Custom widget not showing

[eyrkon]

Hi,

I am new to Qt and I am trying to create some custom widgets for my application.Here is my issue:

I have the following main :

    QApplication a(argc, argv);
    QtWidgetsApplication w;

    QWidget* central_widget = new QWidget(&w);

    QListWidget* listWidget = new QListWidget(central_widget);
    listWidget->setObjectName(QString::fromUtf8("listWidget"));
listWidget->setGeometry(QRect(20, 20, 256, 192));

    w.setCentralWidget(central_widget);

    w.show();
    return a.exec();

This works fine. However if I create a basic custom widget of my own that inherits from QListWidget like this :

class DeviceList : public QListWidget {

public:
	DeviceList(QObject* parent) {}
};

and replace

QListWidget* listWidget = new QListWidget(central_widget);

with

DeviceList* listWidget = new DeviceList(central_widget);

then my widget will not show and the main window is empty.
If however I make the custom widget itself the central one

    w.setCentralWidget(listWidget);

then it will actually appear.

I am not sure what I am doing wrong but I feel like I am missing something fundamental here when it comes to custom widget inheritance.

[jsulm]

@eyrkon said in Custom widget not showing:

DeviceList(QObject* parent) {}

You forgot to call QListWidget constructor here passing it the parent.

[eyrkon]

@jsulm Whoops ,thanks, that works :)

[JonB]

@jsulm
I assume C++ always calls the base constructor which takes no parameters if you do not specify?
And if there is no base with no parameters then it's an error?

[jsulm]

@JonB The base constructor here does not set the parent

[JonB]

@jsulm
I know, that was not the question! :) The question (well, two questions) is about C++ behaviour as I phrased it, please? :) Don't make me go ask ChatGPT, she & I have had some disagreements about facts... !

[Pl45m4]

@JonB said in Custom widget not showing:

The question (well, two questions) is about C++ behaviour as I phrased it, please?

I'm not @jsulm , but you are right. Since most QWidget-based classes have nullptr as default arg for parent in their c'tor, it's basically empty. And a QObject / QWidget with no parent is "standalone" or a toplevel-widget and needs widget.show() to be visible.
When there would be no default and no other empty base/super class c'tor available, I think it wouldn't even compile.

Don't make me go ask ChatGPT, she & I have had some disagreements about facts... !

WDYM?! Elephants do not lay eggs?!
(Source)

:D

[JonB]

@Pl45m4 Sadly ChatGPT no longer gives that answer to that question :(