I need to get access for some QML object property for example two radio button from c++ file , I know how to do it in main.cpp.

this is the code for it

#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <qqmlengine.h>
#include <qqmlcontext.h>
#include <qqml.h>
#include <QQmlComponent>
#include <QStringList>
#include "connect_serial_port.h"
#include "tcpip_client.h"
#include "fileio.h"
#include "cmd_do.h"
#include <QDebug>

//#include "ui_settings.h"

int main(int argc, char *argv[])
{
    QCoreApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    qmlRegisterType<FileIO, 1>("FileIO", 1, 0, "FileIO");

    QGuiApplication app(argc, argv);

    QQmlApplicationEngine engine;
    Connect_Serial_Port connect_serial_port;
    Tcpip_Client tcpip_client;
    Cmd_Do cmd_do;

    //Ui_Settings ui_settings;


    QQmlContext *classContext = engine.rootContext();
    classContext->setContextProperty("connect_serial_port", &connect_serial_port);
    classContext->setContextProperty("tcpip_client", &tcpip_client);
    classContext->setContextProperty("cmd_do", &cmd_do);
    //classContext->setContextProperty("Ui_Settings",&ui_settings);

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

    // Step 1: get access to the root object
        QObject *rootObject = engine.rootObjects().first();
        QObject *qmlObject_serial_radio = rootObject->findChild<QObject*>("serial_radio");
        QObject *qmlObject_tcpip_radio = rootObject->findChild<QObject*>("tcpip_radio");
    // Step 2a: set or get the desired property value for the root object
        rootObject->setProperty("visible", true);
        rootObject->setProperty("width", "1200");
        qDebug() << rootObject->property("visible");
        qDebug() << qmlObject_serial_radio->property("checked");
         qDebug() << qmlObject_tcpip_radio->property("checked");








    connect_serial_port.scan_for_ports();

    return app.exec();
}

but how do I do the same in a different c++ file other than main.cpp.
how t o access rootObject and all of its child?