Parent issue (?) while using derived Qt widget class

Question

The following is the smallest example I could make to present the isuue.

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    QVBoxLayout *vLayout = new QVBoxLayout(this);

    QGroupBox *gb = new QGroupBox;
//    MyGroupBox *gb = new MyGroupBox;
    vLayout->addWidget(gb);

    QPushButton *btB = new QPushButton;
    vLayout->addWidget(btB);
}

The code above produces the image above, it's just a group box and a button in vertical layout.

If I replace QGropBox by MyGroupBox then it doesn't show there anymore. The code below produces the image below.

Widget::Widget(QWidget *parent)
    : QWidget(parent)
{
    QVBoxLayout *vLayout = new QVBoxLayout(this);

//    QGroupBox *gb = new QGroupBox;
    MyGroupBox *gb = new MyGroupBox;
    vLayout->addWidget(gb);

    QPushButton *btB = new QPushButton;
    vLayout->addWidget(btB);
}

Where mygroupbox.h (the constructor body is empty in the .cpp file):

#include <QWidget>

class MyGroupBox : public QWidget
{
    Q_OBJECT
public:
    explicit MyGroupBox(QWidget *parent = 0);

signals:

public slots:

};

Why the group box isn't showing there? How to make it appear?

Answer 1

As an aside, a minimal example could look like the below. Not a single declaration/statement/expression can be removed. The button aids in visualizing the problem, so it should be left in. The use of a failure trigger variable highlights exactly what condition triggers the failure: the code self-documents and you almost need no narrative to explain it. The question could be as concise as the test case below and one sentence "Why is the group box's border not visible when fail is true?". Most likely, had you followed the minimization fully through, you'd realize yourself what the problem was - it becomes rather obvious. It's not so when MyGroupBox is declared in another file!

The technique of putting it all into a single main.cpp file is critical in spotting the problem: all of the code is physically next to each other, making it much easier to spot mistakes! When you minimize, usually the first things that have to go are separate files: cram it all into one file, and then relentlessly remove absolutely everything that's not directly needed in reproducing the issue.

#include <QtWidgets>

struct MyGroupBox : public QWidget {};

int main(int argc, char ** argv) {
   bool fail = true;
   QApplication app{argc, argv};
   QWidget widget;
   QVBoxLayout layout{&widget};
   QGroupBox groupBox;
   MyGroupBox myGroupBox;
   QPushButton button;
   layout.addWidget(fail ? static_cast<QWidget*>(&myGroupBox) : &groupBox);
   layout.addWidget(&button);
   widget.show();
   return app.exec();
}

This concise style is not only for trivial test cases. In your real code, the Widget's header and implementation could look as follows:

// Widget.h
#include <QtWidgets>
#include "MyGroupBox.h"

class Widget : public QWidget {
  Q_OBJECT
  QVBoxLayout layout{this};
  MyGroupBox groupBox;
  QPushButton button{tr("Click Me!")};
public:
  explicit Widget(QWidget * parent = nullptr);
};

// Widget.cpp
#include "Widget.h"

Widget::Widget(QWidget * parent) :
  QWidget{parent} {
  layout.addWidget(&groupBox);
  layout.addWidget(&button);
}

If you insist on shielding the interface from implementation details, don't use pointers to widgets etc., use a PIMPL.

// Widget.h
#include <QWidget>

class WidgetPrivate;
class Widget : public QWidget {
   Q_OBJECT
   Q_DECLARE_PRIVATE(Widget)
   QScopedPointer<WidgetPrivate> const d_ptr;
public:
   explicit Widget(QWidget * parent = nullptr);
};

// Widget.cpp
#include "Widget.h" // should always come first!
#include "MyGroupBox.h"

class WidgetPrivate {
   Q_DECLARE_PUBLIC(Widget)
   Widget * const q_ptr;
public:
   QVBoxLayout layout{q_func()};
   QGroupBox groupBox;
   MyGroupBox myGroupBox;
   QPushButton button{"Click Me!"};
   WidgetPrivate(Widget * q) : q_ptr(q) {
      layout.addWidget(&groupBox);
      layout.addWidget(&button);
   }
};

Widget::Widget(QWidget * parent) :
   QWidget{parent}, d_ptr{new WidgetPrivate{this}}
{}

Answer 2

This is why it doesn't appear:

class MyGroupBox : public QWidget

Your "group box" is basically just a QWidget. Inherit from QGroupBox instead.