Inheritance with QObject (parent?)

[robro]

Hello,

I would like to know what it necessary to proper inherit from QObject.
I have two example classes. Class A which is my base class derived from QObject and Class B which is derived from Class A.

I already found out that I am only allowed to inherit from QObject with the first class. But how do I need to modify the constructors that the parent system is working?

Here is a minimal not working example which tells me that the constructor implementation of B has QObject not as an element and not as Base.

What do I need to do?

Thank you very much!

a.h:

#ifndef A_H
#define A_H

#include <QObject>

class A : public QObject
{
    Q_OBJECT
public:
    explicit A(QObject *parent = nullptr);
};

#endif // A_H

a.cpp:

#include "a.h"

A::A(QObject *parent) : QObject(parent)
{
}

b.h:

#ifndef B_H
#define B_H

#include <QObject>
#include "a.h"

class B : protected A
{
    Q_OBJECT
public:
    explicit B(QObject *parent = nullptr);
};

#endif // B_H

b.cpp:

#include "b.h"

B::B(QObject *parent): QObject(parent)
{
}

main:

#include <QCoreApplication>
#include <b.h>

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);

    B myclass;

    return a.exec();
}

[SGaist]

Hi,

@robro said in Inheritance with QObject (parent?):

B::B(QObject *parent): QObject(parent)

This is wrong, your B object is of A type, therefore:

B::B(QObject *parent): A(parent)

[robro]

Thank you very much :-)

It worked, but now I got another error.
In reality I use the class B togehter with QML.

So I use the following lines:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>
#include "b.h"

int main(int argc, char *argv[])
{

QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling); QGuiApplication app(argc, argv);

qmlRegisterType<B>("com.br.classes", 1, 0, "b");

B b
QQmlApplicationEngine engine;
    engine.rootContext()->setContextProperty("b", &b); //Throws the error
engine.load(QUrl(QLatin1String("qrc:/main.qml")));
return app.exec();

This leads to the following error: "Conversion from B* to QObject* is already existing but it cannot be accesed"

How can I solve that?

[J.Hilk]

@robro said in Inheritance with QObject (parent?):

This leads to the following error: "Conversion from B* to QObject* is already existing but it cannot be accesed"
How can I solve that?

you declared the base class protected:

class B : protected A

And therefore can't be accessed from outside the class, I would change that.