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<td align="right" valign="center"><img src="logo32.png" align="right" width="64" height="32" border="0"></td></tr></table><h1 align=center>Walkthrough: How to use the TQt SAX2 classes</h1>

 
<p> 
<p> For a general discussion of the XML topics in TQt please refer to
the document <a href="xml.html">XML Module.</a>
To learn more about SAX2  see the document describing
<a href="xml.html#sax2">the TQt SAX2 implementation.</a>
<p> Before reading on you should at least be familiar with 
the <a href="xml.html#sax2Intro">Introduction to SAX2.</a>
<p> <a name="tquickStart"></a>
<h2>A tiny parser</h2>
<p> In this section we will present a small example reader that outputs
the names of all elements in an XML document on the command line. 
The element names are indented corresponding to their nesting level.
<p> As mentioned in <a href="xml.html#sax2Intro">Introduction to SAX2</a>
we have to implement the functions of the handler classes that we are
interested in. In our case these are only three:
<a href="qxmlcontenthandler.html#startDocument">TQXmlContentHandler::startDocument</a>(),
<a href="qxmlcontenthandler.html#startElement">TQXmlContentHandler::startElement</a>() and
<a href="qxmlcontenthandler.html#endElement">TQXmlContentHandler::endElement</a>().
<p> For this purpose we use a subclass of the <a href="qxmldefaulthandler.html">TQXmlDefaultHandler</a> (remember
that the special handler classes are all abstract and the default handler class
provides an implementation that does not change the parsing behavior):
<p> <pre>/****************************************************************************
** $Id: qt/structureparser.h   3.3.8   edited Jan 11 14:37 $
**
** Copyright (C) 1992-2007 Trolltech ASA.  All rights reserved.
**
** This file is part of an example program for TQt.  This example
** program may be used, distributed and modified without limitation.
**
*****************************************************************************/

#ifndef STRUCTUREPARSER_H
#define STRUCTUREPARSER_H

#include &lt;<a href="qxml-h.html">qxml.h</a>&gt;

class TQString;

class StructureParser : public <a href="qxmldefaulthandler.html">TQXmlDefaultHandler</a>
{
public:
    bool startDocument();
    bool startElement( const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp; ,
                       const <a href="qxmlattributes.html">TQXmlAttributes</a>&amp; );
    bool endElement( const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp; );

private:
    <a href="qstring.html">TQString</a> indent;
};

#endif
</pre>

<p> Apart from the private helper variable <em>indent</em> that we will use to
get indentation right, there is nothing special about our new 
<em>StructureParser</em> class.
<p> 

<p> Even the implementation is straight-forward: 
<p> <pre>    #include "structureparser.h"

    #include &lt;stdio.h&gt;
    #include &lt;<a href="qstring-h.html">qstring.h</a>&gt;
</pre>
<p> First we overload <a href="qxmlcontenthandler.html#startDocument">TQXmlContentHandler::startDocument</a>() with a non-empty version.
<p> <pre>    <a name="x2137"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startDocument">startDocument</a>()
    {
        indent = "";
        return TRUE;
    }
</pre>
<p> At the beginning of the document we simply 
set <em>indent</em> to an empty string because we
want to print out the root element without any indentation.
Also we return TRUE so that the parser continues without 
reporting an error.
<p> Because we want to be informed when the parser comes
accross a start tag of an element and subsequently print it out, we
have to overload <a href="qxmlcontenthandler.html#startElement">TQXmlContentHandler::startElement</a>(). 
<p> <pre>    <a name="x2138"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startElement">startElement</a>( const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp;,
                                        const <a href="qstring.html">TQString</a>&amp; qName,
                                        const <a href="qxmlattributes.html">TQXmlAttributes</a>&amp; )
    {
        printf( "%s%s\n", (const char*)indent, (const char*)qName );
        indent += "    ";
        return TRUE;
    }
</pre>
<p> This is what the implementation does: The name of the element with
preceding indentation is printed out followed by a linebreak.
Strictly speaking <em>qName</em> contains the local element name 
without an eventual prefix denoting the <a href="xml.html#namespaces">namespace.</a>
<p> If another element follows before the current element's end tag
it should be indented. Therefore we add four spaces to the
<em>indent</em> string.
<p> Finally we return TRUE in order to let the parser continue without
errors.
<p> The last functionality we need to add is the parser's behaviour when an 
end tag occurs. This means overloading <a href="qxmlcontenthandler.html#endElement">TQXmlContentHandler::endElement</a>().
<p> <pre>    <a name="x2136"></a>bool StructureParser::<a href="qxmlcontenthandler.html#endElement">endElement</a>( const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp;, const <a href="qstring.html">TQString</a>&amp; )
    {
        indent.remove( (uint)0, 4 );
        return TRUE;
    }
</pre>
<p> Obviously we then should shorten the <em>indent</em> string by the four
whitespaces added in startElement().
<p> With this we're done with our parser and can start writing the main() 
program.
<p> 

