paint central widget of a QMainWindow???

[jpn]

so for this do i need QStackedWidget ? or simply a obj. of QWidget n set it as central widget???

The central widget can be anything inheriting from QWidget, but I think Qt Designer restricts it to be a QWidget. Just create any custom widget, reimplement the paintEvent(), add corresponding widget in the Designer and promote it.

As I have said previously:

Override QWidget::paintEvent() for the widget you want to paint on and use QPainter to draw the line. For example, if you want to paint on the central widget in a main window, you must reimplement the paintEvent() for the central widget, not for the main window.

So, in another words.
WRONG: "void MyMainWindow::paintEvent(QPaintEvent* event)"
CORRECT: "void MyCentralWidget::paintEvent(QPaintEvent* event)"