class QWidgetChild : public QWidget
{
	Q_OBJECT
public:	
	QWidgetChild(QWidget *parent = 0) : QWidget(parent) {}
	~QWidgetChild() {}
}

MainWindow::MainWindow(QWidget *parent) 
	: QMainWindow(parent)
{
	QWidget *w1 = new QWidget(this);
	w1->setStyleSheet("background-color: red;");
	QWidgetChild *w2 = new QWidgetChild(this);
	w2->setStyleSheet("background-color: green;");
}