How can I draw an ellipse using QPainter?

I want to draw ellipse in UI but my code doesn't work.

QWidget::paintEngine: Should no longer be called QPainter::begin: Paint device returned engine == 0, type: 1

Mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
 : QMainWindow(parent)
 , ui(new Ui::MainWindow), pm(100,100)
{
 ui->setupUi(this);
 //set_form_style();
 draw_ellipse();
}

MainWindow::~MainWindow()
{
 delete ui;
}

void MainWindow::set_form_style(){
 //setWindowFlags(Qt::FramelessWindowHint);
 //setAttribute(Qt::WA_TranslucentBackground);
 //ui->widget->setStyleSheet("background:transparent;");

 setMouseTracking(true);

}

void MainWindow::draw_ellipse(){
 QPainter painter(this);
 painter.setRenderHint(QPainter::Antialiasing);
 painter.setBrush(QBrush(Qt::red, Qt::SolidPattern));
 painter.drawEllipse(100, 50, 150, 150);
}

The issue isn't the way you're using the QPainter, it's the time you're obtaining it.

As the reference documentation says, "the common use of QPainter is inside a widget's paint event". So if you want to do custom painting in your main window, override paintEvent and put your code there.