Kahan summation

#include <iostream>

// Kahan summation algorithm
template<class T>
struct accum
{
    T t, y;
    
    accum(T val = 0): t(val), y() {}
    
    void operator()(T val)
    {
        y -= val;
        val = t - y;
        y += (val - t);
        t = val;
    }
};

int main(int argc, char** argv)
{
    accum<float> sum(6);
    sum(1);
    sum(2);
    sum(3);
    std::cout << sum.t;
    return 0;
}