Naive standard deviation calculation

stddevcalc.cpp

class StdDevCalc {
    private:
        long long m_count;
        double m_sum_sq, m_sum, m_var;
    public:
        StdDevCalc() {
            m_sum_sq = m_sum = m_var = m_count = 0;
        }

        void append(double d) {
            m_count++;
            m_sum_sq += d * d;
            m_sum += d;
            if (m_count > 1) 
                m_var = (m_sum_sq - m_sum * m_sum / m_count) / (m_count - 1);
        }

        double get_std_dev() {
            return sqrt(m_var);
        }
};