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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.

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

The QXML approach

The DOM approach

Drawbacks