The difference between the following codes are indicated by comments, ignore the layout arrangement.
This crashes to: "Access violation reading location 0xcdcdcdd1." in QPicture::play(QPainter *painter), the picture is null.
Qt Code: 
  1. QPrintPreviewWidget *testPreview_1 = new QPrintPreviewWidget();
  2.     QPrintPreviewWidget *testPreview_2 = new QPrintPreviewWidget();
  3.     QPushButton *btn_1 = new QPushButton("zoomIn_1");
  4.     QPushButton *btn_2 = new QPushButton("zoomIn_2");
  5.     //doing layouts
  6.  
  7.     connect(testPreview_2, SIGNAL(paintRequested(QPrinter*)), 
  8.         this, SLOT(testSlot(QPrinter*)));
  9.     connect(testPreview_1, SIGNAL(paintRequested(QPrinter*)),
  10.         this, SLOT(testSlot(QPrinter*)));
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I removed the second connection to render printer's contents, and it works well
Qt Code: 
  1. QPrintPreviewWidget *testPreview_1 = new QPrintPreviewWidget();
  2.     QPrintPreviewWidget *testPreview_2 = new QPrintPreviewWidget();
  3.     QPushButton *btn_1 = new QPushButton("zoomIn_1");
  4.     QPushButton *btn_2 = new QPushButton("zoomIn_2");
  5.     //doing layouts
  6.  
  7.     connect(testPreview_2, SIGNAL(paintRequested(QPrinter*)), 
  8.         this, SLOT(testSlot(QPrinter*)));
  9. //    connect(testPreview_1, SIGNAL(paintRequested(QPrinter*)),
  10. //       this, SLOT(testSlot(QPrinter*)));
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Or else, i removed the buttons, leave 2 print preview widget only, and it works well, too. 2 QPrintpreviewWidget are shown side by side.
Qt Code: 
  1. QPrintPreviewWidget *testPreview_1 = new QPrintPreviewWidget();
  2.     QPrintPreviewWidget *testPreview_2 = new QPrintPreviewWidget();
  3. //    QPushButton *btn_1 = new QPushButton("zoomIn_1");
  4. //    QPushButton *btn_2 = new QPushButton("zoomIn_2");
  5.     //doing layouts
  6.  
  7.     connect(testPreview_2, SIGNAL(paintRequested(QPrinter*)), 
  8.         this, SLOT(testSlot(QPrinter*)));
  9.     connect(testPreview_1, SIGNAL(paintRequested(QPrinter*)),
  10.        this, SLOT(testSlot(QPrinter*)));
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Who has ever came across this problem? is this a but of Qt?