How to share data between threads using QThread

n34rt

I'm trying to learn how to share data between threads using QThread.

In my code below
when I try to print the value of worker_str sometimes i get an exception in the file qdebug.cpp:

Exception thrown: read access violation.
**p** was 0x111011101110111.

and sometimes it prints worker_str as "".

What I'm doing wrong? also, I should call thread->quit(); whenever I don't need that thread running anymore?

#include "worker.h"

App::App(QWidget *parent)
    : QMainWindow(parent)
{
    ui.setupUi(this);


    QThread* thread = new QThread();
    Worker* worker = new Worker();
    worker->moveToThread(thread);

    QString str = "hello world";

    connect(thread, &QThread::started, worker, [worker, str]
    {
        worker->test(str);
    });

    connect(worker, &Worker::finished, thread, [worker, thread]
    {
        qDebug() << "worker_str: " <<  worker->worker_str;

        thread->quit();
    });    

    connect( worker, &Worker::finished, worker, &Worker::deleteLater);
    connect( thread, &QThread::finished, thread, &QThread::deleteLater);
    thread->start();
}

//worker.h
class Worker : public QObject {
    Q_OBJECT
public:
    Worker() {};
    ~Worker() {};

    QString worker_str;

public slots:
    void test(QString str)
    {
        worker_str = str;
        qDebug() << "WORKER: str: " << str;
        emit finished();
    };

signals:
    void finished();

};

Christian Ehrlicher

Your Worker object is already gone when you try to access it in your slot. You can see this when using a debugger and setting a breakpoint in the lambda and the dtor of Worker or use a memory checker tool like e.g. valgrind.

class Worker : public QObject
{
  Q_OBJECT
public:
    ~Worker()
    {
      qWarning() << "DTOR" << this;
    }
    void test(QString str)
    {
        worker_str = str;
        qDebug() << "WORKER: str: " << str << this;
        emit finished();
    }
    QString worker_str;
signals:
    void finished();
};

class App : public QMainWindow
{
  Q_OBJECT
public:
  App(QWidget *parent = nullptr) : QMainWindow(parent)
  {
    QThread* thread = new QThread();
    Worker* worker = new Worker();
    worker->moveToThread(thread);

    QString str = "hello world";
    connect(thread, &QThread::started, worker, [worker, str]
    {
        worker->test(str);
    });
    connect(worker, &Worker::finished, thread, [worker, thread]
    {
        qWarning() << "worker_str: " <<  worker->worker_str;
        thread->quit();
    });
    connect(worker, &Worker::finished, worker, &Worker::deleteLater);
    connect(thread, &QThread::finished, thread, &QThread::deleteLater);
    connect(thread, &QThread::finished, thread, &QCoreApplication::quit);
    thread->start();
  }
};

int main(int argc, char *argv[])
{
  QApplication a(argc, argv);
  App app;
  return a.exec();
}

#include "main.moc"

n34rt

@Christian-Ehrlicher I get the same exception when trying your code

Your Worker object is already gone when you try to access it in your slot.

Could help pointing the proper way to read it then?

Christian Ehrlicher

@n34rt said in How to share data between threads using QThread:

Could help pointing the proper way to read it then?

Either pass the value poer signal/slot out of the thread or don't delete the worker in an own connect() but in the readout function.

n34rt

@Christian-Ehrlicher

or don't delete the worker in an own connect() but in the readout function.

connect(worker, &Worker::finished, thread, [worker, thread]
{

    qWarning() << "worker_str: " <<  worker->worker_str;
    thread->quit();
    thread->deleteLater();
    worker->deleteLater();
    connect(thread, &QThread::finished, thread, &QCoreApplication::quit);
});

like this?

Is this line really necessary?

connect(thread, &QThread::finished, thread, &QCoreApplication::quit);

I mean, do i need to call QCoreApplication::quit for that thread?

Christian Ehrlicher

@n34rt said in How to share data between threads using QThread:

I mean, do i need to call QCoreApplication::quit for that thread?

It's needed to properly end my testcase.

n34rt

@Christian-Ehrlicher I'm asking if I do need to call it whenever I start a thread and don't need it anymore

Christian Ehrlicher

@n34rt said in How to share data between threads using QThread:

I'm asking if I do need to call it whenever I start a thread and don't need it anymore

If you want to quit your application after your thread finished yes - but I doubt so.

n34rt

@Christian-Ehrlicher I see thank you, sorry im a newbie about threading

Either pass the value poer signal/slot out of the thread

Could you point how is it?

jsulm

@n34rt said in How to share data between threads using QThread:

Could you point how is it?

Create a signal with a parameter of the type you want to pass and then a slot with same parameter type. Connect the signal and slot. Each time you want to pass the parameter emit the signal and give it the parameter.