We will learn how to save data into an XML file through the following steps:

1. Add another button to mainwindow.ui, then set its object name as saveXmlButton and its label as Save XML:

2. Right-click on the button and select Go to slot…. A window will pop up with a list of signals available for selection. Select the clicked() option and click OK. A signal function called on_saveXmlButton_clicked() will now be automatically added to both your mainwindow.h and mainwindow.cpp files by Qt:

3. Add the following code to the on_saveXmlButton_clicked() function:

QXmlStreamWriter xml;
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.";
xml.setDevice(&file);
xml.setAutoFormatting(true);
xml.writeStartDocument();

4. Let's also write the first contact element:

xml.writeStartElement("contact");
xml.writeAttribute("category", "Friend");
xml.writeTextElement("name", "John Doe");
xml.writeTextElement("age", "32");
xml.writeTextElement("address", "114B, 2nd Floor, Sterling Apartment, Morrison Town");
xml.writeTextElement("phone", "0221743566");
xml.writeEndElement();

5. Write the second contact element as follows:

xml.writeStartElement("contact");
xml.writeAttribute("category", "Family");
xml.writeTextElement("name", "Jane Smith");
xml.writeTextElement("age", "24");
xml.writeTextElement("address", "13, Ave Park, Alexandria");
xml.writeTextElement("phone", "0025728396");
xml.writeEndElement();
xml.writeEndDocument();

6. Build and run the program and you should see an additional button on the program UI:

7. Click on the Save XML button and a save file dialog will appear on the screen. Type the filename you desire and click the Save button.
8. Open up the XML file you just saved with any text editor. The first part of the file should look like this:

<?xml version="1.0" encoding="UTF-8"?>
<contact category="Friend">
    <name>John Doe</name>
    <age>32</age>
    <address>114B, 2nd Floor, Sterling Apartment, Morrison Town</address>
    <phone>0221743566</phone>
</contact>

9. The second part of the file should look something like this:

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