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void Form3::on_pushButton_clicked()
{
    QString ret;
    ret = m_ui.lineEdit->text();



    QTreeWidgetItem* item =  new QTreeWidgetItem(0);
    m_ui.treeWidget->addTopLevelItem(item);
    item->setText(0,ret);

}

void MainWindow::on_pushButton_2_clicked()
{

 Form3* st = new   Form3;

    st->show();

}

class Form3 : public QWidget {
    Q_OBJECT
public:
    Form3(QWidget *parent = 0);
    Ui::Form3 m_ui;

protected:
    void changeEvent(QEvent *e);



private slots:
    void on_pushButton_clicked();
};

http://doc.trolltech.com/4.5/qsettings.html

 void MyMainWindow::closeEvent(QCloseEvent *event)
 {
     QSettings settings("MyCompany", "MyApp");
     settings.setValue("geometry", saveGeometry());
     settings.setValue("windowState", saveState());
     QMainWindow::closeEvent(event);
 }
 void MainWindow::readSettings()
 {
     QSettings settings("MyCompany", "MyApp");
     restoreGeometry(settings.value("myWidget/geometry").toByteArray());
     restoreState(settings.value("myWidget/windowState").toByteArray());
 }

где-то объявлено QSettings settings;

void saveItem(QTreeWidgetItem* item)
{
    settings.beginGroup(item->data(0, Qt::DisplayRole));
    settings.setValue("1", item->data(1, Qt::DisplayRole));
    settings.setValue("2", item->data(2, Qt::DisplayRole)); //и тд
    for(int i = 0; i < item->childCount(); i++) {
          QTreeWidgetItem* child = item->child(i);
          saveItem(child);
    }
    settings.endGroup();
}

voidSaveTree()
{
    settings.beginGroup("tree");
    saveItem(topItem)
    for(int i = 0; i < treeWidget->topLevelItemCount(); i++) {
          QTreeWidgetItem* item= treeWidget->takeTopLevelItem(i);
          saveItem(item);
    }
    settings.endGroup();
}

void Form3::writeSettings()

{  QTreeWidgetItem* item;
    QSettings settings("Tr", "App");
    //for(int i = 0; i < treeWidget->topLevelItemCount(); i++) {
          //QTreeWidgetItem* item= treeWidget->takeTopLevelItem(i);
          settings.setValue("1", item->data(0, Qt::DisplayRole));
          settings.setValue("2", item->text(0));
   // }

}

void Form3::readSettings()

{
    QSettings settings("Tr", "App");
    QString str = settings.value("2").toString();
    m_ui.treeWidget->topLevelItem(0)->setText(0, str);
}

void Form3::closeEvent(QCloseEvent *event)

{

        writeSettings();
         event->accept();

}

QTreeWidgetItem *item = new QTreeWidgetItem ();
item->setText(0, settings.value("2").toString());
m_ui.treeWidget->addTopLevelItem(item);

    for(int i = 0; i< widget->topLevelItemCount(); i++) {
        QString val = widget->topLevelItem(i)->text(0);
        qDebug() << val;
        settings.setValue(QString::number(i), val);
    }