How to update a variable updated in c++ in qml?

I know how to update qml variables using Q_PROPERTY.

class _data: public QObject {
Q_OBJECT

public:
    _data();
    ~_data();

    Q_PROPERTY(int testflag READ getTestFlag WRITE setTestFlag NOTIFY testFlagChanged);
    int m_testFlag;
    auto getTestFlag() -> int {return m_testFlag;}
    auto setTestFlag(int data) -> void {this->m_testFlag = data; emit testFlagChanged();}
signals:
    void testFlagChanged();
}

When updating the m_testFlag variable in c++ using the code above, the testFlagChanged() signal must be called forcibly to update the value in qml.

_data* mydata;
int main(int argc, char *argv[]) {
    QApplication app(argc, argv);
    QQuickView view;
    mydata = new _data;
    view.rootContext()->setContextProperty("mydata", data);
    view.show();
    app.installEventFilter(new myEventFilter(&view)); // Added an event filter to operate the Update() function when a keyboard event is received. I checked that it works, so I omitted the separate code.
    return app.exec();
}

void update() {
    mydata->m_testFlag = 10;
    mydata->testFlagChanged();
}

Isn't there a way to update the qml variable in a more elegant way other than forcibly calling the testFlagChanged() signal?

I've been looking for something like signal bind, but I haven't been able to find any examples I can use. I need help.

> Edit

I posted a question with test code, but I didn't expect it to receive so much criticism.
I will fix it in the function update() function and write more details about my question. So calm down and take a look at the code.

I am trying to update m_testFlag variable in c++ by declaring it as public in a class called data.
At this time, I want the changed() signal to be generated automatically when the variable value is changed, rather than forcibly generated by the developer. Is there any better way?

"Isn't there a way to update the qml variable in a more elegant way other than forcibly calling the testFlagChanged() signal?" -- A "more elegant way" can be subjective, but I think all you need to do is instead of this:

    mydata->m_testFlag = 10;
    mydata->testFlagChanged();

call this:

    mydata->setTextFlag(10);

Also, don't make m_testFlag public unless you have a really good reason to. It should be private. That's the whole purpose of having getter/setter functions, so outside objects don't need to know the internals of the class.