ReferenceError to root window id when creating QML from C++

[ChrisTof]

I want to load QML files from within C++.
The problem is that the main window cannot be referenced by id from a different QML file.
I create the main window first:

    QGuiApplication  app(argc, argv);
    QQmlApplicationEngine engine;
    engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
    QQuickWindow *window = qobject_cast<QQuickWindow*>(engine.rootObjects().at(0));
    if (!window) {
        qFatal("Error: Your root item has to be a window.");
        return;
    }
    window->show();
    root = window->contentItem();

main.qml:

import QtQuick 2.12
import QtQuick.Window 2.12

Window {
    id: root
    width: 640
    height: 480
}

Then, I add a rectangle:

QQmlComponent component(enginePtr, QUrl("qrc:/rec.qml"));
QQuickItem* object = qobject_cast<QQuickItem*>(component.beginCreate(enginePtr->rootContext()));

    if(object == nullptr) 
         return;

QQmlEngine::setObjectOwnership(object, QQmlEngine::CppOwnership);
object->setParentItem(root);
object->setParent(enginePtr);
component.completeCreate();

rec.qml:

import QtQuick 2.0
Rectangle {
width:100
color:"red"
    Component.onCompleted: {
        height = root.height/Math.max(sourceSize.height, sourceSize.width) * scale;
    }
}

As you can see, rectangle height is calculated from root.height, which is MainWindow height. But, this results in the error:

ReferenceError: root is not defined

How can I refer to the properties of the main window from a different QML file?

[GrecKo]

Do you have a specific reason doing all that in C++?
It's considered bad practice.

[raven-worx]

@GrecKo said in ReferenceError to root window id when creating QML from C++:

It's considered bad practice.

what? why?

@ChrisTof said in ReferenceError to root window id when creating QML from C++:

QQuickItem* object = qobject_cast<QQuickItem*>(component.beginCreate(enginePtr->rootContext()));

you should use the context (or a child context) of the root item, rather the engine's rootContext

QQmlContext* rootItemContext = QQmlEngine::contextForObject(root);

[GrecKo]

@raven-worx said in ReferenceError to root window id when creating QML from C++:

what? why?

Instead of paraphrasing, I'll copy those links:

• https://doc.qt.io/qt-5/qtquick-bestpractices.html#interacting-with-qml-from-c
• http://doc.qt.io/qt-5/qtqml-cppintegration-overview.html#interacting-with-qml-objects-from-c
• https://youtu.be/vzs5VPTf4QQ?t=23m20s

[ChrisTof]

@GrecKo
Yes, I have reasons to do all of that in C++. Even more, I have no choice.

@raven-worx
Thank you very much! That is what I needed. In my case, I have to refer to the window variable:

QQmlContext* rootItemContext = QQmlEngine::contextForObject(window);
QQuickItem *object = qobject_cast<QQuickItem*>(component.beginCreate(rootItemContext))