How to detect and capture an image automatically when person is in front of the camera

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I am learning about face recognition.I do this using opencv, c++ language,visual studio 2015. I want to do when a person come in front of the camera,it capture the face automatically and save in database.how I done it. How it done. Please help me.

this is the code that I use to open the camera

#include <opencv2/opencv.hpp>
#include <opencv2/imgcodecs.hpp>
#include "opencv2/objdetect.hpp"
#include "opencv2/highgui.hpp"
#include "opencv2/imgproc.hpp"
#include <stdio.h>
using namespace std;
using namespace cv;

int main(int, char**)
{
          Mat img;

    VideoCapture cam(0);

    namedWindow("camera");

    while (1) {
        cam >> img;
        imshow("camera", img);

        if (waitKey(1) == 27)
        {
            break;
        }

    }

    cam.release();
    return 0;
}