" Error : Cannot assign [undefined] to QString " while communicating between C++ and QML

[Quentin91]

Hi !

I have a QML code whom I am trying to communicate with with a C++ code.
i have especially some shared object accessed from the QML by :

testText.text=anchor.message

but when doing this, an error is printed in the console:
Error: Cannot assign [undefined] to QString
here is the c++ code of my Anchor class, from QObject

//Anchor.h
class Anchor :public QObject
{
	Q_OBJECT
	Q_PROPERTY(QString message READ message WRITE setMessage NOTIFY messageChanged)
public:
	Anchor();
	Q_INVOKABLE void clicked(/*const QString& clickedStr*/);
	Q_INVOKABLE int getVal(void);

	QString message(void);
	void setMessage(QString);

signals:
	void signalAnchor(QString s);
	void messageChanged();

public slots:
	void slotAnchor(QString s);
private :
	QString msg;
};

#include "Anchor.h"

Anchor::Anchor() :msg("I'm da QString m8")
{
	TRACE_DEBUG("Construction de l'Anchor ")
		if(msg!=nullptr) printQStr(msg)//it works and print the string in the console
}

Q_INVOKABLE void Anchor::clicked(/*const QString& clickedStr*/)
{
	qDebug("Méthode invocable appelee sur click");
}

void Anchor::slotAnchor(QString s)
{
	static int i = 0;
	s.append(" ").append(QString::number(++i));
	qDebug() << "qDebug  : " << s;
	emit signalAnchor(s);
}

Q_INVOKABLE int Anchor::getVal(void)
{
	return 124;
}

QString Anchor::message(void)
{
	printf("getMessage\n");
	return msg;
}
void Anchor::setMessage(QString str)
{
	msg = str;
	emit Anchor::messageChanged();
}

void Anchor::messageChanged(void) {
	printf("messageChanged\n");
}

here is the test functions called in the main:

QGuiApplication app(ai_argc, aipp_argv);

	QQmlApplicationEngine engine;
	engine.addImportPath(QStringLiteral("..\\..\\..\\..\\Tools\\Qt\\5.12.0\\x64\\5.12.0\\msvc2017_64\\qml"));
	Anchor anchor;
	QString  strCppQml = "transmission de chaine du C++ au QML reussie", strQmlCpp = "transmission de chaine du QML au C++ reussie";
	anchor.setProperty("strQmlCpp", strQmlCpp);
	engine.rootContext()->setContextProperty("strCppQml", strCppQml);
//until now, everythin worked. The problem comes from the anchor, from the Q_PROPERTY
	engine.rootContext()->setContextProperty("anchor", &anchor);
	engine.load(QUrl(QML_TEST1_PATH));//the path is correct

	return app.exec();

So i am sure that the QString I'm trying to send is not empty. the object exists
now, the QML

import QtQuick 2.5
import QtQuick.Window 2.5
import "\Items" as Koala
//import "../../Anchor"

Window{
    property var value: anchor.getVal();
    visible: true
    id:mainWindow
    width: 150; height: 150
    color: "#000000"

    Connections{
        target: anchor
        onSignalAnchor:{
            signalText.text=s
            signalText.x=signalText.x+5;
        }
    }
    Text{
       id: testText
       color: "#ffffff"
       text: anchor.message
       anchors.centerIn: parent
       font.pointSize: 10
       visible: false
    }
    Text {
        id: signalText
        x: 27
        y: 102
        color: "#f0f0f0"
        width: 60
        height: 14
        font.pixelSize: 12
    }
    MouseArea{
        anchors.fill: parent

        onClicked:
        {
            anchor.clicked();
            testText.text=anchor.message//there it is!
            testText.visible=true;
            mainWindow.setGeometry(1200,330,500,400)
            signalText.text= value;
        }
        onDoubleClicked: anchor.slotAnchor("calling slotAnchor")
    }
}

Do you know where does that error comes from ? how can I fix it and FINALLY set an efficient communication ?
Thanks for reading

[mranger90]

This is an immediate problem:

void Anchor::messageChanged(void) {
	printf("messageChanged\n");
}

You've provided a definition for the signal. I don't know how you got it to compile because I couldn't.
Once I fixed that the code ran as I suppose it's supposed to.

[Quentin91]

@mranger90 you're right, that's stupid.

I changed it in an empty definition

void Anchor::messageChanged(void) {}

but I have the error anyway.
I think I tried everything now. using qmlRegisterType to import my type, a Q_INVOKABLE method, using the Q_PROPERTY.
In every case, i have this error : cannot assign [undefined] to type
I tried with double, int, QString...

