Passing an object from QML to C++

Question

I am working on a simple app to demonstrate integration of C++ with QML but I have problems. In nutshell I want to create a C++ object in qml on fly and pass it to C++ for processing. Here is my code.

import QtQuick 2.4
import QtQuick.Window 2.2

import test 1.0

Window {
    visible: true
    MainForm {
        anchors.fill: parent

        mouseArea.onClicked: {

            console.log("clicked")

            var student = Qt.createComponent("Student")

            student.name = "Hello frome QML";

            console.log( student.name ) // good

            school.addStudent( student )

         }
   }
}

Student.h

#ifndef STUDENT_H
#define STUDENT_H

#include <QObject>

class Student : public QObject
{
    Q_OBJECT
    QString m_name;

public:
    explicit Student(QObject *parent = 0);
    ~Student();

    Q_PROPERTY(QString name READ name WRITE setName NOTIFY nameChanged)

QString name() const
{
    return m_name;
}

signals:

void nameChanged(QString arg);

public slots:
void setName(QString arg)
{
    if (m_name == arg)
        return;

    m_name = arg;
    emit nameChanged(arg);
}
};

#endif // STUDENT_H

Now when I addStudent() to the school class, there is where there is trouble. The concern .cpp file below.

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

School::School(QObject *parent) : QObject(parent)
{

}

School::~School()
{

}

// why does this not work? it cause compiler error..can't access private members of QObject
//void School::addStudent(Student student)
//{
////    students.append( &student );

////    qDebug() << "added";

////    qDebug() << student.name();// << " was added to school";
//}

void School::addStudent(Student * student)
{
    students.append( student );

    qDebug() << "added";

    qDebug() << student->name();// if this line is there, it breaks the application
}

The questions are also in comments inside the code but the summaries:

1. Why does addStudent() crashes when I try to access student.name? I can access it though in the qml file after I set it.

2. Why does my project not compile if I pass Student by value like seen in the code?

Additional thoughts

It seems like it may have to do with the C++ object getting destroyed by the time C++ function execute. What is the life cycle of C++ object created in QML? When does it gets destroyed? Could the lifetime of the object be an issue here?

Answer 1

Assume that you register the Student type to QML via qmlRegisterType in cpp file.

In your QML, Qt.createComponent("Student") does not create a Student object but a QQmlComponent . Try to set properties other than name and it still works since the object is not a Student . Instead, you should create an object from a pre-defined component via Component.createObject :

MainForm {
    id: mainForm
    mouseArea.onClicked: {
        var student = studentComponent.createObject(mainForm);
        student.name = "Hello frome QML";
        school.addStudent( student );
    }

    Component {
        id: studentComponent
        Student{}
    }
}

And everything works fine now.

---

Back to your code,

//QML
var student = Qt.createComponent("Student")
student.name = "Hello frome QML";
console.log( student.name ) // good

No, it's not good. student.abcdefg = "Hello from QML" works, too. Add console.log(student) and you can see that student is not a Student .

Why does addStudent() crashes when I try to access student.name? I can access it though in the qml file after I set it.

It crashed since the object passed from QML is not a pointer Student . QML engine passes a null pointer to this function. What you accessed in QML is not a Student .

Why does my project not compile if I pass Student by value like seen in the code?

Because Student is an QObject , which is not copyable. Pass by pointer instead.

What is the life cycle of C++ object created in QML? When does it gets destroyed? Could the lifetime of the object be an issue here?

The lifetime is not a issue in your QML since what you created is a component, not an object. When creating Student object in this answer,

var student = studentComponent.createObject(mainForm);

the newly created object sets mainForm as it's parent. The object won't be deleted until mainForm is released. Except you delete it explicitly.

Answer 2

Try moving the declaration of the function QString name() const in the declaration of class Student to before the Q_PROPERTY macro. Or specify "public:" again before the name function. The macro is resetting the access specifier to "private".