<p> <pre>    #include "structureparser.h"
    #include &lt;<a href="qfile-h.html">qfile.h</a>&gt;
    #include &lt;<a href="qxml-h.html">qxml.h</a>&gt;
    #include &lt;<a href="qwindowdefs-h.html">qwindowdefs.h</a>&gt;

    int main( int argc, char **argv )
    {
        if ( argc &lt; 2 ) {
            fprintf( stderr, "Usage: %s &lt;xmlfile&gt; [&lt;xmlfile&gt; ...]\n", argv[0] );
            return 1;
        }
</pre>
<p> This check ensures that we have a sequence of files from the command
line to examine.
<p> <pre>        StructureParser handler;
</pre>
<p> The next step is to create an instance of the <em>StructureParser</em>. 
<p> <pre>        <a href="qxmlsimplereader.html">TQXmlSimpleReader</a> reader;
    <a name="x2140"></a>    reader.<a href="qxmlreader.html#setContentHandler">setContentHandler</a>( &amp;handler );
</pre>
<p> After that we set up the reader. As our <em>StructureParser</em>
class deals with <a href="qxmlcontenthandler.html">TQXmlContentHandler</a> functionality only 
we simply register it as the content handler of our choice.
<p> <pre>        for ( int i=1; i &lt; argc; i++ ) {
</pre>
<p> Successively we deal with all files given as command line arguments.
<p> <pre>            <a href="qfile.html">TQFile</a> xmlFile( argv[i] );
            <a href="qxmlinputsource.html">TQXmlInputSource</a> source( &amp;xmlFile );
</pre>
<p> Then we create a
<a href="qxmlinputsource.html">TQXmlInputSource</a> for the XML file to be parsed. 
<p> <pre>    <a name="x2139"></a>        reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
</pre>
<p> Now we take our input source and start parsing.
<p> <pre>        }
        return 0;
    }
</pre>
<p> Running the program on the following XML file...
<p> <pre>&lt;animals&gt;
&lt;mammals&gt;
  &lt;monkeys&gt; &lt;gorilla/&gt; &lt;orangutan/&gt; &lt;/monkeys&gt;
&lt;/mammals&gt;
&lt;birds&gt; &lt;pigeon/&gt; &lt;penguin/&gt; &lt;/birds&gt;
&lt;/animals&gt;

</pre>

<p> ... produces the following output:
<pre>
animals
    mammals
        monkeys
            gorilla
            orang-utan
    birds
        pigeon
        penguin 
</pre>
 
<p> It will however refuse to produce the correct result if you e.g. insert
a whitespace between a &lt; and the element name in your test-XML file.
To prevent such annoyances
you should always install an error handler with <a href="qxmlreader.html#setErrorHandler">TQXmlReader::setErrorHandler</a>(). This allows you to report
parsing errors to the user.
<p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.

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