boost::thread sample application

#include <iostream>
#include <boost/timer.hpp>
#include <boost/thread.hpp>

using namespace std;

class Calc
{
private:
    double *a;
    int niter;
    int start;
    int end;
    double sum;
public:
    Calc(double *a_ptr, const int &niter_ref, const int &start_ref, const int &end_ref)
    {
        a = a_ptr;
        niter = niter_ref;
        start = start_ref;
        end = end_ref;
        sum = 0.0;
    }
    void run()
    {
        double x;
        for (int i = start; i < end; i++)
        {
            for (int j = 0; j < niter; j++)
            {
                for (int k = 0; k < niter; k++)
                {
                    x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                    sum += x;
                }
            }
        }
    }
    double Sum()
    {
        return sum;
    }
};

double single_thread(double a[], const int &niter)
{
    double sum = 0.0;
    double x;
    for (int i = 0; i < niter; i++)
    {
        for (int j = 0; j < niter; j++)
        {
            for (int k = 0; k < niter; k++)
            {
                x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                sum += x;
            }
        }
    }
    return sum;
}

int main()
{
    boost::timer t;
    const int niter = 2000;
    double a [niter];
    double sum = 0;
    for (long i = 0; i < niter; i++)
    {
        a[i] = (double)i / (double)niter;
    }

    t.restart();
    sum = single_thread(a, niter);
    cout << "Serial. Sum: " << sum << " time: " << t.elapsed() << endl;

    Calc c1(a, niter, 0, niter / 2);
    Calc c2(a, niter, niter / 2, niter);

    t.restart();
    boost::thread calculationThread1(boost::bind( &Calc::run, &c1 ));
    boost::thread calculationThread2(boost::bind( &Calc::run, &c2 ));
    calculationThread1.join();
    calculationThread2.join();
    cout << "Two boost threads. Sum: " << c1.Sum() + c2.Sum() << " time: " << t.elapsed() << endl;
    return 0;
}