///  Get the mass centers:
vector<Point2f> mc( contours.size() );
for( int i = 0; i < contours.size(); i++ )
    { mc[i] = Point2f( mu[i].m10/mu[i].m00 , mu[i].m01/mu[i].m00 ); }

#include "opencv2opencv.hpp"
#include <vector>

using namespace std;
using namespace cv;

int main()
{

    Mat1b gray = imread("path_to_image", IMREAD_GRAYSCALE);

    Moments mu = moments(gray, true);
    Point center;
    center.x = mu.m10 / mu.m00;
    center.y = mu.m01 / mu.m00;

    Mat3b res;
    cvtColor(gray, res, CV_GRAY2BGR);

    circle(res, center, 3, Scalar(0,0,255));

    imshow("Result", res);
    waitKey();

    return 0;
}