QT C++ - Using MainWindow class QString in a thread class

Question

I'm trying to learn about threading, and the basics of C++ and QT. I have a mainWindow button method that will essentially run a thread, and store the value of a ui combobox text:

void MainWindow::on_Request_Raw_Button_clicked(bool checked)
{
  if(checked)
  {
      // Store the current request into var.
      curr_request = ui->comboBox_2->currentText();
      qDebug() << curr_request << " started.\n";

      mThread->start();
  }
  else
  {
      qDebug() << "Stopped.\n";
      mThread->Stop = true;
  }
}

And in the thread call, when it runs I want to use the data member in MainWindow , specifically curr_request.

test_thread::test_thread(QObject *parent) : QThread(parent)
{

}

void test_thread::run()
{
    this->Stop = false;
    QMutex mutex;
    while(true)
    {
        qDebug() << "I started.\n";
        if( this->Stop ) {
            break;
            mutex.unlock();
        }
        /* Do stuff here */
        qDebug() << "Test: " << curr_request;
        QString temp = curr_request;

        mutex.unlock();
        emit temp;

        this->usleep(900000);
    }
}

In my mainwindow.h

public:
  explicit MainWindow(QWidget *parent = 0);
  ~MainWindow();
  test_thread *mThread;
  QString curr_request = "";

My thread includes the mainwindow.h file. I get the error: Invalid use of non-static data member 'curr_request'.

Answer 1

Since curr_request is a member of your class MainWindow , you have to access it through an object of MainWindow , eg

MainWindow mainWindow;
mainWindow.curr_request = "something";

Or do something like this:

MainWindow* pMainWindow = QCoreApplication::instance()->findChild<MainWindow>();
QString temp = pMainWindow->curr_request;

Beware that this is possibly not thread-safe and may or may not lead to a race condition, depending on how you access the instance.

(Source: A somewhat similar question was has been asked here , where this possibility was commented).