 'n', L'\u03bc', 'm', L'\u200b', 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y' };
    static const size_t zero_degree_index = 8;

    int degree = (int)floor(log10(fabs(number)) / 3);
    double scaled = number * pow(1000, -degree);

    ss << scaled << prefixes[degree + zero_degree_index];
  }
  else
  {
    ss << 0.0;
  }
  return ss.str();
}

// Non-Unicode `u` for micro (10e-6)
std::string to_si_string(double number, int precision)
{
  std::ostringstream ss;
  ss << std::setprecision(precision);

  if (number != 0.0)
  {
    static const std::array<char, 17> prefixes = { 'y', 'z', 'a', 'f', 'p', 'n', 'u', 'm', ' ', 'k', 'M', 'G', 'T', 'P', 'E', 'Z', 'Y' };
    static const size_t zero_degree_index = 8;

    int degree = (int)floor(log10(fabs(number)) / 3);
    double scaled = number * pow(1000, -degree);

    ss << scaled;
    if (degree != 0)
    {
      ss << prefixes[degree + zero_degree_index];
    }
  }
  else
  {
    ss << 0.0;
  }
  return ss.str();
}