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:

    /*
     parser.h - demonstration of a parser in C++
    */
    
    #ifndef PARSER_H
    #define PARSER_H
    
    #include <qstring.h>
    #include <QtXml/QXmlDefaultHandler>
    #include <QtXml/QXmlAttributes>
    
    class Parser : public QXmlDefaultHandler
    {
    public:
    
      Parser();
    
      /** given by the framework from qxml. Called when parsing the xml-document starts.          */
      bool startDocument();
    
      /** given by the framework from qxml. Called when the reader occurs an open tag (e.g. \<b\> ) */
      bool startElement( const QString&, const QString&, const QString& qName, const QXmlAttributes& att );
    
    };
    
    
    #endif
    

    parser.cpp:

    /*
     parser.cpp - demonstration of a parser in C++
    */
    
    #include "parser.h"
    #include <kdebug.h>
    
      Parser::Parser()
      {
      }
      
      bool Parser::startDocument()
      {
        kDebug() << "Searching document for tags";
        return true;
      }
      
      bool Parser::startElement( const QString&, const QString&, const QString& qName, const QXmlAttributes& att )
      {
        kDebug() << "Found Element" << qName;
        return true;
     }
    

    hello.cpp:

    /*
    hello.cpp
    compile it with
    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
    */
    
    
    #include <qstring.h>
    #include <QXmlInputSource>
    #include <qfile.h>
    #include <parser.h>
    
    int main()
    {  
      Parser* handler=new Parser();
      QXmlInputSource* source=new QXmlInputSource(new QFile("hello.htm"));
      QXmlSimpleReader reader;
      reader.setContentHandler( handler );
      reader.parse( source );
    }
    

    The DOM approach

    /*
       dom.cpp
       A demonstration how to use the dom parsing framework.
       Prints the first subnode of an HTML file, i.e. typically 
       "head" or "body".
       compile it like this:
       g++ -I. -I/opt/kde3/include -I/usr/lib/qt3/include dom.cpp \
       -L/opt/kde3/lib -L/usr/lib/qt3/lib -lqt-mt -lkdeui   
    */
    #include <qdom.h>
    #include <qfile.h>
    #include <kdebug.h>
    
    int main()
    {
      QDomDocument doc( "myDocument" );
      QFile qf("hello.htm");
      doc.setContent( &qf );
      QDomElement docElement = doc.documentElement(); 
      QDomNode node;
      node = docElement.firstChild();
      kdDebug() << node.nodeName() << endl;
    }
    

    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.