Show dialog before main window - c++

Question

I have a windowed application, which crashes after showing information dialog only before QMainWindow is activated.
Information dialog is shown only if passed data is invalid, however it might be a user interaction (file select / drag) or passed as argument, which causes problems. When / how should I show such error dialog than?
Note: When dialog is only shown (with show() method rather than exec()) it doesn't crash, but dialog gets discarded right away even with setModal( true ).
Any ideas? Thanks,
EDIT:
Some code:
int WinMain(HINSTANCE hInstance, HINSTANCE prevInstance, LPSTR lpCmdLine, int nShowCmd)
{
QApplication app(__argc, __argv);
MBViewer viewer;
viewer.show();
return app.exec();
}
MBViewer::MBViewer()
{
setAcceptDrops(true);
m_ui.setupUi(this);
m_viewer = new Viewer_Widget();
m_ui.preview_layout->addWidget(m_viewer);
parse_parameters();
connect_controls();
connect_actions();
}
void MBViewer::connect_controls()
{
(...)
connect( m_viewer, SIGNAL( view_initialized()), this, SLOT( open_file() ));
(...)
}
void MBViewer::open_file()
{
// somefile is set in parse_parameters or by user interaction
if (!somefile.is_valid()) {
m_viewer->reset();
// This will crash application after user clicked OK button
QMessageBox::information( this, "Error", "Error text", QMessageBox::Ok );
return;
}
(...)
}

Answer 1

Try a message box without pointer to your main window like in this example:
QMessageBox msgBox;
msgBox.setText(text.str().c_str());
msgBox.setIcon(QMessageBox::Question);
QPushButton *speed = msgBox.addButton("Speed optimization", QMessageBox::AcceptRole);
QPushButton *memory = msgBox.addButton("Memory optimization", QMessageBox::AcceptRole);
QPushButton *close = msgBox.addButton("Close", QMessageBox::RejectRole);
msgBox.setDefaultButton(speed);
msgBox.exec();
if (msgBox.clickedButton() == memory)
return true;
if (msgBox.clickedButton() == close)
exit(4);
It works even before creating any window (but after QApplication initialization).

Answer 2

When you call app.exec( ),it starts the main message handler loop which is required to be running before you start displaying dialogs. QMessageBox is a modal dialog when used with exec, so will prevent the app.exec function being called. Therefore, it's likely that messages are being sent before the message handler has been initialised and so a crash is observed.
When show() is used, execution of app.exec is allow to process, which is why the crash doesn't happen.
If you want a modal MessageBox at startup, you'll need to launch it after the message handler has been created / initialised. Not the cleanest way, but you could try launching it on a timer to delay the call to exec.