OpenCv and Visual C++ Face detection

edit

I recently came across this video which shows how to write a face recognition program and i tried using his codes to study how the program works. He indicated to change one of the lines and i followed as he said. I tried changing the line but it sill showed this error.

Assertion failed (!empty()) in cv::CascadeClassifier::detectMultiScale and
cv::Exception at memory location 0x000000000029EE10

Unhandled exception at 0x000007FEFD5CA06D in opencvTry.exe: Microsoft C++ exception: cv::Exception at memory location 0x000000000029EE10. occurred

My program - try.cpp:

#include <opencv2/core.hpp>
#include <opencv2/highgui.hpp>
#include <opencv2/videoio.hpp>
#include <opencv2/objdetect.hpp>
#include <opencv2/imgproc.hpp>
#include <iostream>

using namespace cv;
using namespace std;


int main(int argc, char** argv)
{
    // capture from web camera init

    VideoCapture cap(0);
    cap.open(0);

    Mat img;

    // Initialize the inbuilt Harr Cascade frontal face detection
    // Below mention the path of where your haarcascade_frontalface_alt2.xml file is located

    CascadeClassifier face_cascade;
    face_cascade.load("C:\OpenCV\sources\data\haarcascades\haarcascade_frontalface_alt2.xml");    
    // i tried changing this line to match my folder in C Drive

    for (;;)
    {

        // Image from camera to Mat

        cap >> img;

        // obtain input image from source
        cap.retrieve(img, CV_CAP_OPENNI_BGR_IMAGE);

        // Just resize input image if you want
        resize(img, img, Size(1000, 640));

        // Container of faces
        vector<Rect> faces;


        // Detect faces
        face_cascade.detectMultiScale(img, faces, 1.1, 2, 0 | CV_HAAR_SCALE_IMAGE, Size(140, 140));
        // error message appears here


        //Show the results
        // Draw circles on the detected faces

        for (int i = 0; i < faces.size(); i++)
        {
            Point center(faces[i].x + faces[i].width*0.5, faces[i].y + faces[i].height*0.5);
            ellipse(img, center, Size(faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar(255, 0, 255), 4, 8, 0);
        }

        // To draw rectangles around detected faces
        /* for (unsigned i = 0; i<faces.size(); i++)
        rectangle(img,faces[i], Scalar(255, 0, 0), 2, 1);*/


        imshow("wooohooo", img);
        int key2 = waitKey(20);

    }
    return 0;
}