Exposing a C++ variable to QML

Kawa7

Is there a way to expose a C++ variable to QML without risking a memory leak? I have reproduced the problem in this example:

Bug .pro:

QT += quick \
    widgets

CONFIG += c++17

# The following define makes your compiler emit warnings if you use
# any Qt feature that has been marked deprecated (the exact warnings
# depend on your compiler). Refer to the documentation for the
# deprecated API to know how to port your code away from it.
DEFINES += QT_DEPRECATED_WARNINGS

# You can also make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
# You can also select to disable deprecated APIs only up to a certain version of Qt.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
        connection.cpp \
        device.cpp \
        main.cpp

RESOURCES += qml.qrc

# Additional import path used to resolve QML modules in Qt Creator's code model
QML_IMPORT_PATH =

# Additional import path used to resolve QML modules just for Qt Quick Designer
QML_DESIGNER_IMPORT_PATH =

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

HEADERS += \
    connection.h \
    device.h

Device.h:

#ifndef DEVICE_H
#define DEVICE_H

#include <QObject>

class Device : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString name READ name CONSTANT)

private:
    QString m_name;

public:
    explicit Device(QObject *parent = nullptr);
    Device(const Device & device);
    Device(QString name);
    QString name() const;
};

//Q_DECLARE_METATYPE(Device)

#endif // DEVICE_H

Device.cpp:

#include "device.h"

Device::Device(QObject *parent) : QObject(parent)
{
    m_name = "Device name";
}

Device::Device(const Device & device) : QObject()
{
    m_name = device.m_name;
}

Device::Device(QString name) : QObject()
{
    m_name = name;
}

QString Device::name() const
{
    return m_name;
}

Connection.h:

#ifndef CONNECTION_H
#define CONNECTION_H

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

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

public slots:
    void call();

signals:
    void signalWithoutPointer(Device device);
    void signalWithPointer(Device * device);
};

#endif // CONNECTION_H

Connection.cpp:

#include "connection.h"
#include <QDebug>

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

void Connection::call()
{
    qDebug() << "Function call emitted";
    Device device = Device("Device");
    Device * pointer = new Device("Pointeur");
    emit signalWithoutPointer(device); //undefined or converted to QVariant
    emit signalWithPointer(&device); //use after free (only if the receiver is in another thread i.e. Qt::QueuedConnection)
    emit signalWithPointer(pointer); //memory leak
}

main.cpp:

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQuickItem>

#include "connection.h"
#include "device.h"

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

    QGuiApplication app(argc, argv);

    qmlRegisterSingletonType<Connection>("com.bug.connection", 1, 0, "Connection",
                                    [](QQmlEngine * engine, QJSEngine * scriptEngine) -> QObject * {
                                        Q_UNUSED(engine)
                                        Q_UNUSED(scriptEngine)
                                        Connection * connection = new Connection();
                                        return connection;
                                    });
    qmlRegisterType<Device>("com.bug.device", 1, 0, "Device");

    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);

    QQmlComponent component(&engine, QUrl(QStringLiteral("qrc:/main.qml")));
    component.create();
    int exec = app.exec();
    return exec;
}

main.qml:

import QtQuick 2.12
import QtQuick.Controls 2.12
import QtQuick.Layouts 1.12
import QtQuick.Window 2.12
import com.bug.connection 1.0
import com.bug.device 1.0

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

    Text {
        id: text1
        text: qsTr("Default")
    }

    Connections {
        target: Connection
        onSignalWithoutPointer: (device) =>{
            console.log("Signal emitted without pointer : " + device)
            console.log("Device's name without pointer : " + device.name)
            text1.text = device.name
        }
        onSignalWithPointer: (device) =>{
             console.log("Signal emitted with pointer : " + device)
             console.log("Device's name with pointer : " + device.name)
             text1.text = device.name
         }
    }

    Button {
        id: button
        x: 270
        y: 220
        text: qsTr("Button")
        onClicked: {
            console.log("clicked button")
            Connection.call()
        }
    }
}

GrecKo

@Kawa7 Why do you create an instance of an object, emit it and don't keep it around?

You have to define who gets to own it.
Put it in a model/list, return it in a function, etc.

Kawa7

@GrecKo In the context of my project, the event is temporary and I have no reason to keep the instance of this object.

Unsolved Exposing a C++ variable to QML