[mranger90]

No, the point is that you have it defined at all. Signals should never have a body.

[Ronak5]

@Quentin91
hey did you try marking your qml strings with "qsTr()" ?
could you try qsTr(anchor.message)?

Also, you can't define signals as mentioned by @mranger90 as it's already defined in moc code. You will get error as multiple definition of the signal function.
Thanks.

[Quentin91]

@Ronak5 no, I didn't. that's probably not the problem any current test is trying to share a bool with QML and the problem is the same. but I'll remember to try it when sharing a string :)

So here is a simpler project and use a Singleton. One of the problem was that I have to declare my object in the QML when using qmlRegisterType. And I don't want to. So here is the new code, the error is the same.
I followed the example of the doc here https://doc.qt.io/qt-5/qtqml-cppintegration-definetypes.html

miniModel.h (the singleton)

#ifndef _MINIMODEL_H_
#define _MINIMODEL_H_
#include <stdlib.h>
#include <stdio.h>
#include <iostream>
#include <string>
#include <QtGui/qguiapplication.h> 
#include <QtQml/qqmlcontext.h>
#include <QtQml/qqmlapplicationengine.h>
#include <QtCore/qdebug.h>
#include <QtCore/qobject.h>
#include <QtCore/qvariant.h>
class MiniModel : public QObject
{
	Q_OBJECT
	Q_PROPERTY(bool miniboule READ miniboule WRITE setMiniboule NOTIFY minibouleChanged)
public:
	MiniModel();
	bool miniboule();
	void setMiniboule(bool bouboule);
signals:
	void minibouleChanged();
private:
	bool m_miniboule;
};
#endif

main.cpp v1 : singleton using a QObject

#include "miniModel.h"

//defining a miniModel instance as a singleton
static QObject* mp_singleton(QQmlEngine* engine, QJSEngine* scriptEngine)
{
	Q_UNUSED(engine)
	Q_UNUSED(scriptEngine)

	MiniModel* miniSingleton = new MiniModel();
	return miniSingleton;
}

int main(int argc=0, char* argv[]=nullptr)
{
	printf("\n launching \n");
	QGuiApplication app(argc, argv);

	qmlRegisterSingletonType<MiniModel>("myModel.miniModel", 1, 0, "MiniModel",mp_singleton);

	QQmlApplicationEngine engine;

	engine.addImportPath(QStringLiteral("..\\..\\..\\..\\Tools\\Qt\\5.12.0\\x64\\5.12.0\\msvc2017_64\\qml"));
	engine.load(QUrl(QStringLiteral("..\\..\\..\\miniModel.qml")));

	return app.exec();
}

main.cpp v2 : singletin using QJSValue

#include "miniModel.h"

static QJSValue m_singletonModel(QQmlEngine* engine, QJSEngine* scriptEngine) {
	Q_UNUSED(engine)

	static bool m_miniboule;
	QJSValue miniModel = scriptEngine->newObject();
	miniModel.setProperty("miniboule", m_miniboule);
	return miniModel;
}

int main(int argc = 0, char* argv[] = nullptr)
{
	printf("\n launching \n");
	QGuiApplication app(argc, argv);

	qmlRegisterSingletonType("myModel.miniModel", 1, 0, "MiniModel", m_singletonModel);

	QQmlApplicationEngine engine;

	engine.addImportPath(QStringLiteral("..\\..\\..\\..\\Tools\\Qt\\5.12.0\\x64\\5.12.0\\msvc2017_64\\qml"));
	engine.load(QUrl(QStringLiteral("..\\..\\..\\miniModel.qml")));

	return app.exec();
}

and the QML. be careful, it's tough

import QtQuick 2.5
import QtQuick.Window 2.5
import QtQuick.Controls 1.4

import myModel.miniModel 1.0 as MyModel
ApplicationWindow {
    id: root
    width: 300
    height: 480
    visible:true
    Text{
       id: textTest
       x: 62
       y: 75
       color: "#d21616"
       text:  "vanilla"
       visible: false//the text is supposed to appear when clicking in the mouseArea
    }
    MouseArea{
        anchors.fill: parent
        onClicked: textTest.visible=  MyModel.Minimodel.miniboule//the boolean I want to acess, defined to true
    }
}

now, the error changed, since I called MyModel.MiniModel.miniboule instead of just MiniModel.miniboule
the error is TypeError: Cannot read property 'miniboule' of undefined