How do I interact with QML from C++ in QtQuick 2.0

[Itehnological]

Let's say that we have a very simple QML file, like this one:

@import QtQuick 2.0

Rectangle {
width: 800
height: 600
color: '#000'

Text {
    text: qsTr("Hi all")
    anchors.centerIn: parent
}

}
@

The QML File is loaded with the QtQuick2ApplicationViewer helper class, like this:
@QtQuick2ApplicationViewer viewer;
viewer.setMainQmlFile(QStringLiteral("qml/MyApp/Login/Window.qml"));
viewer.showFullScreen();
@

How should I proceed, if for example I would like to change the Rectangle's color to white, from C++. My guess was:
@QQuickItem *window = viewer.rootObject();
window->setProperty("color", "#fff");
@
But all that does is the following compiler error:
@invalid use of incomplete type 'struct QQuickItem'
forward declaration of 'struct QQuickItem'
@

[hipersayan_x]

You forgot the:

@
#include <QQuickItem>
@

before the use.

[Itehnological]

Yhea. Didn't noticed I was missing that library. I spent a whole day digging on why that doesn't work. Thanks a lot!