Seriously Noob question: QCoreApplication::exec hangs

[jbarnesweb]

Would appreciate someone's help with this newbie question.

With Qt 4.7, 4.8 and 5.0, my console application hangs when I run it. I'm expecting it to run to completion.

// from app.h
class App : public QObject
{
    Q_OBJECT
public:
    explicit App(QCoreApplication *app, QObject *parent = 0);
 
    void exec()
    {
        emit finished();
    }
 
signals:
    void finished();
 
public slots:
 
};
 
// from app.cpp
App::App(QCoreApplication *app, QObject *parent) :
    QObject(parent)
{
    connect(this, SIGNAL(finished()), app, SLOT(quit()));
}
 
// from main.cpp
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
 
    App app(&a);
    app.exec();
 
    return a.exec();
}

// from app.h
class App : public QObject
{
    Q_OBJECT
public:
    explicit App(QCoreApplication *app, QObject *parent = 0);
    
    void exec()
    {
        emit finished();
    }

signals:
    void finished();
    
public slots:
    
};

// from app.cpp
App::App(QCoreApplication *app, QObject *parent) :
    QObject(parent)
{
    connect(this, SIGNAL(finished()), app, SLOT(quit()));
}

// from main.cpp
int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    App app(&a);
    app.exec();
    
    return a.exec();
}

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Why doesn't this app just immediately terminate and return control to the command line?

[anda_skoa]

Why doesn't this app just immediately terminate and return control to the command line?

a.exec()

a.exec()

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runs the Qt event loop. This does not return until quit() or exit() are called on the same object.
Since you program doesn't call either after event loop processing has started, it will continue to do event processing forever.

Cheers,
_

[jbarnesweb]

Ok. Next question. Since a.exec() blocks, how do I call a.quit() after a.exec()?

[ChrisW67]

From another object, ui or a timer.

The point here, however, is that you have two separate calls to exec(), only the first of which is terminated.

If you really want an application that exits immediately then don't call exec() at all, or connect a zero length timer to qApp quit()

The exec() function is not designed to be overridden (not virtual)

[anda_skoa]

The point here, however, is that you have two separate calls to exec(), only the first of which is terminated.

Actually only once, the first exec() is just a normal C++ method on QObject derived class "App".
Got me confused as well

It is basically just a no-op, it emits a signal, which calls quit() but since the application is not running yet it doesn't do anything.

Cheers,
_

[ChrisW67]

I stand corrected. My brain saw "class App", assumed it sub-classed QCoreApplication, and then refused to read differently