#include <iostream>
#include <chrono>
#include <ratio>


// m/s - SI derived unit
struct Speed
{
    long double value;
};

// m - SI unit
template <class Ratio = std::ratio<1>>
struct Length
{
    unsigned long long value;
};

auto operator ""_m(unsigned long long n)  { return Length<>{n}; }

auto operator ""_km(unsigned long long n) { return Length<std::kilo>{n}; }

template <class Ratio, class Rep, class Period>
auto operator / (
    const Length<Ratio> &len,
    const std::chrono::duration<Rep, Period> &dur)
{
    const auto len_ratio = static_cast<double>(Ratio::num) / Ratio::den;
    const auto dur_sec = std::chrono::duration_cast<std::chrono::seconds>(dur);
    return Speed{len_ratio * len.value / dur_sec.count()};
}

int main()
{
    using namespace std::chrono_literals;
    const Speed s1 =      30_m  /     1s;
    const Speed s2 = 108'000_m  / 3'600s;
    const Speed s3 =     108_km / 3'600s;
    const Speed s4 =     108_km /     1h;
    std::cout << "s1 =      30_m  /     1s == " << s1.value << " m/s\n";
    std::cout << "s2 = 108'000_m  / 3'600s == " << s2.value << " m/s\n";
    std::cout << "s3 =     108_km / 3'600s == " << s3.value << " m/s\n";
    std::cout << "s4 =     108_km /     1h == " << s4.value << " m/s\n";
    return 0;
}