Development/Tutorials/QtDOM Tutorial

Short introduction to XML

XML is a general structured format to store and exchange hierarchical data.

If you know HTML, you'll find XML quite similar (in fact, after some small modifications, a HTML file is a valid XML file): XML uses nested tags of the form <tagname>...</tagname> for tags with contents and <tagname/> for tags without content. Each tag can contain other tags, and the tag itself can have attributes of the form <tagname attribute=value>...</tagname>.

The name of the tags is not restricted (unlike HTML, which only defines a given set of proper HTML tags), so you can choose whatever name fits your needs.

As an example, let us assume that you want to store holiday information into a file and use Qt to load or modify it. To get a feeling for how XML looks like, here is one possible format for such a holiday file: <?xml version='1.0' encoding='UTF-8'?> <holidayset country="at">

 <name>Holidays for Austria</name>
 <holiday>
   <name>New Year's Day</name>
   <date>2007-01-01</date>
 </holiday>
 <holiday>
   <name>Christmas</name>
   <date>2007-12-24</date>
 </holiday>

</holidayset> This file defines a holiday set for Austria (notice the country="at" attribute to the holidayset tag). The holiday set, enclosed in <holidayset>...</holidayset> contains two holidays, each enclosed with <holiday>...</holiday>. Each of these holiday elements contains the settings for that holiday enclosed in appropriately named tag.

Notice that we did use the same tag <name> inside the <holidayset> and inside the <holiday> tags. Also note that we implicitly used specially formated (ISO-formatted) contents for the date tags, without yet specifying it. Also note that we used quite generic names for the tags, which might become a problem with complexer structure, when we want to use the same name for different purposes.

We will use this example throughout this tutorial. In our application, we want to store the holiday set in the following class:

class Holiday { public:

 Holiday() {}
 ~Holiday() {}

 QDate mDate;
 QString mName;

};

class HolidaySet { public:

 HolidaySet( const QString &c ) : mCountry( c ) {}
 ~HolidaySet() {}

 QString mCountry, mName;
 QList<Holiday> mHolidays;

};

In production code, you would not make the member variables public and directly access them, but rather add accessors and setter functions: QDate date() { return mDate; } void setDate( const QDate &date ) { mDate = date; } To save space, I decided to neglect that rule of thumb here in this example. As this is a tutorial for XML and Qt DOM, I want to concentrate on the basics of Qt DOM and not on a good general programming style.

As there are only so many sensible names, sooner or later you will find out that you will use the same tagname or attribute name for different cases with different meanings. That is the point where namespaces come in.

Creating a simple XML file with Qt DOM

Let us first look at how to use the Qt classes to generate the XML for the holiday file from the HolidaySet class that you have in memory. For this purpose, Qt offers the classes QDomDocument to represent the whole document and QDomNode and QDomElement to represent each individual tag and attribute. As a line of code says more then a thousand words, let us look at some sample code to generate the DOM tree from the HolidaySet class:

/* Helper function to generate a DOM Element for the given DOM document

  and append it to the children of the given node. */

QDomElement addElement( QDomDocument &doc, QDomNode &node,

                       const QString &tag, 
                       const QString &value = QString::null )

{

 QDomElement el = doc.createElement( tag );
 node.appendChild( el );
 if ( !value.isNull() ) {
   QDomText txt = doc.createTextNode( value );
   el.appendChild( txt );
 }
 return el;

}

QString holidaySetToXML( const HolidaySet &hs ) {

 QDomDocument doc;

 // generate the <holidayset> tag as the root tag, add the country 
 // attribute if needed
 QDomElement holidaySetElement = addElement( doc, doc, "holidayset" );
 if ( !hs.mCountry.isEmpty() ) 
   holidaySetElement.setAttribute( "country", hs.mCountry );
 
 // Add the <name> and <comment> elements to the holidayset
 if ( !hs.mName.isEmpty() ) 
   addElement( doc, holidaySetElement, "name", hs.mName );

 // Add each holiday as a <holiday>..</holiday> element
 QList<Holiday>::iterator i;
 for ( i = hs.mHolidays.begin(); i != hs.mHolidays.end(); ++i) {
    QDomElement h = addElement( doc, holidaySetElement, "holiday" );
    addElement( doc, h, "name", (*i).mName );
    addElement( doc, h, "date", (*i).mDate.toString( Qt::ISODate ) );
 }

 return doc.toString();

}

Introduction to XML Namespaces

<?xml version='1.0' encoding='UTF-8'?> <h:holidays xmlns:h="urn:kde:developer:tutorials:QtDom:holidays" h:country="at">

 <h:holiday>
   <h:name>New Year's Day</h:name>
   <h:date>2007-01-01</h:date>
 </h:holiday>
 <h:holiday>
   <h:name>Christmas</h:name>
   <h:date>2007-12-24</h:date>
 </h:holiday>

</h:holidays>

Loading XML documents using Qt and the SAX parser

Initial Author: Reinhold Kainhofer