I am learning exchanging data between qml and C++ class. So I write this simple test program.

I wrote a simple QML file

import QtQuick 2.7
import QtQuick.Controls 2.0
import QtQuick.Layouts 1.0

//import testing 1.0

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

    property string count: "hello"

    Rectangle
    {
        width: parent.width
        height: parent.height
        Text {
            id: text
            text: qsTr("text")
            anchors.horizontalCenter: parent.horizontalCenter
            anchors.verticalCenter: parent.verticalCenter
        }
    }
    MouseArea
    {
        anchors.fill: parent
        onClicked:
        {
            console.log("count " + count)
        }
    }
}

and simple main.cpp

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include "updatecounter.h"

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

//    qmlRegisterType<UpdateCounter>("testing", 1, 0, "UpdateCounter");

    QQmlApplicationEngine engine;
    engine.load(QUrl(QLatin1String("qrc:/main.qml")));

    return app.exec();
}

Everything is fine up to here. Then I added my C++ class

#ifndef UPDATECOUNTER_H
#define UPDATECOUNTER_H

#include <QObject>

class UpdateCounter : public QObject
{
    Q_OBJECT
    Q_PROPERTY(QString counter READ counter WRITE setCounter NOTIFY counterChanged)
public:
    explicit UpdateCounter(QObject *parent = 0);

    QString counter() const {return mcounter;}
    void setCounter(const QString &counter);
    Q_INVOKABLE void sendCounterUpdate(const QString &counter);

signals:
    void counterChanged();

private:
    QString mcounter;
};

#endif // UPDATECOUNTER_H

#include "updatecounter.h"

UpdateCounter::UpdateCounter(QObject *parent) : QObject(parent)
{

}

void UpdateCounter::setCounter(const QString &counter)
{
    if(counter == mcounter)
        return;
    mcounter = counter;
}

void UpdateCounter::sendCounterUpdate(const QString &counter)
{
    if(counter == mcounter)
        return;
    mcounter = counter;
    emit counterChanged();
}

The program behaves as expected. Then I register my class using qmlRegisterType. I get linking errors.

main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual struct QMetaObject const * __cdecl UpdateCounter::metaObject(void)const " (?metaObject@UpdateCounter@@UEBAPEBUQMetaObject@@XZ)
main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual void * __cdecl UpdateCounter::qt_metacast(char const *)" (?qt_metacast@UpdateCounter@@UEAAPEAXPEBD@Z)
main.obj:-1: error: LNK2001: unresolved external symbol "public: virtual int __cdecl UpdateCounter::qt_metacall(enum QMetaObject::Call,int,void * *)" (?qt_metacall@UpdateCounter@@UEAAHW4Call@QMetaObject@@HPEAPEAX@Z)
main.obj:-1: error: LNK2019: unresolved external symbol "public: __cdecl UpdateCounter::UpdateCounter(class QObject *)" (??0UpdateCounter@@QEAA@PEAVQObject@@@Z) referenced in function "public: __cdecl QQmlPrivate::QQmlElement<class UpdateCounter>::QQmlElement<class UpdateCounter>(void)" (??0?$QQmlElement@VUpdateCounter@@@QQmlPrivate@@QEAA@XZ)
main.obj:-1: error: LNK2001: unresolved external symbol "public: static struct QMetaObject const UpdateCounter::staticMetaObject" (?staticMetaObject@UpdateCounter@@2UQMetaObject@@B)

Please let me know what I did wrong.

Thank you,
Anthony

[Added code tags ~kshegunov]