Development/Tutorials/Programming Tutorial KDE 4/How to write an XML parser

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A parser is used to distinguish between formal language and bulk data of a given grammar. See http://en.wikipedia.org/wiki/Parser for more information. There are two ways to write a parser: to split up the content of a file into an object as known from object-oriented programming (the DOM approach) or to trigger a function everytime a reader occurs a given syntax tag (the QXML approach).

The QXML approach

parser.h:

 1 /*
 2  parser.h - demonstration of a parser in C++
 3 */
 4 
 5 #ifndef PARSER_H
 6 #define PARSER_H
 7 
 8 #include <qstring.h>
 9 #include <QtXml/QXmlDefaultHandler>
10 #include <QtXml/QXmlAttributes>
11 
12 class Parser : public QXmlDefaultHandler
13 {
14 public:
15 
16   Parser();
17 
18   /** given by the framework from qxml. Called when parsing the xml-document starts.          */
19   bool startDocument();
20 
21   /** given by the framework from qxml. Called when the reader occurs an open tag (e.g. \<b\> ) */
22   bool startElement( const QString&, const QString&, const QString& qName, const QXmlAttributes& att );
23 
24 };
25 
26 
27 #endif

parser.cpp:

 1 /*
 2  parser.cpp - demonstration of a parser in C++
 3 */
 4 
 5 #include "parser.h"
 6 #include <kdebug.h>
 7 
 8   Parser::Parser()
 9   {
10   }
11   
12   bool Parser::startDocument()
13   {
14     kDebug() << "Searching document for tags";
15     return true;
16   }
17   
18   bool Parser::startElement( const QString&, const QString&, const QString& qName, const QXmlAttributes& att )
19   {
20     kDebug() << "Found Element" << qName;
21     return true;
22  }

hello.cpp:

 1 /*
 2 hello.cpp
 3 compile it with
 4 g++ -I. -I/home/kde-devel/kde/include -I/home/kde-devel/qt-unstable/include/Qt -I/home/kde-devel/qt-unstable/include /home/kde-devel/qt-unstable/include/QtXml parser.h parser.cpp hello.cpp -L/home/kde-devel/kde/lib -L/home/kde-devel/qt-unstable/lib -lQtCore_debug -lQtXml_debug -lkdeui
 5 */
 6 
 7 
 8 #include <qstring.h>
 9 #include <QXmlInputSource>
10 #include <qfile.h>
11 #include <parser.h>
12 
13 int main()
14 {  
15   Parser* handler=new Parser();
16   QXmlInputSource* source=new QXmlInputSource(new QFile("hello.htm"));
17   QXmlSimpleReader reader;
18   reader.setContentHandler( handler );
19   reader.parse( source );
20 }

The DOM approach

 1 /*
 2    dom.cpp
 3    A demonstration how to use the dom parsing framework.
 4    Prints the first subnode of an HTML file, i.e. typically 
 5    "head" or "body".
 6    compile it like this:
 7    g++ -I. -I/opt/kde3/include -I/usr/lib/qt3/include dom.cpp \
 8    -L/opt/kde3/lib -L/usr/lib/qt3/lib -lqt-mt -lkdeui   
 9 */
10 #include <qdom.h>
11 #include <qfile.h>
12 #include <kdebug.h>
13 
14 int main()
15 {
16   QDomDocument doc( "myDocument" );
17   QFile qf("hello.htm");
18   doc.setContent( &qf );
19   QDomElement docElement = doc.documentElement(); 
20   QDomNode node;
21   node = docElement.firstChild();
22   kdDebug() << node.nodeName() << endl;
23 }

Drawbacks

HTML parsing only works for "legal" html documents. For example, look at this code:

<html>
  <body>
      <a href="http://www.kde.org/"></a>
      <a href="/index.php?title=Special:User&returnto=Main_Page">Log in</a>
      <a href="http://www.gmx.de"></a>
  </body>
</html>

This code contains a & and will bring your parser to an error.

See here:

<html>
  <body>
      <a href="http://www.kde.org/"></a>
      <a href="/index.php" nowrap>Log in</a>
      <a href="http://www.gmx.de"></a>
  </body>
</html>

This code will throw an error because of the nowrap that is not xml-conform.


This page was last modified on 29 June 2011, at 20:54. Content is available under Creative Commons License SA 3.0 as well as the GNU Free Documentation License 1.2 unless otherwise noted.