class MyXmlWriter {
  public:
    MyXmlWriter();
    ~MyXmlWriter();
    QString writeToString();
  private:
    QXmlStreamWriter writer;
}

QString MyXmlWriter::writeToString(){
   QString result;
   writer(&result); // at this became the error: no match for call to '(QXmlStreamWriter) (QString*)'
   xmlWriter.setAutoFormatting(true);
}