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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><html><head><meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1"><title>Qt Toolkit -  The Qt SAX2 implementation</title><style type="text/css"><!--h3.fn,span.fn { margin-left: 1cm; text-indent: -1cm; }a:link { color: #004faf; text-decoration: none }a:visited { color: #672967; text-decoration: none }body { background: white; color: black; }--></style></head><body bgcolor="#ffffff"><p><table width="100%"><tr><td><a href="index.html"><img width="100" height="100" src="qtlogo.png"alt="Home" border="0"><img width="100"height="100" src="face.png" alt="Home" border="0"></a><td valign="top"><div align="right"><img src="dochead.png" width="472" height="27"><br><a href="classes.html"><b>Classes</b></a>- <a href="annotated.html">Annotated</a>- <a href="hierarchy.html">Tree</a>- <a href="functions.html">Functions</a>- <a href="index.html">Home</a>- <a href="topicals.html"><b>Structure</b>  <font face="Arial,Helvetica,Geneva,Swiss,SunSans-Regular" align="center" size=32>Qte</font></a></div></table><h1 align="center"> The Qt SAX2 implementation</h1><br clear="all"><h2><a name="introSAX2">Introduction to SAX2</a></h2><p>The SAX2 interface is an event-driven mechanism to provide the user withdocument information. "Event" in this context has nothing to do with theterm "event" you probably know from windowing systems; it means that theparser reports certain document information while parsing the document.These reported information is referred to as "event".<p>To make it less abstract consider the following example:<pre>&lt;quote&gt;To make it less abstract consider the following example:&lt;/quote&gt;</pre><p>Whilst reading (a SAX2 parser is usually referred to as "reader") the above document three events would be triggered:<OL><LI> A start tag occurs (<tt>&lt;quote&gt;</tt>).<LI> Character data (i.e. text) is found.<LI> An end tag is parsed (<tt>&lt;/quote&gt;</tt>).</OL><p>Each time such an event occurs the parser reports it so that a suitable event handling routine can be invoked.<p>Whilst this is a fast and simple approach to read XML documentsmanipulation is difficult because data are not stored, simply handledand discarded serially. This is when the <a href="xml-dom.html">DOM interface</a> comes handy.<p>The Qt XML module provides anabstract class, <a href="qxmlreader.html">QXmlReader</a>, that defines the interface for potential SAX2 readers.At the moment Qt ships with one reader implementation, <a href="qxmlsimplereader.html">QXmlSimpleReader</a>.<p>The reader reports parsing events through special handler classes. In Qtthe following ones are available:<ul><li><a href="qxmlcontenthandler.html">QXmlContentHandler</a>    reports events related to the content of a document (e.g. the start tag    or characters).<li><a href="qxmldtdhandler.html">QXmlDTDHandler</a>    reports events related to the DTD (e.g. notation declarations).<li><a href="qxmlerrorhandler.html">QXmlErrorHandler</a>    reports errors or warnings that occurred during parsing.<li><a href="qxmlentityresolver.html">QXmlEntityResolver</a>    reports external entities during parsing and allows the user to resolve    external entities him- or herself instead of leaving it to the reader.<li><a href="qxmldeclhandler.html">QXmlDeclHandler</a>    reports further DTD related events (e.g. attribute declarations).    Usually users are not interested in them, but under certain circumstances    this class comes handy.<li><a href="qxmllexicalhandler.html">QXmlLexicalHandler</a>    reports events related to the lexical structure of the document    (the beginning of the DTD, comments etc.). Occasionally this    might be useful.</ul><p>These classes are abstract classes describing the interface. The<a href="qxmldefaulthandler.html">QXmlDefaultHandler</a> class provides a "do nothing" default implementation forall of them. Therefore users need to overload only theQXmlDefaultHandler functions they are interested in. <p>To read input XML data a special class <a href="qxmlinputsource.html">QXmlInputSource</a> is used.<p>Apart from the already mentioned ones the following SAX2 support classes provide the user with useful functionality:<ul><li> <a href="qxmlattributes.html">QXmlAttributes</a>     is used to pass attributes in a start element event.<li> <a href="qxmllocator.html">QXmlLocator</a>     is used to obtain the actual parsing position of an event.<li> <a href="qxmlnamespacesupport.html">QXmlNamespaceSupport</a>     is used to easily implement <a href="xml.html#namespaces">namespace</a>     support for a reader.     Note that namespaces do not change the parsing      behavior. They are only reported through the handler.</ul><p><h2><a name="features">Features</a></h2><p>The behaviour of an XML reader depends on whether it supports certainoptional features or not. As an example a reader can have the feature "report attributes used for <a href="xml.html#namespaces">namespace</a>  declarationsand prefixes along with the local name of a tag".  Like every other feature this has a unique name represented by a URI:it is called <em>http://xml.org/sax/features/namespace-prefixes.</em><p>The Qt SAX2 implementation allows you to find out whether the reader has this ability using <a href="qxmlreader.html#2e0d44">QXmlReader::hasFeature()</a>.If the return value is TRUE it is possible toturn the relevant feature on and off.To do this use <a href="qxmlreader.html#57cdb9">QXmlReader::setFeature()</a>. Whether a supported featureis on or off (TRUE or FALSE) can be queried using  <a href="qxmlreader.html#be21af">QXmlReader::feature()</a>.