Subclass of subclass of qobject

Alart

Hi,
I want to inherite QObject class in another class. But in this new class I want to add some Q_PROPERTY-ies too and register both as type to qml.
Like that:
baseclass.h

#ifndef BASECLASS_H
#define BASECLASS_H

#include <QObject>

class baseclass : public QObject
{
	protected:
		Q_OBJECT
		Q_PROPERTY(QString baseProperty READ baseProperty WRITE setBaseProperty NOTIFY basePropertyChanged)
		QString m_baseProperty;

	public:
		explicit baseclass(QObject *parent = 0) :
			QObject(parent)
			{}
		explicit baseclass(QString baseProperty, QObject *parent = 0) :
			QObject(parent), m_baseProperty(baseProperty)
			{}

		~baseclass() {}

		QString baseProperty() const
		{
			return m_baseProperty;
		}

	signals:
		void basePropertyChanged(QString baseProperty);

	public slots:
		void setBaseProperty(QString baseProperty)
		{
			if (m_baseProperty == baseProperty)
				return;

			m_baseProperty = baseProperty;
			emit basePropertyChanged(baseProperty);
		}
};

#endif // BASECLASS_H

inheritingclass.h

#ifndef INHERITINGCLASS_H
#define INHERITINGCLASS_H

#include "baseclass.h"

class inheritingclass : public baseclass
{
		Q_PROPERTY(QString newProperty READ newProperty WRITE setNewProperty NOTIFY newPropertyChanged)
		QString m_newProperty;

	public:
		explicit inheritingclass(QObject *parent = 0) :
			baseclass(parent)
			{}
		explicit inheritingclass(QString baseProperty, QString newProperty, QObject *parent = 0) :
			baseclass(baseProperty, parent), m_newProperty(newProperty)
			{}

		~inheritingclass() {}

		QString newProperty() const
		{
			return m_newProperty;
		}

	signals:
		void newPropertyChanged(QString newProperty);

	public slots:
		void setNewProperty(QString newProperty)
		{
			if (m_newProperty == newProperty)
				return;

			m_newProperty = newProperty;
			emit basePropertyChanged(newProperty);
		}
};

#endif // INHERITINGCLASS_H

main.cpp:

#include <QApplication>
#include <QQmlApplicationEngine>
#include <QtQml>

#include "baseclass.h"
#include "inheritingclass.h"

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

	qmlRegisterType<baseclass>("example.inheritance",1,0,"baseClass");
	qmlRegisterType<inheritingclass>("example.inheritance",1,0,"inheritingClass");

	baseclass BaseClass("baseProperty of base class");
	inheritingclass InheritingClass ("baseProperty of inheriting class", "new Property of inheriting clas");

	QQmlApplicationEngine engine;

	engine.rootContext()->setContextProperty("BaseClass", &BaseClass);
	engine.rootContext()->setContextProperty("InheritingClass", &InheritingClass);

	engine.load(QUrl(QStringLiteral("qrc:/main.qml")));

	return app.exec();
}

main.qml:

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Dialogs 1.2

ApplicationWindow {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")

    menuBar: MenuBar {
        Menu {
            title: qsTr("File")
            MenuItem {
                text: qsTr("Exit")
                onTriggered: Qt.quit();
            }
        }
    }

    MainForm {
        anchors.fill: parent
        baseProperty.text:BaseClass.baseProperty
        inherBaseProp.text: InheritingClass.baseProperty
        newProperty.text: InheritingClass.newProperty
    }
}

MainForm.ui.qml

import QtQuick 2.5
import QtQuick.Controls 1.4
import QtQuick.Layouts 1.2

Item {
    id: item1
    width: 640
    height: 480
    property alias newProperty: newProperty
    property alias inherBaseProp: inherBaseProp
    property alias baseProperty: baseProperty

    ColumnLayout {
        id: columnLayout1
        anchors.fill: parent

        Text {
            id: baseProperty
            text: qsTr("this.id")
            Layout.alignment: Qt.AlignHCenter | Qt.AlignVCenter
            font.pixelSize: 12
        }

        Text {
            id: inherBaseProp
            text: this.id
            Layout.alignment: Qt.AlignHCenter | Qt.AlignVCenter
            font.pixelSize: 12
        }

        Text {
            id: newProperty
            text: this.id
            Layout.alignment: Qt.AlignHCenter | Qt.AlignVCenter
            font.pixelSize: 12
        }
    }

}

But there inherining class isn't registered as a new type, and newProperty of inheriting class isn't displayed. My question is:

How to register class that inherits from another custom qobject class as a type?

? Offline

Hi! I think you forgot the Q_OBJECT macro in inheritingclass. Does this really compile?

kshegunov

@Alart
In addition to what @Wieland wrote, Q_OBJECT is supposed to appear in the private section of the class, not as protected. So this:

class baseclass : public QObject
{
protected:
    Q_OBJECT
    Q_PROPERTY(QString baseProperty READ baseProperty WRITE setBaseProperty NOTIFY basePropertyChanged)
    QString m_baseProperty;
    // ...
};

Would go like this:

class baseclass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString baseProperty READ baseProperty WRITE setBaseProperty NOTIFY basePropertyChanged)

protected:
    QString m_baseProperty;
    // ...
};

Alart

It compiles obviously. I did "a lot of work" to provide that truncated but working example. Q_OBJECT macro is inherited from baseclass.
You can try yourself... I can provide example for earlier versions of Qt.
I can't add new Q_OBJECT macro, because inheritingclass does not inherit from QObject. If I add multiple inheritance it does not compile, because of ambiguous QObject.

In fact Q_OBJECT macro can be put even in public section - it does not affect anything. Maybe that macro forces private somewhere and despite I make it protected base QObject is private?

