Latest revision as of 12:18, 15 May 2019

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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.