Qt5 Tutorial QtXML DOM Reading - 2020

In this tutorial, we will learn how to read in QDOM of QtXML.

To include the definitions of the module's classes, use the following directive:

#include <QtXml>

To link against the module, add this line to our qmake .pro file:

QT += xml

Though the QtXML docuemnt says:

The module is not actively maintained anymore.
Please use the QXmlStreamReader and QXmlStreamWriter classes in Qt Core instead.

We still need to understand QtXML to move on to the other topics.

In this example, we will read in from the following XML document we made in the previous tutorial: QtXML - Writing to a file.

<Dorms>
 <Dorm ID="0" Name="Dorm Building 0">
  <Room ID="0" Name="Room 0"/>
  <Room ID="1" Name="Room 1"/>
  <Room ID="2" Name="Room 2"/>
 </Dorm>
...
</Dorms>

To retrieve the info, the following code will be called by retrievElements(root, "Dorm", "Name").

void retrievElements(QDomElement root, QString tag, QString att)
{
    QDomNodeList nodes = root.elementsByTagName(tag);

    qDebug() << "# nodes = " << nodes.count();
    for(int i = 0; i < nodes.count(); i++)
    {
        QDomNode elm = nodes.at(i);
        if(elm.isElement())
        {
            QDomElement e = elm.toElement();
            qDebug() << e.attribute(att);
        }
    }
}

So, "Dorm" is for tag and "Name" is for attribute.

First, we get QDomNodeList using "Dorm" as tag by calling elementsByTagName(tag), then with that node, we get QDomElement using "Name" as an attribute for the call attribute(att).

The whole code looks like this:

#include <QtCore>
#include <QtXML>
#include <QDebug>

void retrievElements(QDomElement root, QString tag, QString att)
{
    QDomNodeList nodes = root.elementsByTagName(tag);

    qDebug() << "# nodes = " << nodes.count();
    for(int i = 0; i < nodes.count(); i++)
    {
        QDomNode elm = nodes.at(i);
        if(elm.isElement())
        {
            QDomElement e = elm.toElement();
            qDebug() << e.attribute(att);
        }
    }
}

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

    // Create a document to write XML
    QDomDocument document;

    // Open a file for reading
    QFile file("C:/Test/myXLM.xml");
    if(!file.open(QIODevice::ReadOnly | QIODevice::Text))
    {
        qDebug() << "Failed to open the file for reading.";
        return -1;
    }
    else
    {
        // loading
        if(!document.setContent(&file;))
        {
            qDebug() << "Failed to load the file for reading.";
            return -1;
        }
        file.close();
    }

    // Getting root element
    QDomElement root = document.firstChildElement();

    // retrievelements(QDomElement root, QString tag, QString att)
    retrievElements(root, "Dorm", "Name");

    qDebug() << "Reading finished";

    return a.exec();
}

If we run the code now, we get:

# nodes =  5
"Dorm Building 0"
"Dorm Building 1"
"Dorm Building 2"
"Dorm Building 3"
"Dorm Building 4"
Reading finished

We get the 2nd level element but the the 3rd level element ("Room") yet.

    // Getting root element
    QDomElement root = document.firstChildElement();

    // retrievelements(QDomElement root, QString tag, QString att)
    retrievElements(root, "Dorm", "Name");

    qDebug() << "\nGoing deeper level - getting the 'Room'";
    QDomNodeList dorms = root.elementsByTagName("Dorm");
    // Looping through each dorm
    for(int i = 0; i < dorms.count(); i++)
    {
        QDomNode dormnode = dorms.at(i);
        if(dormnode.isElement())
        {
            QDomElement dorm = dormnode.toElement();
            qDebug() << "Rooms in " << dorm.attribute("Name");
            /*
             *-<Dorm Name="Dorm Building 0" ID="0">
             *   <Room Name="Room 0" ID="0"/>
             *   <Room Name="Room 1" ID="1"/>
             *   <Room Name="Room 2" ID="2"/>
             */
            retrievElements(dorm, "Room", "Name");
        }
    }

Run the code, and we get:

# nodes =  5
"Dorm Building 0"
"Dorm Building 1"
"Dorm Building 2"
"Dorm Building 3"
"Dorm Building 4"

Going deeper level - getting the 'Room'
Rooms in  "Dorm Building 0"
# nodes =  3
"Room 0"
"Room 1"
"Room 2"
Rooms in  "Dorm Building 1"
# nodes =  3
"Room 0"
"Room 1"
"Room 2"
Rooms in  "Dorm Building 2"
# nodes =  3
"Room 0"
"Room 1"
"Room 2"
Rooms in  "Dorm Building 3"
# nodes =  3
"Room 0"
"Room 1"
"Room 2"
Rooms in  "Dorm Building 4"
# nodes =  3
"Room 0"
"Room 1"
"Room 2"
Reading finished

Get the source code: main.cpp.

#include <QtCore>
#include <QtXML>
#include <QDebug>

void retrievElements(QDomElement root, QString tag, QString att)
{
    QDomNodeList nodes = root.elementsByTagName(tag);

    qDebug() << "# nodes = " << nodes.count();
    for(int i = 0; i < nodes.count(); i++)
    {
        QDomNode elm = nodes.at(i);
        if(elm.isElement())
        {
            QDomElement e = elm.toElement();
            qDebug() << e.attribute(att);
        }
    }
}

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

    // Create a document to write XML
    QDomDocument document;

    // Open a file for reading
    QFile file("C:/Test/myXLM.xml");
    if(!file.open(QIODevice::ReadOnly | QIODevice::Text))
    {
        qDebug() << "Failed to open the file for reading.";
        return -1;
    }
    else
    {
        // loading
        if(!document.setContent(&file;))
        {
            qDebug() << "Failed to load the file for reading.";
            return -1;
        }
        file.close();
    }

    // Getting root element
    QDomElement root = document.firstChildElement();

    // retrievelements(QDomElement root, QString tag, QString att)
    retrievElements(root, "Dorm", "Name");

    qDebug() << "\nGoing deeper level - getting the 'Room'";
    QDomNodeList dorms = root.elementsByTagName("Dorm");
    // Looping through each dorm
    for(int i = 0; i < dorms.count(); i++)
    {
        QDomNode dormnode = dorms.at(i);
        if(dormnode.isElement())
        {
            QDomElement dorm = dormnode.toElement();
            qDebug() << "Rooms in " << dorm.attribute("Name");
            /*
             *-<Dorm Name="Dorm Building 0" ID="0">
             *   <Room Name="Room 0" ID="0"/>
             *   <Room Name="Room 1" ID="1"/>
             *   <Room Name="Room 2" ID="2"/>
             */
            retrievElements(dorm, "Room", "Name");
        }
    }

    qDebug() << "Reading finished";

    return a.exec();
}