Segfault when clicking menu in QT

Question

I'm pretty new to QT development, and I'm trying to create my first window with a menu. The problem is that I get a segmentation fault whenever I actually click on the File menu when trying to debug it.

This is QT 5.10, running on Fedora Linux 64-bit

My header file has:

private:
    QApplication app;
    Ui::MainWindow ui;
    QMainWindow mainWindow;

public:
    explicit ProgName(int argc, char *argv[], QObject *parent = nullptr);
    int run();

...and the rest is just standard QT boilerplate omitted for brevity. My main source file:

#include "progname.h"

int main(int argc, char *argv[])
{
    ProgName pn(argc, argv, nullptr);

    return pn.run();
}

ProgName::ProgName(int argc, char *argv[], QObject *parent) :
    QObject(parent),
    app(argc, argv)
{
    ui.setupUi(&mainWindow);
}

int ProgName::run()
{
    mainWindow.show();

    return app.exec();
}

And the really long one, the UI file, made with QT creator:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <layout class="QFormLayout" name="formLayout">
    <item row="0" column="0">
     <widget class="QTreeView" name="mainTreeView"/>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="mainMenuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>400</width>
     <height>28</height>
    </rect>
   </property>
   <widget class="QMenu" name="menuFile">
    <property name="title">
     <string>File</string>
    </property>
   </widget>
   <addaction name="menuFile"/>
  </widget>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

Any ideas?

Edit: Here's the stack trace:

1  __strlen_avx2                                                                                                                                0x7ffff5832c37 
2  QCoreApplication::arguments()                                                                                                                0x7ffff6a7a63b 
3  argv0BaseName()                                                                                                                              0x7fffe88d0101 
4  QXcbIntegration::wmClass() const                                                                                                             0x7fffe88d05fd 
5  QXcbWindow::create()                                                                                                                         0x7fffe88e596b 
6  QXcbIntegration::createPlatformWindow(QWindow *) const                                                                                       0x7fffe88d153e 
7  QWindowPrivate::create(bool, unsigned long long)                                                                                             0x7ffff6fd32fe 
8  QWidgetPrivate::create_sys(unsigned long long, bool, bool)                                                                                   0x7ffff7714ced 
9  QWidget::create(unsigned long long, bool, bool)                                                                                              0x7ffff77153ad 
10 QMenuPrivate::adjustMenuScreen(QPoint const&)                                                                                                0x7ffff785bf68 
11 QMenu::popup(QPoint const&, QAction *)                                                                                                       0x7ffff785f801 
12 QMenuBarPrivate::popupAction(QAction *, bool)                                                                                                0x7ffff786c402 
13 QMenuBarPrivate::setCurrentAction(QAction *, bool, bool)                                                                                     0x7ffff786e508 
14 QMenuBar::mousePressEvent(QMouseEvent *)                                                                                                     0x7ffff786ee72 
15 QWidget::event(QEvent *)                                                                                                                     0x7ffff7722baf 
16 QMenuBar::event(QEvent *)                                                                                                                    0x7ffff787014b 
17 QApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                      0x7ffff76e392c 
18 QApplication::notify(QObject *, QEvent *)                                                                                                    0x7ffff76eb6cf 
19 QCoreApplication::notifyInternal2(QObject *, QEvent *)                                                                                       0x7ffff6a76be7 
20 QApplicationPrivate::sendMouseEvent(QWidget *, QMouseEvent *, QWidget *, QWidget *, QWidget * *, QPointer<QWidget>&, bool)                   0x7ffff76ea6a2 
21 QWidgetWindow::handleMouseEvent(QMouseEvent *)                                                                                               0x7ffff773d47b 
22 QWidgetWindow::event(QEvent *)                                                                                                               0x7ffff773fb1f 
23 QApplicationPrivate::notify_helper(QObject *, QEvent *)                                                                                      0x7ffff76e392c 
24 QApplication::notify(QObject *, QEvent *)                                                                                                    0x7ffff76eb174 
25 QCoreApplication::notifyInternal2(QObject *, QEvent *)                                                                                       0x7ffff6a76be7 
26 QGuiApplicationPrivate::processMouseEvent(QWindowSystemInterfacePrivate::MouseEvent *)                                                       0x7ffff6fc98a3 
27 QGuiApplicationPrivate::processWindowSystemEvent(QWindowSystemInterfacePrivate::WindowSystemEvent *)                                         0x7ffff6fcb495 
28 QWindowSystemInterface::sendWindowSystemEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                        0x7ffff6fa479b 
29 userEventSourceDispatch(_GSource *, int ( *)(void *), void *)                                                                                0x7fffe892cb60 
30 g_main_context_dispatch                                                                                                                      0x7ffff2195b77 
31 g_main_context_iterate.isra                                                                                                                  0x7ffff2195f20 
32 g_main_context_iteration                                                                                                                     0x7ffff2195fac 
33 QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>)                                                                   0x7ffff6ac7c2f 
34 QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>)                                                                                      0x7ffff6a7596a 
35 QCoreApplication::exec()                                                                                                                     0x7ffff6a7e094 
36 ProgName::run                                                                                                            progname.cpp 21 0x401aaf       
37 main                                                                                                                       progname.cpp 7  0x401961       

Answer 1

Please have a look at the constructors of QCoreApplication and QApplication.

The constructors expect the argc argument to be a reference to an integer.

In your case, you pass the argc argument by value to the constructor of your ProgName class. Inside, you pass a reference to this (local) value down to the constructor of QApplication. The issue is that the reference will be invalid/dangling, as soon as the constructor call is finished. In particular, when calling exec() later in the run method, the application object will try to access the reference which fails and produces the observed crash.

Long story short: as a workaround, just pass through the argc argument as a reference:

ProgName::ProgName(int &argc, char *argv[], QObject *parent) :
    QObject(parent),
    app(argc, argv)
{
    ui.setupUi(&mainWindow);
}