QT + How to call slot from custom C++ code running in a different thread

stribika got it almost right:

QMetaObject::invokeMethod( textEdit, "append", Qt::QueuedConnection,
                           Q_ARG( QString, myString ) );

cjhuitt's right, though: You usually want to declare a signal on the thread and connect it to the append() slot, to get object lifetime management for free (well, for the price of a minor interface change). On a sidenote, the additional argument:

               Qt::QueuedConnection ); // <-- This option is important!

from cjhuitt's answer isn't necessary anymore (it was, in Qt <= 4.1), since connect() defaults to Qt::AutoConnection which now (Qt >= 4.2) does the right thing and switches between queued and direct connection mode based on QThread::currentThread() and the thread affinity of the receiver QObject at emit time (instead of sender and receiver affinity at connect time).

In addition to stribika's answer, I often find it easier to use a signal/slot connection. You can specify that it should be a queued connection when you connect it, to avoid problems with the thread's signals being in the context of its owning object.

class mythread : public QThread
{
signals:
    void appendText( QString );
public:

    mythread(mywindow* win){this->w = win;};
    mywindow* w;
    void run()
    {
        emit ( appendText( "Hello" ) );
    };
};

int main(int argc, char *argv[])
{
    QApplication* a = new QApplication(argc, argv);
    mywindow* w = new mywindow();

    w->show();
    mythread* thr = new mythread(w);
    (void)connect( thr, SIGNAL( appendText( QString ) ),
                   w->ui.textEdit, SLOT( append( QString ) ),
                   Qt::QueuedConnection ); // <-- This option is important!
    thr->start();

    return a->exec();
}

You need to use QMetaObject::invokeMethod. For example:

void MyThread::run() {
    QMetaObject::invokeMethod(label, SLOT(setText(const QString &)), Q_ARG(QString, "Hello"));
}

(The above code comes from here: http://www.qtforum.org/article/26801/qt4-threads-and-widgets.html)