I got this piece of code :

Qt Code: 
  1. void MainWindow::wheelEvent(QWheelEvent* event) {
  2.  
  3.     //Get the position of the mouse before scaling, in scene coords
  4.     QPointF pointBeforeScale(ui->graphWidget->mapToScene(event->pos()));
  5.  
  6.     //Get the original screen centerpoint
  7.     QPointF screenCenter = GetCenter(); //CurrentCenterPoint; //(visRect.center());
  8.  
  9.     //Scale the view ie. do the zoom
  10.     double scaleFactor = 1.15; //How fast we zoom
  11.     if(event->delta() > 0) {
  12.         //Zoom in
  13.         ui->graphWidget->scale(scaleFactor, scaleFactor);
  14.     } else {
  15.         //Zooming out
  16.         ui->graphWidget->scale(1.0 / scaleFactor, 1.0 / scaleFactor);
  17.     }
  18.  
  19.     //Get the position after scaling, in scene coords
  20.     QPointF pointAfterScale(ui->graphWidget->mapToScene(event->pos()));
  21.  
  22.     //Get the offset of how the screen moved
  23.     QPointF offset = pointBeforeScale - pointAfterScale;
  24.  
  25.     //Adjust to the new center for correct zooming
  26.     QPointF newCenter = screenCenter + offset;
  27.     SetCenter(newCenter);
  28. }
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As you can see, the GraphicsView is ui->graphWidget . But it is a part of the mainwindow, and not built programattically. It is part of the ui form of the main window.