Data back from QVariant to QJsonDocument/QJsonObject does not work.

#include <QCoreApplication>
#include <QString>
#include <QJsonObject>
#include <QBuffer>
#include <QSettings>
#include <QJsonArray>
#include <QJsonDocument>
class User
{
public:
 User() {}
 User(const QString& ruser_name, const int& ruser_age)
 {
 user_name = ruser_name;
 user_age = ruser_age;
 }

QString get_user_name() const \{ return user_name; }

void write(QJsonObject &json) const
 {
 json["user_name"] = user_name;
 json["user_age"] = user_age;
 }

void read(const QJsonObject &json)
 {
 if (json.contains("user_name") && json["user_name"].isString())
 user_name = json["user_name"].toString();

if (json.contains("user_age"))
 user_age = json["user_name"].toInt();
 }
private:
 QString user_name;
 int user_age;
};

int main(int argc, char *argv[])
{
 QCoreApplication a(argc, argv);

QMap<QString, User> user_list;
 user_list["Tom"] = User("Tom",10);
 user_list["Henry"] = User("Henry",20);

QJsonArray userListArray;

QMapIterator<QString,User> iterator(user_list);

while (iterator.hasNext())
 {
 iterator.next();
 QJsonObject userJsonObject;
 User userObj = iterator.value();
 userObj.write(userJsonObject);
 userListArray.append(userJsonObject);
 }

QJsonObject user_list_object_wrote;
 user_list_object_wrote["user_list"] = userListArray;

QJsonDocument user_list_doc_wrote(user_list_object_wrote);

QVariant user_list_variant = user_list_doc_wrote.toVariant();

// Now read the data back into QJsonDocument/QJsonObject
 QJsonDocument user_list_doc_read = user_list_variant.toJsonDocument();

QJsonObject user_list_object_read = user_list_variant.toJsonObject();

return a.exec();
}