用户定义的文字，具有长双精度值

#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;
}

该程序的输出如下：

3 km = 3000 m
3 mi = 4828.03 m