ASSERT failure in QCoreApplication::sendEvent: 'Cannot send events to objects owened by a different thread

[Domenico]

hello to the whole community Qt Forum,
I need a help I can not understand the problem that is reporting in my code .. as the codes of my application is too long .. I bring here a similar code and I know that the error comes from there .. I can you tell what's wrong? thank you very much to those who will help me

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

#include "afxwin.h"
#include <iostream>
#include <thread>
#include <QDebug>

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

    static void MainThread(LPVOID lp);

public slots:
    void stopLoop();

private:
    Ui::MainWindow *ui;
    std::thread t;
};

#endif // MAINWINDOW_H

MainWindow.cpp

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

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);
    t = std::thread(MainThread,(void*)this);
}

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

void MainWindow::MainThread(LPVOID lp)
{
    MainWindow *parent = (MainWindow*)lp;

    qDebug()<<"Hello Concurrent World\n";
    parent->stopLoop();
}

void MainWindow::stopLoop()
{
    t.detach();
    ui->pushButton->setEnabled(false);
}

main.cpp

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


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

   return a.exec();
}

the error that brings me back is:

what's wrong?

[Added code tags ~kshegunov]

[Domenico]

@kshegunov

Thank you very much!

I simply replaced

QMetaObject:invokeMethod(parent,"stopLoop",Qt::QueueConnection);

on

parent->stopLoop();

and the problem is solved!

[kshegunov]

This:

ui->pushButton->setEnabled(false);

is a race condition. You mustn't touch any UI object from a thread different than the GUI one. As a matter of fact, you shouldn't execute methods of GUI classes in different threads, that's basically asking for trouble.

[Domenico]

@kshegunov

Thanks for the reply! so how can I disable the pushButton?

[kshegunov]

Start reading here. There are also examples throughout the documentation how threads are to interact with the GUI (and in the forum as well).

In a nutshell you have to emit a signal that's connected to a slot, which in turn is going to disable the button. Before doing that, however, you need a QObject that's going to emit it, and which is living in the worker thread.

PS. Here's some skeleton code how to spin a thread Qt-style.

[Domenico]

@kshegunov

Thank you very much!

I simply replaced

QMetaObject:invokeMethod(parent,"stopLoop",Qt::QueueConnection);

on

parent->stopLoop();

and the problem is solved!

[SGaist]

Hi,

Except that it's the wrong thing to do. You are currently just lucky it works. As already stated, don't modify GUI elements from another thread.

Sorry, I misunderstood what you wrote. invokeMethod is a solution.

However, you should still consider clean separation between UI updates and thread processing.