How to drawline in a QWidget or any onther?

I want to drawline in a QWidget or any other suitable controls.

Somebody tolds me can overload paintEvent().

1.If I use QT designer to create UI, I cannot drawline with this codes:

Code:

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void iLCDEmulator::paintEvent(QPaintEvent *)
{
    QPainter paint(ui.displayWidget); //ui.displayWidget is a QWidget.
    //If use "QPainter paint(this) " ,it will drawline in my main windows.
    paint.setPen(Qt::red); 
    paint.drawLine(0,0,50,50);
}

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2.If overload paintEvent(), this function will be called at the program statrs or "the widget was obscured and has now been uncovered".
I just want to drawline when I need. And I want to control the line's positon uncontrolled.

You must overload paintEvent() in class which you want to drawline.

Quote:

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Originally Posted by yangyunzhao

I just want to drawline when I need. And I want to control the line's positon uncontrolled.

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Could you elaborate on that?