Edit:
I previously worked around it by creating all properties (private or protected macros, protected variables) in base class and defining methods or whatever I needed in subclass. Then I used that in qml as base class even if it was subclass.
But I want to define Q_PROPERTY in subclass too if it's possible, because that's more neat. I don't need to control unnecessary properties when I use base class and all special informations are stored in one class, not scattered between base and inheriting class.

kshegunov

@Alart
Hello,

Q_OBJECT macro is inherited from baseclass.

And if that were the proper way of doing it a lot of people writing Qt have gone way out of their way for nothing including it in every class that derives from QObject, whether directly or indirectly.

I can't add new Q_OBJECT macro, because inheritingclass does not inherit from QObject. If I add multiple inheritance it does not compile, because of ambiguous QObject.

Well, it does through baseclass, and it gives you ambiguity because Q_OBJECT is not in the private section of your base class.

I previously worked around it by creating all properties (private or protected macros, protected variables) in base class and defining methods or whatever I needed in subclass.

This would defeat the whole purpose of having inheritance, wouldn't it?

Then I used that in qml as base class even if it was subclass.
But I want to define Q_PROPERTY in subclass too if it's possible

What you want has nothing to do with QML, but is a matter of the Qt meta-object system, and you can define properties in your derived classes, if you do that properly.

Kind regards.

Chris Kawa

@Alart said:

I can't add new Q_OBJECT macro, because inheritingclass does not inherit from QObject. If I add multiple inheritance it does not compile, because of ambiguous QObject.

inheritingclass inherits baseclass which inherits QObject, so it is a QObject already. There's no multiple inheritance present or needed here. And yes, you need Q_OBJECT macro in the inheritingclass, because it declares signals and slots. If you don't specify the Q_OBJECT macro you are limited to signals and slots declared in baseclass only, and you can't add new ones.
Q_OBJECT macro is needed on every "level" of inheritance that adds any moc dependent features (signals, slots, properties etc.).

Alart

OK, I tried around it already, because this seems like a proper way.
This is what I got, however it does not compile:

#ifndef BASECLASS_H
#define BASECLASS_H

#include <QObject>

class baseclass : public QObject
{
	private: // if neccesary
		Q_OBJECT
		Q_PROPERTY(QString baseProperty READ baseProperty WRITE setBaseProperty NOTIFY basePropertyChanged)
		QString m_baseProperty; // this can be private or protected

	public:
		explicit baseclass(QObject *parent = 0) :
			QObject(parent)
			{}
		explicit baseclass(QString baseProperty, QObject *parent = 0) :
			QObject(parent), m_baseProperty(baseProperty)
			{}

		~baseclass() {}

		QString baseProperty() const
		{
			return m_baseProperty;
		}

	signals:
		void basePropertyChanged(QString baseProperty);

	public slots:
		void setBaseProperty(QString baseProperty)
		{
			if (m_baseProperty == baseProperty)
				return;

			m_baseProperty = baseProperty;
			emit basePropertyChanged(baseProperty);
		}
};

#endif // BASECLASS_H

#ifndef INHERITINGCLASS_H
#define INHERITINGCLASS_H

#include "baseclass.h"

class inheritingclass : public baseclass
{
		Q_OBJECT
		Q_PROPERTY(QString newProperty READ newProperty WRITE setNewProperty NOTIFY newPropertyChanged)
		QString m_newProperty;

	public:
		explicit inheritingclass(QObject *parent = 0) :
			baseclass(parent)
			{/*setParent(parent);*/}
		explicit inheritingclass(QString baseProperty, QString newProperty, QObject *parent = 0) :
			baseclass(baseProperty, parent), m_newProperty(newProperty)/*, m_baseProperty(baseProperty)*/
			{/*setParent(parent);*/}

		~inheritingclass() {}

		QString newProperty() const
		{
			return m_newProperty;
		}

	signals:
		void newPropertyChanged(QString newProperty);

	public slots:
		void setNewProperty(QString newProperty)
		{
			if (m_newProperty == newProperty)
				return;

			m_newProperty = newProperty;
			emit basePropertyChanged(newProperty);
		}
};

#endif // INHERITINGCLASS_H

Now I'm getting:
/home/alart/QTworek/InheritanceQobject/inheritingclass.h:14: error: undefined reference to `vtable for inheritingclass'
The same in both constructors and destructors. I remember that problem, it had something to do with macros...

I already tried clean and build.

kshegunov

This is a very different thing altogether. Now, try without inlining your constructors, and I think you'll be good to go. Like this:

class baseclass : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString baseProperty READ baseProperty WRITE setBaseProperty NOTIFY basePropertyChanged)

protected:
    QString m_baseProperty; // this can be private or protected

public:
     // Define those in your cpp:
    explicit baseclass(QObject *parent = 0); //< Don't inline this!
    ~baseclass(); //< Don't inline this since it's virtual and virtual methods can't be inlined by the compiler anyway
};

and also the same for inheritingclass.

Alart

OK, now it works.
BTW. with protected macros in baseclass it works too. But anyway I will keep it private unless I read whole documentation.

Thank you all.

Chris Kawa

The Q_OBJECT macro can be put in public, protected or private section and it will do the same. It doesn't matter for moc but I highly recommend to always put it at the top of your class for the following reason:

The Q_OBJECT macro is defined like this:

#define Q_OBJECT \
public: \
   //stuff... \
private: \
  //other stuff...

So it's easy to make your other members private by mistake like this:

class Foo
{
public:
    Q_OBJECT
    int bar; //oh no! bar is private
};

If you put the macro at the top it won't affect access declarations below:

class Foo
{
    Q_OBJECT
public:
    int bar; //yes! bar is public as intended
};

Feedback Wanted: Qt Hub

Subclass of subclass of qobject