here is the code below:
Qt Code: 
  1. const char *filename="C://OpenCV2.2//data//haarcascades//haarcascade_frontalface_alt.xml";
  2.     cascade = ( CvHaarClassifierCascade* )cvLoad(filename,0,0,0);
  3.     if( !cascade )
  4.         {
  5.             fprintf( stderr, "ERROR: Could not load classifier cascade\n" );
  6.             return -1;
  7.         }
  8.     storage=cvCreateMemStorage(0);
  9.     capture=cvCaptureFromCAM(0);
  10.    // assert( cascade && storage && capture );
  11.     IplImage* frame= cvQueryFrame(capture);
  12.     m_i=QImage(QSize(frame->width,frame->height),QImage::Format_RGB888);
  13.     ui->frame->setMinimumSize(m_i.width(),m_i.height());
  14.     ui->frame->setMaximumSize(ui->frame->minimumSize());
  15.  
  16.     m_opencvimg=cvCreateImageHeader(cvSize(m_i.width(),m_i.height()),8,3);
  17.     m_opencvimg->imageData=(char*)m_i.bits();
  18.  
  19.     if(frame->origin== IPL_ORIGIN_TL)
  20.         cvCopy(frame,m_opencvimg,0);
  21.     else
  22.         cvFlip(frame,m_opencvimg,0);
  23.  
  24.     cvCvtColor(m_opencvimg,m_opencvimg,CV_BGR2RGB);
  25.  
  26.     m_timer=new QTimer(this);
  27.     connect(m_timer,SIGNAL(timeout()),this,SLOT(queryFrame()));
  28.     m_timer->start(50);
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Error's i was getting while compiling this code:

C:/Users/mind_freak/facedetect-build-desktop/../facedetect/widget.cpp:15: error: returning a value from a constructor

i dont know i have given the correct path to the xml file but unable to load.

help me out of this....i am stuck with this....

thanks..