Question

I’ve designed some construction like this:

template<class Ui_Class>
class Base_Dialog : virtual public QDialog, protected Ui_Class
{

protected:

    QDialog* caller_;

public:
    template<class Implementation>
    Base_Dialog(Implementation*const & imp,QDialog *caller,QWidget* parent = nullptr);

};

template<class Ui_Class>
template<class Implementation>
Base_Dialog<Ui_Class>::Base_Dialog(Implementation*const& imp,QDialog *caller,QWidget* parent):
    QDialog(parent),
    caller_(caller)
{
    setupUi(imp);
}

I’m using it like so:

class My_Class : public **Base_Dialog<Ui::My_Class>**
{
    Q_OBJECT


public slots:
    void display_me()
    {/*THIS IS NOT GETTING CONNECTED*/
        QMessageBox::warning(this,"Aha!","Aha!");
    }

public:
    explicit My_Class(QDialog* caller = nullptr,QWidget *parent = nullptr);

};

Line_Counter::Line_Counter(QDialog* caller,QWidget *parent) :
    Base_Dialog(this,caller,parent)

{
    //setupUi(this);//THIS WORKS BUT I'D RATHER CALL IT FROM Base_Dialog


}

this above construct is suppose to ease and simplify the way of inheriting from QDialog and from Ui class. And this works, except that when slots and signals are introduced in my class the base class for some reason doesn’t see them (slots/signals). If I call setupUi in My_Class ctor everything works correctly but I’d prefer to call it in Base_Class. Is there a way to do it?

Answer 1

The constructor for lineCounter should be renamed to My_Class as well, am I right?

The reason for this behavior is that the meta object returned in constructor of Base_Dialog from virtual const QMetaObject * metaObject() const is meta-object for Base_Dialog, as at this stage it is not yet instance of My_Class instance. My_CLass overrides this virtual method (invisibly in Q_OBJECT macro) but only after My_Class constructor code starts to execute – means after Base_Dialog code is finished. Signals and slots internally use meta-objects while connecting, so it seems this way it will not work.

Summarizing – you cannot do it, because constructor of Base_Dialog doesn’t know that My_Class instance is being created and therefore cannot have acces to any of it’s contents.

Sometimes this problem can be bypassed using CRTP, but in this case I have no idea if it’s aplicable. I would rather opt for doing it Qt’s way – call setupUi from My_Class constructor.