Qt signaling across threads, one is GUI thread?

Take a look at Signals and slots across threads. If you always use signals and slots to communicate with the worker thread, Qt handles the moveToThread for you if it’s needed and you used the correct connection.

Edit: I would guess the article’s author was seeing his problem since he was calling start in the constructor before the thread was actually created. In other words, don’t trust third-party code blindly.

Edit: In response to your comment, look at the Mandelbrot example, under the MandelbrotWidget Class Implementation header:

With queued connections, Qt must store a copy of the arguments that were passed to the signal so that it can pass them to the slot later on. Qt knows how to take of copy of many C++ and Qt types, but QImage isn’t one of them. We must therefore call the template function qRegisterMetaType() before we can use QImage as parameter in queued connections.

I believe this is slightly outdated, here are the valid meta types. Since signals and slots across threads use queued connections, you should not have to do the moveToThread calls in most cases.

Edit:
I will try to explain things with a similar example:

mythread.h:

#ifndef MYTHREAD_H
#define MYTHREAD_H

#include <QThread>
#include <QMutex>

class MyThread : public QThread
{
   Q_OBJECT

protected:
   virtual void run();

signals:
   void signalGUI(QString);
};

#endif // MYTHREAD_H

mythread.cpp:

#include "mythread.h"
#include <QString>

void MyThread::run()
{
   qDebug("Thread id inside run %d",(int)QThread::currentThreadId());
   static int run = 0;
   QString temp = QString("Run: %1").arg(run++);
   qDebug("String address inside run %p", &temp);
   emit signalGUI(temp);
}

mylineedit.h

#ifndef MYLINEEDIT_H
#define MYLINEEDIT_H

#include <QLineEdit>

class MyLineEdit : public QLineEdit
{
Q_OBJECT
public:
    explicit MyLineEdit(QWidget *parent = 0);

public slots:
    void setText(const QString &string);

};

#endif // MYLINEEDIT_H

mylineedit.cpp

#include "mylineedit.h"
#include <QThread>

MyLineEdit::MyLineEdit(QWidget *parent) :
    QLineEdit(parent)
{
}

void MyLineEdit::setText(const QString &string)
{
   qDebug("Thread id inside setText %d",(int)QThread::currentThreadId());
   qDebug("String address inside setText %p\n", &string);
   QLineEdit::setText(string);
}

main.cpp:

#include <QApplication>
#include <QPushButton>
#include <QHBoxLayout>
#include "mythread.h"
#include "mylineedit.h"

//GUI calls a thread to do some job and sub update the text box once it is done
int main(int argc, char *argv[])
{
   QApplication a(argc, argv);
   QWidget w;
   QHBoxLayout * pH = new QHBoxLayout(&w);
   QPushButton * pushButton = new QPushButton("Run Thread", &w);
   MyLineEdit * lineEdit = new MyLineEdit(&w);

   pH->addWidget(pushButton);
   pH->addWidget(lineEdit);
   w.show();

   MyThread thread;
   qDebug("Thread id %d",(int)QThread::currentThreadId());
   QObject::connect(pushButton,SIGNAL(clicked()),&thread,SLOT(start())) ;
   QObject::connect(&thread,SIGNAL(signalGUI(const QString&)),lineEdit,SLOT(setText(const QString&)));
   return a.exec();
}

Sample output after clicking button:

Thread id 1088110320
Thread id inside run 1093176208
String address inside run 0x41288350
Thread id inside setText 1088110320
String address inside setText 0x974af58

As you can see, the run thread is different than the main GUI thread. Also, even though you pass a const reference to a QString, since it crosses thread boundaries it copies it.
I strongly encourage you to read Threads and QObject.

2. The QThread::start() method creates the thread and calls your run() implementation. If you want to handle events or received signals on the thread you have to call QThread::exec() inside your run() implementation. You should never call run() explicitly and you should never call exec() outside of run().

3. The owner thread makes a difference only when a slot is connected to a signal with the connection type other than Qt::DirectConnection. Then Qt will ensure that the slot runs on the owner thread, but for that the owner thread must be running an event loop with QThread::exec(). In this case calling myObj.moveToThread(myThread) will ensure that myObj slots run on the thread myThread.

4. The thread object belongs to the thread where it was created, not on the thread that it manages (and where the run method will run). So when you connect a signal to a thread object’s slot, that slot will run in the thread where the thread object was created unless you call moveToThread().