为什么 Qt5 会出现奇怪的崩溃，这取决于我创建应用程序实例的方式？

Question

Here is a reduced version of my code:

#include <QtWidgets/QApplication>
#include <QtWidgets/QMainWindow>


class MainView : public QMainWindow
{
    public:
        static MainView *Initialise(int argc, char **argv);

        int Run() { return qApp->exec(); }

    private:
        MainView() { }
        virtual ~MainView() { }

};


namespace
{
    QApplication *s_app      {nullptr};
    MainView     *s_instance {nullptr};
};


MainView *MainView::Initialise(int argc, char **argv)
{
    if (nullptr == s_app) {
        s_app = new QApplication(argc, argv);
    }

    if (nullptr == s_instance) {
        s_instance = dynamic_cast<MainView *>(new MainView());
    }

    return s_instance;
}


class Framework
{
    public:
        Framework() { }
        ~Framework() { }

        int Initialise(int argc, char **argv)
        {
            m_main_view = MainView::Initialise(argc, argv);

            return 0;
        }

        int Run()
        {
            m_main_view->show();
            return m_main_view->Run();
        }

    private:
        MainView *m_main_view;

};


int main(int argc, char **argv)
{
    Framework  framework;
    int        result = 0;

    // Weirdness here!!!!
    MainView::Initialise(argc, argv);  // Crashes even with this line.

    result = framework.Initialise(argc, argv);
    if (0 == result) {
        result = framework.Run();
    }

    return result;
}

This code crashes when showing the MainWindow instance. In my full version of this code, where MainView inherits from a virtual interface class (so as to allow me to decouple the view and substitute a command line interface or a test interface) it doesn't crash unless I comment out the code marked as "weirdness here".

In this code, if I replace:

MainView::Initialise(argc, argv);  // Crashes even with this line.

with:

if (nullptr == s_app) {
    s_app = new QApplication(argc, argv);
}

then it works fine and doesn't crash.

Essentially if the application is instantiated within MainView::Initialise() , whether called from main() or from Framework::Initialise() then the program crashes, but if I initialise the application in main() first then it doesn't crash.

I don't want my main() function to have any knowledge of the UI at all, if possible, even taking my virtual base class interface into account. There should be no need and it makes main() messier than necessary.

So why is this crashing and how do I stop it?

Answer 1

There's a subtle difference between your invocation of the QApplication constructor in main() and the invocation in MainView::Initialise() .

The documentation for the QApplication constructor says:

QApplication:: QApplication ( int &argc, char **argv)

<... >

Note: argc and argv might be changed as Qt removes command line arguments that it recognizes.

So when invoking the constructor in MainView::Initialise() , you are actually passing a copy of the argc that came from main. Presumably, Qt stores a reference to this internally somewhere, causing it to crash when the QApplication later dereferences it during the construction of the window.

If you change the signature of both Initialise() functions to take a reference to argc, the program works without crashing.