Access an attribute c++ object from QML

Question

I am having a problem understanding how to use a c++ singleton object from qml. I know that I have to inherit my classes from the QObject class and that I have to expose the properties. And that in order for them to be usable in the qml I have to do setContextProperty("ClassName", &class name).

However, if we admit that this class contains another object and that I want to be able to use it from the qml, I get errors like "cannot call method name" from undefined object.

Example:

APi.h

 class API : public QObject {
  Q_OBJECT
  Q_PROPERTY(User *user READ getUser WRITE setUser NOTIFY userChanged)
public:
  Q_INVOKABLE inline User *getUser() const {
    qDebug() << "get called";
    return user;
  };
  inline void setUser(User *new_user) { this->user = new_user; };

signals:
  void userChanged();

private:
  User *user;
};

User.h

#ifndef USER_H
#define USER_H

#include <QObject>
#include <QString>

class User: public QObject{
Q_OBJECT
public:
    Q_INVOKABLE inline QString &getName(){return name;}; // qrc:/main.qml:63: TypeError: Cannot call method 'getName' of undefined
private:
    QString name;
};
#endif // USER_H

main.cpp

#include <QQmlContext>
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QtQuick3D/qquick3d.h>
#include "API.h"
#include "User.h"

int main(int argc, char *argv[]) {
  QGuiApplication app(argc, argv);
  QQmlApplicationEngine engine;
  API api;
  User user;
  api.setUser(&user);
  engine.rootContext()->setContextProperty("API", &api);

  qmlRegisterType<User>("User", 1, 0, "User");

  engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
  if (engine.rootObjects().isEmpty())
    return -1;

  return app.exec();
}

main.qml

import QtQuick
import QtQuick3D
import QtQuick.Controls
import QtQuick.Layouts

Window {

    Item{
        id: test
        Component.onCompleted: {
            console.log("Completed")
            API.getUser() // no error, getUser is called without error
            console.log(API.getUser().getName())
        }
    }
}

What possibilities do I have to access the User object from the qml through API?

Please tighten up your example (use the edit button) to remove typo's like Q_Object. — Adriaan de Groot
– Adriaan de Groot, Commented Jan 17, 2022 at 22:07
Your example is not complete. Please provide a minimal reproducible example. As an example of why this is needed, you aren't showing us where you are loading your qml. — JarMan
– JarMan, Commented Jan 17, 2022 at 22:41
OK, I edited it and created a simple reproducible example as you intented for — jdenozi
– jdenozi, Commented Jan 17, 2022 at 23:26

Arun Kumar B · Accepted Answer · 2022-01-18 09:32:01Z

1

You could do any of the following:

Add a public slot in API class:

API.cpp:
QString API::getUserName()
{
    return user.getName();
}

main.qml:
Component.onCompleted: console.log( API.getUserName() )

Make User a Q_PROPERTY in API class:

API.h:
Q_PROPERTY( User* user READ user NOTIFY userChanged)

main.qml:
Component.onCompleted: console.log( API.user.getName() )

Register User with QML:

main.cpp:
qmlRegisterType<User>("MyUser", 1, 0, "MyUser");

main.qml:
Component.onCompleted: console.log( API.getUser().getName() )

edited Jan 18, 2022 at 9:32

answered Jan 18, 2022 at 9:16

Arun Kumar B

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1 Comment

jdenozi Over a year ago

Ok perfect, the second one is what I looking for. But I noticed that I can't used &getName cause of the reference instead of it just use getName

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