cvQueryFrame returns null - c++

Question

i am trying to run this OpenCV tutorial code using CodeBlocks on windows :
#include <iostream>
#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <opencv2/objdetect/objdetect.hpp>
#include <opencv2/imgproc/imgproc.hpp>
using namespace std;
using namespace cv;
// Function Headers
void detectAndDisplay( Mat frame );
// Global variables
String face_cascade_name = "haarcascade_frontalface_alt.xml";
String eyes_cascade_name = "haarcascade_eye_tree_eyeglasses.xml";
CascadeClassifier face_cascade;
CascadeClassifier eyes_cascade;
string window_name = "Capture - Face detection";
RNG rng(12345);
// #function main
int main( int argc, const char** argv ){
CvCapture* capture;
Mat frame;
//-- 1. Load the cascades
if( !face_cascade.load( face_cascade_name ) ){ std::cout << "--(!)Error loading Face cascade\n"; return -1; };
if( !eyes_cascade.load( eyes_cascade_name ) ){ std::cout << "--(!)Error loading Eyes cascade\n"; return -1; };
//-- 2. Read the video stream
capture = cvCaptureFromCAM( 0 );
if( capture ){
while( true ){
frame = cvQueryFrame( capture );
//-- 3. Apply the classifier to the frame
if( !frame.empty() ){
detectAndDisplay( frame );
}
else{
std::cout << " --(!) No captured frame -- Break!"; break;
}
int c = waitKey(10);
if( (char)c == 'c' ) { break; }
}
}
return 0;
}
// #function detectAndDisplay
void detectAndDisplay( Mat frame ){
std::vector<Rect> faces;
Mat frame_gray;
cvtColor( frame, frame_gray, CV_BGR2GRAY );
equalizeHist( frame_gray, frame_gray );
//-- Detect faces
face_cascade.detectMultiScale( frame_gray, faces, 1.1, 2, 0|CV_HAAR_SCALE_IMAGE, Size(30, 30) );
for( size_t 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( frame, center, Size( faces[i].width*0.5, faces[i].height*0.5), 0, 0, 360, Scalar( 255, 0, 255 ), 4, 8, 0 );
Mat faceROI = frame_gray( faces[i] );
std::vector<Rect> eyes;
//-- In each face, detect eyes
eyes_cascade.detectMultiScale( faceROI, eyes, 1.1, 2, 0 |CV_HAAR_SCALE_IMAGE, Size(30, 30) );
for( size_t j = 0; j < eyes.size(); j++ ){
Point center( faces[i].x + eyes[j].x + eyes[j].width*0.5, faces[i].y + eyes[j].y + eyes[j].height*0.5 );
int radius = cvRound( (eyes[j].width + eyes[j].height)*0.25 );
circle( frame, center, radius, Scalar( 255, 0, 0 ), 4, 8, 0 );
}
}
//-- Show what you got
imshow( window_name, frame );
}
But the Output is " --(!) No captured frame -- Break!"
During the execution the camera flash blinks then the output is displayed.