Thank you wysota for your reply.

But could you please be more specific?

Someone suggested me with the following code:
Qt Code: 
  1. int Dlg::exec()
  2. {
  3.     functionToBeCalled();
  4.     return QDialog::exec();
  5. }
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but the problem is that, my functionToBeCalled() is a function that does some animation (inside a QGraphicsView), so if we call this function before the exec() function, the user will not see the animation.
I have got an idea but could not make it work. In the following code, I re-implement completely the exec() function, by just copying the source code of QDialog::exec() and adding functionToBeCalled() after show():
Qt Code: 
  1. int Dlg::exec()
  2. {
  3.     Q_D(QDialog);
  4.  
  5.     if (d->eventLoop) {
  6.         qWarning("QDialog::exec: Recursive call detected");
  7.         return -1;
  8.     }
  9.  
  10.     bool deleteOnClose = testAttribute(Qt::WA_DeleteOnClose);
  11.     setAttribute(Qt::WA_DeleteOnClose, false);
  12.  
  13.     d->resetModalitySetByOpen();
  14.  
  15.     bool wasShowModal = testAttribute(Qt::WA_ShowModal);
  16.     setAttribute(Qt::WA_ShowModal, true);
  17.     setResult(0);
  18.  
  19. //On Windows Mobile we create an empty menu to hide the current menu
  20. #ifdef Q_WS_WINCE_WM
  21. #ifndef QT_NO_MENUBAR
  22.     QMenuBar *menuBar = 0;
  23.     if (!findChild<QMenuBar *>())
  24.         menuBar = new QMenuBar(this);
  25.     if (qt_wince_is_smartphone()) {
  26.         QAction *doneAction = new QAction(tr("Done"), this);
  27.         menuBar->setDefaultAction(doneAction);
  28.         connect(doneAction, SIGNAL(triggered()), this, SLOT(_q_doneAction()));
  29.     }
  30. #endif //QT_NO_MENUBAR
  31. #endif //Q_WS_WINCE_WM
  32.  
  33.     bool showSystemDialogFullScreen = false;
  34. #ifdef Q_OS_SYMBIAN
  35.     if (qobject_cast<QFileDialog *>(this) || qobject_cast<QFontDialog *>(this) ||
  36.         qobject_cast<QWizard *>(this)) {
  37.         showSystemDialogFullScreen = true;
  38.     }
  39. #endif // Q_OS_SYMBIAN
  40.  
  41.     if (showSystemDialogFullScreen) {
  42.         setWindowFlags(windowFlags() | Qt::WindowSoftkeysVisibleHint);
  43.         setWindowState(Qt::WindowFullScreen);
  44.     }
  45.     show();
  46.  
  47. /****** MY ANIMATION HERE *******/
  48. functionToBeCalled(); 
  49.  
  50. #ifdef Q_WS_MAC
  51.     d->mac_nativeDialogModalHelp();
  52. #endif
  53.  
  54.     QEventLoop eventLoop;
  55.     d->eventLoop = &eventLoop;
  56.     QPointer<QDialog> guard = this;
  57.     (void) eventLoop.exec(QEventLoop::DialogExec);
  58.     if (guard.isNull())
  59.         return QDialog::Rejected;
  60.     d->eventLoop = 0;
  61.  
  62.     setAttribute(Qt::WA_ShowModal, wasShowModal);
  63.  
  64.     int res = result();
  65.     if (deleteOnClose)
  66.         delete this;
  67. #ifdef Q_WS_WINCE_WM
  68. #ifndef QT_NO_MENUBAR
  69.     else if (menuBar)
  70.         delete menuBar;
  71. #endif //QT_NO_MENUBAR
  72. #endif //Q_WS_WINCE_WM
  73.     return res;
  74. }
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The first error I got is _'QDialog::d_func' : cannot access private member declared in class 'QDialog'_.
Hope somebody can help.
Have a nice day !