First Create Your C++ Class

This will be the class that has a method that gets called from QML.

This class must be derived from QObject.
Decorate your method with Q_INVOKABLE .
Nothing special in the .cpp file.

class MyClass : public QObject
{
	Q_OBJECT

public: 
	explicit MyClass(QObject *parent = nullptr);

	Q_INVOKABLE void helloWorld();
	Q_INVOKABLE QString echo(QString value);
}

In my CPP file, I just qDebug evidence that these methods are called.

Set the Context Property

In main, we have to tell Qt that the method will be called.

#include <QGuiApplication>
#include <QQmlContext>
#include <QQmlApplicationEngine>
#include "myclass.h"

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

    QQmlApplicationEngine engine;

	QQmlContext *context = engine.rootContext();
	
    MyClass my_obj;
	conttext->setContextProperty("MyObj", &my_obj);

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

    return app.exec();
}

The “MyObj” value will be the name of the instantiated object we use to call methods from in the QML file.
setContextProperty is the bridge that joins the class with QML/JavaScript.

Call the Method from QML

You can now call the method as if you were using a JavaScript object method.

import QtQuick 2.12
import QtQuick.Window 2.12
import QtQuick.Controls 2.5

Window {
    visible: true
    title: qsTr("Hello World")

    Column {
            Button{
                text : "Hello World"
                onClicked: {
                    MyObj.helloWorld();
                }
            }
            Rectangle {
                width: textToShowId.implicitWidth + 20
                height: 50
                color: "white"
                Text{
                    id : textToShowId
                    text : MyObj.echo("Rectangle text will be the return " +
	                    "value of this method.");
                }
            }
        }
}