QQmlListProperty is deprecated, but how to migrate?

[Yujeonja]

I have a project that extending QML using C++ which is using QQmlListProperty for default property. But from Qt 5.15 it complains deprecated warning.
This is a minimal reproducing refer to this example. My project is very similar.

// person.h
#ifndef PERSON_H
#define PERSON_H

#include <QObject>

class Person : public QObject
{
    Q_OBJECT

    Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)
public:
    explicit Person(QObject *parent = nullptr);

    QString name() const;
    void setName(const QString& name);

signals:
    void nameChanged(QString name);

private:
    QString m_name;
};

#endif // PERSON_H

// person.cpp
#include "person.h"

Person::Person(QObject *parent) : QObject(parent)
{

}

QString Person::name() const
{
    return this->m_name;
}

void Person::setName(const QString &name)
{
    if (this->m_name != name) {
        this->m_name = name;

        emit this->nameChanged(name);
    }
}

// birthdayparty.h
#ifndef BIRTHDAYPARTY_H
#define BIRTHDAYPARTY_H

#include <QObject>

#include <QQmlListProperty>

#include "person.h"

class BirthdayParty : public QObject
{
    Q_OBJECT
    Q_PROPERTY(Person* host READ host WRITE setHost)
    Q_PROPERTY(QQmlListProperty<Person> guests READ guests)
    Q_CLASSINFO("DefaultProperty", "guests")
//    QML_ELEMENT
public:
    BirthdayParty(QObject *parent = nullptr);

    Person* host() const;
    void setHost(Person *host);

    QQmlListProperty<Person> guests();
    int guestCount() const;
    Person* guest(int) const;

private:
    Person *m_host;
    QList<Person*> m_guests;
};

#endif // BIRTHDAYPARTY_H

// birthdayparty.cpp
#include "birthdayparty.h"

BirthdayParty::BirthdayParty(QObject *parent) : QObject(parent)
{

}

Person* BirthdayParty::host() const
{
    return this->m_host;
}

void BirthdayParty::setHost(Person *host)
{
    this->m_host = host;
}

QQmlListProperty<Person> BirthdayParty::guests()
{
    return QQmlListProperty<Person>(this, this->m_guests);
}

int BirthdayParty::guestCount() const
{
    return this->m_guests.length();
}

Person* BirthdayParty::guest(int i) const
{
    return this->m_guests[i];
}

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>

#include "person.h"
#include "birthdayparty.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    const QUrl url(QStringLiteral("qrc:/main.qml"));
    QObject::connect(&engine, &QQmlApplicationEngine::objectCreated,
                     &app, [url](QObject *obj, const QUrl &objUrl) {
        if (!obj && url == objUrl)
            QCoreApplication::exit(-1);
    }, Qt::QueuedConnection);

    qmlRegisterType<Person>("ExtendingExample", 1, 0, "Person");
    qmlRegisterType<BirthdayParty>("ExtendingExample", 1, 0, "BirthdayParty");

    engine.load(url);

    return app.exec();
}

and using in QML

// main.qml
import QtQuick 2.12
import QtQuick.Window 2.12

import ExtendingExample 1.0

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

  BirthdayParty {
    host: Person {
      name: "John Doe"
    }

    Person {
      name: "Jane Doe"
    }
    Person {
      name: "Mary Smith"
    }
    Person {
      name: "Robert Brown"
    }
  }
}

This works except the warning in compile time.

warning: ‘QQmlListProperty<T>::QQmlListProperty(QObject*, QList<T*>&) [with T = Person]’ is deprecated: Use constructor taking QList pointer, and gain improved performance [-Wdeprecated-declarations]

so I replaced QQmlListProperty<Person> to QList<Person*>.

Q_PROPERTY(QList<Person*> guests READ guests)
...
QList<Person*> guests();
...
QList<Person*> BirthdayParty::guests()
{
    return this->m_guests;
}

This time, an error occured in QML.

Invalid property assignment: "guests" is a read-only property

again, I added WRITE method to the property.

Q_PROPERTY(QList<Person*> guests WRITE setGuests READ guests)
                                 ^
...
void BirthdayParty::setGuests(QList<Person*> guests)
{
    this->m_guests = guests;
}

But QML error message is,

Cannot assign to property of unknown type "QList<Person*>".

How can I solve this? I don't want to see deprecated warnings because Qt 5.15 is the last Qt5 version so my project may won't work next Qt6 and very difficult to migrate to.

[J.Hilk]

@Yujeonja replace QList<Person*> with QList<QObject*> thats supported and on the qml side happens an "implicit cast" when you try to access Person functions

[GrecKo]

The warning tells you exactly what to do.

replace return QQmlListProperty<Person>(this, this->m_guests); with return QQmlListProperty<Person>(this, &this->m_guests);

[Yujeonja]

I misread the documents and warning. The deprecated is constructor not class. That constructor is removed from the QQmlListProperty documentation and replaced with new one and are very similar..