Example for https://essential-cpp.programming-books.io/user-defined-literals-d3696e4f87894401af6c2f073d356228 (made with https://codeeval.dev)

main.cc

#include <iostream>

long double operator"" _km(long double val)
{
    return val * 1000.0;
}

long double operator"" _mi(long double val)
{
    return val * 1609.344;
}

int main()
{
    std::cout << "3 km = " << 3.0_km << " m\n";
    std::cout << "3 mi = " << 3.0_mi << " m\n";
    return 0;
}