QJsonValue::isUndefined does not work correctly
<pre><code>#include &lt;QCoreApplication&gt;
#include &lt;QJsonDocument&gt;
#include &lt;QJsonValue&gt;
#include &lt;QJsonObject&gt;

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    char json[] = "{ id: 12}";
    QJsonDocument doc = QJsonDocument::fromJson(json);
    QJsonValue v = doc.object()["no-defined"];
    Q_ASSERT(v.isUndefined());  // assert failed.

    return a.exec();
}
</code></pre>

<p>I want to use <code>QJsonValue::isUndefined()</code> to judge if I define the key, but in the test code, <code>v.isUndefined()</code> return false, not true. I don't know the reason, is there any  explanation?</p>