error jumps :( QPainter::begin: Paint device returned engine == 0, type: 1

timob256

I am trying to draw with the mouse, but why does the error jump out?

Here is the whole code:

mainwindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
}

MainWindow::~MainWindow()
{

}

void MainWindow::mousePressEvent(QMouseEvent *event)
{
    QPainter painter(this); // Создаём объект отрисовщика

     float rad = 1;

     QPointF pt = event->localPos(); // event->Pos() и так и так пробовал
     QPen pen_abris(Qt::black, 2, Qt::SolidLine, Qt::FlatCap); 

     painter.setPen(pen_abris);
     painter.drawEllipse(pt.x()-rad, pt.y()-rad, rad*2.0, rad*2.0);
     this->update();
 }

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QDebug>
#include <QPainter>
#include <QPen>
#include <QFont>
#include <QFontMetrics>
#include <QKeyEvent>
#include <QRegion>
#include <QPointF>
#include <QGraphicsView>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
     MainWindow(QWidget *parent = 0);
    ~MainWindow();

     QPoint pos; //координаты точки для рисования
protected:

     void mousePressEvent(QMouseEvent *event);
};
#endif // MAINWINDOW_H

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

here is the error

QWidget :: paintEngine: Should no longer be called QPainter :: begin: Paint device returned engine == 0, type: 1 QPainter :: setPen: Painter not active QWidget :: paintEngine: Should no longer be called QPainter :: begin: Paint device returned engine == 0, type: 1 QPainter :: setPen: Painter not active

Why doesn't it draw, I can't understand (I set the background)

jsulm

@timob256 You can only paint inside paintEvent, see https://doc.qt.io/qt-5/qpainter.html
"Warning: When the paintdevice is a widget, QPainter can only be used inside a paintEvent() function or in a function called by paintEvent().".
So, move the painting into paintEvent.