impement operator [] like in QVariantMap for QML - qt

Question

Main goal is make ContextProperty for QML like QVariantMap with access operator []:
For example cpp file:
...
QVariantMap qvm; qvm["zero"] = "it's OK";
view.rootContext()->setContextProperty("qvm",qvm);
...
qml file:
console.log("qvm[zero] === "+qvm["zero"]); // output: qvm[zero] === it's OK
but with my class:
class MyStore : public QObject
{
Q_OBJECT
public:
MyStore(QObject* parent=nullptr):QObject(parent){
}
MyStore(const MyStore &other,QObject* parent=nullptr):QObject(parent){ store = other.store; }
~MyStore(){}
inline const QVariant &operator[](QString i) const {
static const QVariant val(store[i]);
return val;
}
QVariantMap store;
};
Q_DECLARE_METATYPE(MyStore);
cpp file:
store.store["zero"] = "OK only in cpp";
qDebug("main: store[\"zero\"] == %s", store["zero"].toString().toAscii().data()); // output: main: store["zero"] == "OK only in cpp"
view.rootContext()->setContextProperty("mystore",&store);
qml file:
console.log("qml: mystore[\"zero\"] === "+mystore["zero"]); // output: qml: mystore["zero"] === undefined
How to implement for output:
qml: mystore["zero"] === "OK only in cpp"
?