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| 19 | <a href="index.html">
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| 31 | </td>
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| 32 | <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 Qt SAX2 classes</h1>
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| 33 |
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| 34 |
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| 35 | <p>
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| 36 | <p> For a general discussion of the XML topics in Qt please refer to
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| 37 | the document <a href="xml.html">XML Module.</a>
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| 38 | To learn more about SAX2 see the document describing
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| 39 | <a href="xml.html#sax2">the Qt SAX2 implementation.</a>
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| 40 | <p> Before reading on you should at least be familiar with
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| 41 | the <a href="xml.html#sax2Intro">Introduction to SAX2.</a>
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| 42 | <p> <a name="quickStart"></a>
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| 43 | <h2>A tiny parser</h2>
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| 44 | <p> In this section we will present a small example reader that outputs
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| 45 | the names of all elements in an XML document on the command line.
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| 46 | The element names are indented corresponding to their nesting level.
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| 47 | <p> As mentioned in <a href="xml.html#sax2Intro">Introduction to SAX2</a>
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| 48 | we have to implement the functions of the handler classes that we are
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| 49 | interested in. In our case these are only three:
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| 50 | <a href="qxmlcontenthandler.html#startDocument">QXmlContentHandler::startDocument</a>(),
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| 51 | <a href="qxmlcontenthandler.html#startElement">QXmlContentHandler::startElement</a>() and
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| 52 | <a href="qxmlcontenthandler.html#endElement">QXmlContentHandler::endElement</a>().
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| 53 | <p> For this purpose we use a subclass of the <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a> (remember
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| 54 | that the special handler classes are all abstract and the default handler class
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| 55 | provides an implementation that does not change the parsing behavior):
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| 56 | <p> <pre>/****************************************************************************
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| 57 | ** $Id: xml-sax-walkthrough.html 2051 2007-02-21 10:04:20Z chehrlic $
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| 58 | **
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| 59 | ** Copyright (C) 1992-2007 Trolltech ASA. All rights reserved.
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| 60 | **
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| 61 | ** This file is part of an example program for Qt. This example
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| 62 | ** program may be used, distributed and modified without limitation.
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| 63 | **
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| 64 | *****************************************************************************/
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| 65 |
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| 66 | #ifndef STRUCTUREPARSER_H
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| 67 | #define STRUCTUREPARSER_H
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| 68 |
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| 69 | #include <<a href="qxml-h.html">qxml.h</a>>
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| 70 |
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| 71 | class QString;
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| 72 |
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| 73 | class StructureParser : public <a href="qxmldefaulthandler.html">QXmlDefaultHandler</a>
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| 74 | {
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| 75 | public:
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| 76 | bool startDocument();
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| 77 | bool startElement( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& ,
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| 78 | const <a href="qxmlattributes.html">QXmlAttributes</a>& );
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| 79 | bool endElement( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& );
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| 80 |
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| 81 | private:
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| 82 | <a href="qstring.html">QString</a> indent;
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| 83 | };
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| 84 |
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| 85 | #endif
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| 86 | </pre>
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| 87 |
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| 88 | <p> Apart from the private helper variable <em>indent</em> that we will use to
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| 89 | get indentation right, there is nothing special about our new
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| 90 | <em>StructureParser</em> class.
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| 91 | <p>
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| 92 |
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| 93 | <p> Even the implementation is straight-forward:
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| 94 | <p> <pre> #include "structureparser.h"
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| 95 |
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| 96 | #include <stdio.h>
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| 97 | #include <<a href="qstring-h.html">qstring.h</a>>
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| 98 | </pre>
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| 99 | <p> First we overload <a href="qxmlcontenthandler.html#startDocument">QXmlContentHandler::startDocument</a>() with a non-empty version.
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| 100 | <p> <pre> <a name="x2137"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startDocument">startDocument</a>()
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| 101 | {
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| 102 | indent = "";
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| 103 | return TRUE;
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| 104 | }
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| 105 | </pre>
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| 106 | <p> At the beginning of the document we simply
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| 107 | set <em>indent</em> to an empty string because we
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| 108 | want to print out the root element without any indentation.
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| 109 | Also we return TRUE so that the parser continues without
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| 110 | reporting an error.
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| 111 | <p> Because we want to be informed when the parser comes
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| 112 | accross a start tag of an element and subsequently print it out, we
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| 113 | have to overload <a href="qxmlcontenthandler.html#startElement">QXmlContentHandler::startElement</a>().
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| 114 | <p> <pre> <a name="x2138"></a>bool StructureParser::<a href="qxmlcontenthandler.html#startElement">startElement</a>( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&,
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| 115 | const <a href="qstring.html">QString</a>& qName,
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| 116 | const <a href="qxmlattributes.html">QXmlAttributes</a>& )
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| 117 | {
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| 118 | printf( "%s%s\n", (const char*)indent, (const char*)qName );
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| 119 | indent += " ";
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| 120 | return TRUE;
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| 121 | }
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| 122 | </pre>
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| 123 | <p> This is what the implementation does: The name of the element with
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| 124 | preceding indentation is printed out followed by a linebreak.
