[Solved] Do I need to run Qt even loop (QApplication.exec()) to display QMessage::critical()?

[Zingam]

While this code seems to work. Is it correct?
I thought that no Qt functinality would work if I didn't use a.exec().

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

try {
    MainWindow w;
    w.show();

    return a.exec();
}
catch (ExceptionInitialization& exception) {
    qDebug() << exception.getMessage();
    QMessageBox::critical(nullptr,
                          QObject::tr("ERROR"),
                          exception.getMessage());

    return (-1);
}

}@

[dheerendra]

Not required. It is dialog. Instead of passing nullptr, try passing 0. What is the issue you are facing ? It it not shown ?

[JKSH]

Hi,

[quote author="Zingam" date="1407675034"]While this code seems to work. Is it correct?
I thought that no Qt functinality would work if I didn't use a.exec().[/quote]Yes, it's fine.

exec() starts and event loop. Without it, you can't handle events and signals, but other things in Qt still work. (For example, even if I don't call exec() I can still use QFile to read/write files)

Note that there are many ways to start an event loop:

• QCoreApplication::exec()
• QThread::exec()
• QEventLoop::exec()
• QDialog::exec()

What Dheerendra meant is that QMessage::critical() calls QDialog::exec() internally, so you do get a running event loop until you close the dialog.

[quote author="Dheerendra" date="1407677087"]Instead of passing nullptr, try passing 0.[/quote]Why? nullptr was invented specifically to avoid the ambiguity caused by passing 0.

We should encourage each other to use C++11.

[jazzycamel]

@
#include <QtGui/QApplication>
#include <QtGui/QMessageBox>
#include <exception>
using namespace std;

#if __cplusplus<201103L // naive C++11 support test
#define nullptr 0
#endif

class MyException: public exception {
virtual const char *what() const throw(){
return "My Exception Happened";
}
} myException;

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

try { throw myException; }
catch(exception &e) {
    QMessageBox::critical(nullptr, "Exception!", e.what());
}

}
@

Works perfectly for me (Qt4.8) ;o)

[Zingam]

Thank you!
I wasn't sure if the MessageBox would work without issues. I do a great deal of initialization in MainWindow like copying files, socket initialization, xml parsing, etc. And if some of this fails I need to warn the user and exit.

So I assume my doubts are solved.

Note: It's a C++11 app.