To learn how to save data into an XML file using the QDomDocument class, let's do the following:

1. Add the second button to the UI, called Save XML:

2. Right-click on the Save XML button and select Go to slot…. Then, pick the clicked() option and click OK. A new clicked() slot function will be added to your source files.
3. Write the following code within the button's clicked() slot function:

void MainWindow::on_saveXmlButton_clicked() {
    QString filename = QFileDialog::getSaveFileName(this, "Save Xml", ".", "Xml files (*.xml)");
    QFile file(filename);
    if (!file.open(QFile::WriteOnly | QFile::Text)) {
        qDebug() << "Error saving XML file.";
        file.close();
        return;
    }

4. The previous code continues:

    QDomDocument xml("contact");

    // John Doe
    QDomElement root = xml.createElement("contact");
    root.setAttribute("category", "Family");
    xml.appendChild(root);

    QDomElement tagName = xml.createElement("name");
    root.appendChild(tagName);
    QDomText textName = xml.createTextNode("John Doe");
    tagName.appendChild(textName);

5. Save the age and address elements:

    QDomElement tagAge = xml.createElement("age");
    root.appendChild(tagAge);
    QDomText textAge = xml.createTextNode("32");
    tagAge.appendChild(textAge);

    QDomElement tagAddress = xml.createElement("address");
    root.appendChild(tagAddress);
    QDomText textAddress = xml.createTextNode("114B, 2nd Floor, Sterling Apartment, Morrisontown");
    tagAddress.appendChild(textAddress);

6. Let's continue with the phone element. Proceed to the next contact:

    QDomElement tagPhone = xml.createElement("phone");
    root.appendChild(tagPhone);
    QDomText textPhone = xml.createTextNode("0221743566");
    tagPhone.appendChild(textPhone);
    
    // Jane Smith
    QDomElement root2 = xml.createElement("contact");
    root2.setAttribute("category", "Friend");
    xml.appendChild(root2);

7. Save the name and age:

    QDomElement tagName2 = xml.createElement("name");
    root2.appendChild(tagName2);
    QDomText textName2 = xml.createTextNode("Jane Smith");
    tagName2.appendChild(textName2);

    QDomElement tagAge2 = xml.createElement("age");
    root2.appendChild(tagAge2);
    QDomText textAge2 = xml.createTextNode("24");
    tagAge2.appendChild(textAge2);

8. Save the address and phone:

    QDomElement tagAddress2 = xml.createElement("address");
    root2.appendChild(tagAddress2);
    QDomText textAddress2 = xml.createTextNode("13, Ave Park, Alexandria");
    tagAddress2.appendChild(textAddress2);

    QDomElement tagPhone2 = xml.createElement("phone");
    root2.appendChild(tagPhone2);
    QDomText textPhone2 = xml.createTextNode("0025728396");
    tagPhone2.appendChild(textPhone2);

9. Stream all the data into the XML file:

    // Save to file
    QTextStream output(&file);
    output << xml.toString();
    file.close();
}

10. Compile and run the program and click on the Save XML button. Enter your desired filename in the save file dialog and click Save.
11. Open the XML file you saved in step 4 with any text editor and you should see something like this:

<!DOCTYPE contact>
<contact category="Family">
    <name>John Doe</name>
    <age>32</age>
    <address>114B, 2nd Floor, Sterling Apartment, Morrisontown</address>
    <phone>0221743566</phone>
</contact>

12. We can also see the second contact under it:

<contact category="Friend">
    <name>Jane Smith</name>
    <age>24</age>
    <address>13, Ave Park, Alexandria</address>
    <phone>0025728396</phone>
</contact>