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| 125 | Strictly speaking <em>qName</em> contains the local element name
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| 126 | without an eventual prefix denoting the <a href="xml.html#namespaces">namespace.</a>
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| 127 | <p> If another element follows before the current element's end tag
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| 128 | it should be indented. Therefore we add four spaces to the
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| 129 | <em>indent</em> string.
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| 130 | <p> Finally we return TRUE in order to let the parser continue without
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| 131 | errors.
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| 132 | <p> The last functionality we need to add is the parser's behaviour when an
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| 133 | end tag occurs. This means overloading <a href="qxmlcontenthandler.html#endElement">QXmlContentHandler::endElement</a>().
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| 134 | <p> <pre> <a name="x2136"></a>bool StructureParser::<a href="qxmlcontenthandler.html#endElement">endElement</a>( const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>&, const <a href="qstring.html">QString</a>& )
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| 135 | {
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| 136 | indent.remove( (uint)0, 4 );
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| 137 | return TRUE;
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| 138 | }
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| 139 | </pre>
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| 140 | <p> Obviously we then should shorten the <em>indent</em> string by the four
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| 141 | whitespaces added in startElement().
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| 142 | <p> With this we're done with our parser and can start writing the main()
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| 143 | program.
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| 144 | <p>
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| 145 |
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| 146 | <p> <pre> #include "structureparser.h"
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| 147 | #include <<a href="qfile-h.html">qfile.h</a>>
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| 148 | #include <<a href="qxml-h.html">qxml.h</a>>
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| 149 | #include <<a href="qwindowdefs-h.html">qwindowdefs.h</a>>
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| 150 |
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| 151 | int main( int argc, char **argv )
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| 152 | {
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| 153 | if ( argc < 2 ) {
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| 154 | fprintf( stderr, "Usage: %s <xmlfile> [<xmlfile> ...]\n", argv[0] );
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| 155 | return 1;
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| 156 | }
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| 157 | </pre>
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| 158 | <p> This check ensures that we have a sequence of files from the command
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| 159 | line to examine.
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| 160 | <p> <pre> StructureParser handler;
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| 161 | </pre>
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| 162 | <p> The next step is to create an instance of the <em>StructureParser</em>.
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| 163 | <p> <pre> <a href="qxmlsimplereader.html">QXmlSimpleReader</a> reader;
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| 164 | <a name="x2140"></a> reader.<a href="qxmlreader.html#setContentHandler">setContentHandler</a>( &handler );
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| 165 | </pre>
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| 166 | <p> After that we set up the reader. As our <em>StructureParser</em>
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| 167 | class deals with <a href="qxmlcontenthandler.html">QXmlContentHandler</a> functionality only
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| 168 | we simply register it as the content handler of our choice.
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| 169 | <p> <pre> for ( int i=1; i < argc; i++ ) {
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| 170 | </pre>
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| 171 | <p> Successively we deal with all files given as command line arguments.
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| 172 | <p> <pre> <a href="qfile.html">QFile</a> xmlFile( argv[i] );
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| 173 | <a href="qxmlinputsource.html">QXmlInputSource</a> source( &xmlFile );
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| 174 | </pre>
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| 175 | <p> Then we create a
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| 176 | <a href="qxmlinputsource.html">QXmlInputSource</a> for the XML file to be parsed.
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| 177 | <p> <pre> <a name="x2139"></a> reader.<a href="qxmlsimplereader.html#parse">parse</a>( source );
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| 178 | </pre>
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| 179 | <p> Now we take our input source and start parsing.
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| 180 | <p> <pre> }
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| 181 | return 0;
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| 182 | }
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| 183 | </pre>
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| 184 | <p> Running the program on the following XML file...
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| 185 | <p> <pre><animals>
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| 186 | <mammals>
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| 187 | <monkeys> <gorilla/> <orangutan/> </monkeys>
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| 188 | </mammals>
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| 189 | <birds> <pigeon/> <penguin/> </birds>
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| 190 | </animals>
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| 191 |
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| 192 | </pre>
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| 193 |
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| 194 | <p> ... produces the following output:
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| 195 | <pre>
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| 196 | animals
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| 197 | mammals
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| 198 | monkeys
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| 199 | gorilla
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| 200 | orang-utan
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| 201 | birds
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| 202 | pigeon
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| 203 | penguin
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| 204 | </pre>
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| 205 |
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| 206 | <p> It will however refuse to produce the correct result if you e.g. insert
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| 207 | a whitespace between a < and the element name in your test-XML file.
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| 208 | To prevent such annoyances
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| 209 | you should always install an error handler with <a href="qxmlreader.html#setErrorHandler">QXmlReader::setErrorHandler</a>(). This allows you to report
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| 210 | parsing errors to the user.
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| 211 | <p> <p>See also <a href="step-by-step-examples.html">Step-by-step Examples</a>.
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| 212 |
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| 213 | <!-- eof -->
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| 214 | <p><address><hr><div align=center>
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| 215 | <table width=100% cellspacing=0 border=0><tr>
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| 216 | <td>Copyright © 2007
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| 217 | <a href="troll.html">Trolltech</a><td align=center><a href="trademarks.html">Trademarks</a>
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| 218 | <td align=right><div align=right>Qt 3.3.8</div>
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| 219 | </table></div></address></body>
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| 220 | </html>
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