Solved QQmlContext::setContextProperty: property is not defined.

[Ibrahim]

Hi;

// main.cpp
#include <QGuiApplication>
#include <QQmlApplicationEngine>
#include <QQmlContext>

int main(int argc, char* argv[])
{
  QGuiApplication app(argc, argv);

  QQmlApplicationEngine engine;
  engine.load(QUrl(QStringLiteral("qrc:/main.qml")));
  QQmlContext* context = new QQmlContext(engine.rootContext());
  context->setContextProperty("num", 77);

  return app.exec();
}

// MainForm.ui.qml
import QtQuick 2.5

Rectangle {
  width: 360
  height: 360
  property alias text0: text0.text
  Text {
    id: text0
    x: parent.width / 2
    y: parent.height / 2
  }
}
// main.qml
import QtQuick 2.5
import QtQuick.Window 2.2

Window {
  visible: true

  MainForm {
    anchors.fill: parent
    text0: num
  }
}

This code is getting this error message: qrc:/main.qml:9: ReferenceError: num is not defined but I defined num property in main.cpp. What is the problem? Thanks.

[Charby]

The problem is that you don't provide the property to the root context but to a copy...
you should rather use this :

QQmlContext* context = engine.rootContext();
context->setContextProperty("num", 77);
//engine.rootContext()->setContextProperty("num", 77); //even simpler

[Ibrahim]

@Charby Thanks. The problem solved.

[jpnurmi]

Context properties must be set before loading QML that references the context properties.

[zeFreelance]

I copied the (non-working) code from a Qt 5.12.3 help page.
I am not sure why they put it in examples if that code doesn't work. :(
@Charby your code works seamlessly. Thanks a ton for clarifying this to all of us!

[SGaist]

Hi @zeFreelance and welcome to devnet,

What example would that be ?

[fcarney]

@SGaist said in QQmlContext::setContextProperty: property is not defined.:

What example would that be ?

I have seen this bad code and copy and pasted it myself without realizing I was creating a new context object. It was on stackoverflow if I remember right. I don't think I have seen this as an example on Qt docs pages though.