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    <p>Take a look at <a href="https://doc.qt.io/qt-5/threads-qobject.html#signals-and-slots-across-threads" rel="nofollow noreferrer">Signals and slots across threads</a>. 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.</p> <p>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.</p> <p>Edit: In response to your comment, look at the <a href="https://doc.qt.io/qt-5/qtcore-threads-mandelbrot-example.html" rel="nofollow noreferrer">Mandelbrot</a> example, under the <code>MandelbrotWidget Class Implementation</code> header:</p> <blockquote> <p>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.</p> </blockquote> <p>I believe this is slightly outdated, here are the <a href="https://doc.qt.io/qt-5/qmetatype.html#Type-enum" rel="nofollow noreferrer">valid meta types</a>. Since signals and slots across threads use queued connections, you should not have to do the moveToThread calls in most cases.</p> <p>Edit: I will try to explain things with a similar example:</p> <p>mythread.h:</p> <pre><code>#ifndef MYTHREAD_H #define MYTHREAD_H #include &lt;QThread&gt; #include &lt;QMutex&gt; class MyThread : public QThread { Q_OBJECT protected: virtual void run(); signals: void signalGUI(QString); }; #endif // MYTHREAD_H </code></pre> <p>mythread.cpp:</p> <pre><code>#include "mythread.h" #include &lt;QString&gt; 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", &amp;temp); emit signalGUI(temp); } </code></pre> <p>mylineedit.h</p> <pre><code>#ifndef MYLINEEDIT_H #define MYLINEEDIT_H #include &lt;QLineEdit&gt; class MyLineEdit : public QLineEdit { Q_OBJECT public: explicit MyLineEdit(QWidget *parent = 0); public slots: void setText(const QString &amp;string); }; #endif // MYLINEEDIT_H </code></pre> <p>mylineedit.cpp</p> <pre><code>#include "mylineedit.h" #include &lt;QThread&gt; MyLineEdit::MyLineEdit(QWidget *parent) : QLineEdit(parent) { } void MyLineEdit::setText(const QString &amp;string) { qDebug("Thread id inside setText %d",(int)QThread::currentThreadId()); qDebug("String address inside setText %p\n", &amp;string); QLineEdit::setText(string); } </code></pre> <p>main.cpp:</p> <pre><code>#include &lt;QApplication&gt; #include &lt;QPushButton&gt; #include &lt;QHBoxLayout&gt; #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(&amp;w); QPushButton * pushButton = new QPushButton("Run Thread", &amp;w); MyLineEdit * lineEdit = new MyLineEdit(&amp;w); pH-&gt;addWidget(pushButton); pH-&gt;addWidget(lineEdit); w.show(); MyThread thread; qDebug("Thread id %d",(int)QThread::currentThreadId()); QObject::connect(pushButton,SIGNAL(clicked()),&amp;thread,SLOT(start())) ; QObject::connect(&amp;thread,SIGNAL(signalGUI(const QString&amp;)),lineEdit,SLOT(setText(const QString&amp;))); return a.exec(); } </code></pre> <p>Sample output after clicking button:</p> <pre>Thread id 1088110320 Thread id inside run 1093176208 String address inside run 0x41288350 Thread id inside setText 1088110320 String address inside setText 0x974af58</pre> <p>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 <strong>strongly</strong> encourage you to read <a href="https://doc.qt.io/qt-5/threads-qobject.html" rel="nofollow noreferrer">Threads and QObject</a>.</p>
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    1. COI did not see any documentation stating about Qt handling moveToThread, I read about Qt handles signal slot connection type based on where the object live.
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    2. COUpdated response, I may have over simplified a bit, but in most cases it will handle the work.
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    3. CO@Adam What do you mean by " Qt handles the moveToThread for you if it's needed and you used the correct connection". As per the example `connect(&thread, SIGNAL(renderedImage(const QImage &, double)), this, SLOT(updatePixmap(const QImage &, double)));` the `thread` and `this` are all objects from same thread but how the connection is queued connection?
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