//QMyWidget.h

#pragma once

#include <QWidget>
class QPaintEvent;

class QMyWidget : public QWidget
{
    Q_OBJECT
protected:
    virtual void paintEvent ( QPaintEvent * );
};


//QMyWidget.cpp

#include <QPainter>

QMyWidget::QMyWidget(QWidget *parent):QWidget(parent)
{
}

void QMyWidget ::paintEvent( QPaintEvent * )
{
    QPainter P(this);

    P.  ... рисуем
}