Using Singleton Class contains QComboxBox , crashed

trreeee

show the widget and select the item, program will crash on exit

class SettingWidget : public QWidget
{
    Q_OBJECT

public:
    ~SettingWidget(){}
    static SettingWidget& instance(){
        static SettingWidget mInstance;
        return mInstance;
    }
private:
    explicit SettingWidget(QWidget *parent = 0) :
        QWidget(parent)
    {
        cbox = new QComboBox(this);
        cbox->addItem("1");
    }

    QComboBox * cbox;

};
main.cpp
 QApplication a(argc, argv);
  SettingWidget::instance().show();
return a.exec();

console:

QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
15:18:50: C:/Users/black/Documents/build-untitled29-Desktop_Qt_5_15_2_MinGW_64_bit-Debug/untitled29.exe crashed.

sorry ,i do not how to post stack. I do not use thread

jsulm

@trreeee How should anybody know why it crashes if you do not post relevant code? How does your instance() look like? Did you run through debugger to see where exactly it crashes and how the stack trace looks like?

JonB

@trreeee said in Using singleton contain QComboxBox , crashed:

static SettingWidget& instance();

You may know C++ rules for this better than I, but although this is a static method returning a reference to a SettingWidget object you have not actually created any such static object for it to return. The code does not show the definition of instance().

jsulm

@JonB said in Using singleton contain QComboxBox , crashed:

you have not actually created any such static object for it to return

It could be a static variable inside instance() - that's why I asked for its implementation :-)

JonB

@jsulm Yes I changed my reply to make this clear.

JonB

@trreeee
You keep altering your code, so I don't know how accurate to what you really have it is or when the updates are going to end....

One thing: it may not be relevant, but I don't think you should go cbox->show(); during the constructor for SettingWidget, that is not a place to put show()s.

trreeee

@JonB said in Using singleton contain QComboxBox , crashed:

: it may not be rel

sorry, This is the first time to ask a question here

JonB

@trreeee
Did you at least test your code with SettingWidget sw; instead of SettingWidget::instance() to verify that it really is the static which is the issue?

console:

QBasicTimer::start: QBasicTimer can only be used with threads started with QThread
15:18:50: C:/Users/black/Documents/build-untitled29-Desktop_Qt_5_15_2_MinGW_64_bit-Debug/untitled29.exe crashed.

Now that you post this it looks like you are using threds and you issue lies there, nothing to do with static.... Is the code you are posting your whole program, or part of something else?

jsulm

@trreeee Please run through debugger and post stack trace here

Paul Colby

I tried out your sample code, and for me, it runs fine :) However, it does crash on exit, because SettingWidget::cbox is destroyed twice. Once, because its a child of the single SettingWidget instance (this is done by Qt), and then because its a static variable (this done by C++ specifications).

The fix is to unset the parent-child relationship in the SettingWidget destructor, like:

   ~SettingWidget() {
        if ((cbox) && (cbox->parent() == this)) {
            cbox->setParent(nullptr);
        }
    }

The if conditions aren't really necessary here, but good practice anyway :)

Cheers.

trreeee

@Paul-Colby said in Using singleton contain QComboxBox , crashed:

if ((cbox) && (cbox->parent() == this)) {
cbox->setParent(nullptr);
}

thank you, I crash on exit also. I tried out "if code", the program still crash.

if I do not select the QComboBox 's item, the program can exit normally

trreeee

I solved , I used exit(0) to exit program. the program can exit normally, and ~SettingWidget(){} can be called normally,
return a.exec() crashed when ~SettingWidget(){} be called, qApp->exit() same as return a.exe()

JonB

@trreeee said in Using Singleton Class contains QComboxBox , crashed:

exit(0) to exit program. the program can exit normally, and ~SettingWidget(){} can be called normally,

Are you sure ~SettingWidget() is called in this case? Sounds more to me like you are just avoiding the (incorrect) destruction behaviour this way? But up to you.

My 2 cents? If you must have this singleton widget, I would have newed a static SettingWidget *mInstance; rather than having a static SettingWidget. Your way the (static) destruction comes after the parent has been destroyed, not a good idea.

trreeee

@JonB said in Using Singleton Class contains QComboxBox , crashed:

@trreeee said in Using Singleton Class contains QComboxBox , crashed:

exit(0) to exit program. the program can exit normally, and ~SettingWidget(){} can be called normally,

Are you sure ~SettingWidget() is called in this case? Sounds more to me like you are just avoiding the (incorrect) destruction behaviour this way? But up to you.

My 2 cents? If you must have this singleton widget, I would have newed a static SettingWidget *mInstance; rather than having a static SettingWidget. Your way the (static) destruction comes after the parent has been destroyed, not a good idea.

Thank you, I tested it, it did work
I also found this way

#include <QCoreApplication>
static Widget* instance(){
    static Widget* impl = new Widget;
	connect(qApp, &QCoreApplication::aboutToQuit, impl, &Widget::deleteLater);
	return impl;
}

JonB

@trreeee Good, I think that is much better!