Signals and slots in Qt console app

Question

I'm new to this but am wanting to write a Qt console app which uses Qt's features including signals and slots and therefore want an application event loop. Following this question How do I create a simple Qt console application in C++? I seem to be heading in the right direction but why does anything get executed after emitting finished in the following code:

// main.cpp
#include <QtCore>
#include <QDebug>

class Task : public QObject
{
    Q_OBJECT
public:
    Task(QObject *parent = 0) : QObject(parent) {}

public slots:
    void run()
    {
        // Do processing here
        qDebug() << "Hello World";
        emit finished();
        qDebug() << "I thought I'd finished!";
    }

signals:
    void finished();
};

#include "main.moc"

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    // Task parented to the application so that it
    // will be deleted by the application.
    Task *task = new Task(&a);

    // This will cause the application to exit when
    // the task signals finished.
    QObject::connect(task, SIGNAL(finished()), &a, SLOT(quit()));

    // This will run the task from the application event loop.
    QTimer::singleShot(0, task, SLOT(run()));

    return a.exec();
}

Answer 1

Try to subclass the QCoreApplication and reimplement the quit() signal with inserting a debug print in.

You will see that the quit function is called immediately before your second print in the "run" slot, provided you are using DirectConnection.

Also, worth nothing that quit() is slightly different to the GNU C exit() as per documentation:

Note that unlike the C library function of the same name, this function does return to the caller -- it is event processing that stops.

This means quit() dos not mean that application quits right away without terminating gracefully in the middle of your method execution.