Get QML root object from anywhere in a code in Qt 5.3

Question

1. How to retrieve a root object (the code below returns 0, rootContext()->findChild() also returns 0) from anywhere in a C++ code (a class method where the class is a registered QML type and the class component definition is a direct child of the root, see objectName) and add a generated QQuickItem at runtime?
2. There is myclass registered as QML type. How to add a QML Button to the scene using the bottommost code?

In the main.qml (fragment)

import QtQuick 2.3
import QtQuick.Window 2.2

Window {
    objectName: root

    visible: true
    width: 360
    height: 100

    // ...
}

myclass.cpp

void myclass::add_hip()
{
    setProperty("hip", 87);

    QQmlEngine *engine = QtQml::qmlEngine(this);

//    QObject *root = engine->rootContext()->findChild<QObject*>("womanObj");
    QQuickWindow *window = qobject_cast<QQuickWindow *>(root);
    QObject *wobj = window->findChild<QObject *>("womanObj");
    // 1. Define a QML component.
    QQmlComponent *readHipComp = new QQmlComponent(engine);
    readHipComp->setData("import QtQuick.Controls 1.2\n\
                         Button {\n\
                             anchors.top: addHipBtn.top\n\
                             anchors.left: addHipBtn.left\n\
                             anchors.margins: 3\n\
                             text: \"Hip value\"\n\
                             onClicked: {\n\
                                 msgDlg.text = myclass.hip\n\
                                 msgDlg.open()\n\
                             }\
                         }", QUrl());
    // 2. Create the component instance.
    QQuickItem *readHipBtn = qobject_cast<QQuickItem *>(readHipComp->create());
    // 3. Add the instance to the scene.
//    readHipBtn->setParentItem(qobject_cast<QQuickItem *>(engine->rootContext()->contextObject()));

//    QObject *root = QtQml::qmlContext(this)->findChild<QQuickItem *>("root");
//    readHipBtn->setParent(root);
}

UPDATE

extern QQuickWindow *window;
void myclass::add_hip()
{ 

   //...
 readHipBtn->setParentItem(window->contentItem());
//...
}

worked, but 3.) It does not see other main.qml objects.

Answer 1

You don't need the objectName.

Get root element and check that it's a window:

QQmlApplicationEngine engine;
engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
QObject *root = engine.rootObjects()[0];

QQuickWindow *window = qobject_cast<QQuickWindow *>(root);
if (!window) {
    qFatal("Error: Your root item has to be a window.");
    return -1;
}
window->show();