<p>Consider the example <pre>&lt;document xmlns:book = 'http://trolltech.com/fnord/book/'          xmlns      = 'http://trolltech.com/fnord/' &gt;</pre><p>A reader not supporting the<em>http://xml.org/sax/features/namespace-prefixes</em> feature would clearlyreport the element name <em>document</em> but not its attributes <em>xmlns:book</em> and <em>xmlns</em>with their values. Areader with the feature <em>http://xml.org/sax/features/namespace-prefixes</em>reports the namespace attributes if <a href="qxmlreader.html#be21af">QXmlReader::feature()</a> is TRUE and disregards them if the feature is FALSE.<p>Other features include <em>http://xml.org/sax/features/namespace</em> (namespace processing, implies <em>http://xml.org/sax/features/namespace-prefixes)</em> or<em>http://xml.org/sax/features/validation</em> (the ability to report validationerrors). <p>Whilst SAX2 leaves it to the user to define and implement whateverfeatures are required, support for <em>http://xml.org/sax/features/namespace</em>(and thus <em>http://xml.org/sax/features/namespace-prefixes)</em> ismandantory. Accordingly <a href="qxmlsimplereader.html">QXmlSimpleReader</a>, the implementationof <a href="qxmlreader.html">QXmlReader</a> that comes with the Qt XML module, supports both of them,and therefore can do namespace processing.<p>Being a non-validating parser <a href="qxmlsimplereader.html">QXmlSimpleReader</a> does not support <em>http://xml.org/sax/features/validation</em>and other features.<p><h2><a name="namespaces">Namespace support via features</a></h2><p>As we have seen in the previous sectionwe can configure the behavior of the reader when it comes to namespaceprocessing. This is done by setting and unsetting the <em>http://xml.org/sax/features/namespaces</em> and<em>http://xml.org/sax/features/namespace-prefixes</em> features.<p>They influence the reporting behavior in the following way:<ol><li>Namespace prefixes and local parts of elements and attributes can be    reported.<li>The qualified names of elements and attributes are reported.<li><a href="qxmlcontenthandler.html#a587d5">QXmlContentHandler::startPrefixMapping()</a> and <a href="qxmlcontenthandler.html#3a4e72">QXmlContentHandler::endPrefixMapping()</a>     are called by the reader.<li>Attributes that declare namespaces (i.e. the attribute <em>xmlns</em> and    attributes starting with <em>xmlns:</em> ) are reported.</ol><p>Consider the following element:<p><pre>&lt;author xmlns:fnord = 'http://trolltech.com/fnord/'             title="Ms"              fnord:title="Goddess"              name="Eris Kallisti"/&gt;</pre><p>With <em>http://xml.org/sax/features/namespace-prefixes</em> set to TRUE the reader will report four attributes, with the <em>namespace-prefixes</em>feature set to FALSE only three: The <em>xmlns:fnord</em> attribute defininga namespace is then "unvisible" for the reader.<p>The <em>http://xml.org/sax/features/namespaces</em> feature on the other hand is responsible for reporting local names, namespace prefixes and -URIs.With <em>http://xml.org/sax/features/namespaces</em> set to TRUEthe parser will report <em>title</em> as the local name of <em>fnord:title</em>attribute, <em>fnord</em> being the namespace prefix and <em>http://trolltech.com/fnord/</em>as the namespace URI.When <em>http://xml.org/sax/features/namespaces</em> is FALSE none of them arereported.<p>In the current implementation the Qt XML classes follow the definitionthat the prefix <em>xmlns</em> itself isn't associated with any namespace at all(see <a href="http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using"> http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using</a>).Therefore even with <em>http://xml.org/sax/features/namespaces</em> and<em>http://xml.org/sax/features/namespace-prefixes</em> both set to TRUEthe reader won't return either a local name, a namespace prefix ora namespace URI for <em>xmlns:fnord.</em><p>This might be changed in the future following the W3C suggestion<a href="http://www.w3.org/2000/xmlns/">http://www.w3.org/2000/xmlns/</a> to associate <em>xmlns</em> with the namespace <em>http://www.w3.org/2000/xmlns.</em> <p>As the SAX2 standard suggests <a href="qxmlsimplereader.html">QXmlSimpleReader</a> by default has<em>http://xml.org/sax/features/namespaces</em> set to TRUE and<em>http://xml.org/sax/features/namespace-prefixes</em> set to FALSE.When changing this behavior using <a href="qxmlsimplereader.html#1f7766">QXmlSimpleReader::setFeature()</a> note that the combination of both features set toFALSE is illegal.<p>For a practical demonstration of how the two features affect the output of the reader run the <a href="xml-tagreader-with-features-tagreader-cpp.html">tagreader with features example</a>.<p><h3><a name="sax2namespaces">Summary</a></h3><p><a href="qxmlsimplereader.html">QXmlSimpleReader</a> implements the following behavior (the value in parentheses denotes to the SAX2 requirements if they differ from theQt implementation:<table border="1"><tr>    <th>namespaces</th>    <th>namespace-prefixes</th>    <th>Namespace prefix and local part</th>    <th>Qualified names</th>    <th>Prefix mapping</th>    <th>xmlns attributes</th></tr><tr>    <td>TRUE</td>    <td>FALSE</td>    <td>Yes</td>    <td>Yes <font color="#808080">(Unknown)</font></td>    <td>Yes</td>    <td>No</td></tr><tr>    <td>TRUE</td>    <td>TRUE</td>    <td>Yes</td>    <td>Yes</td>    <td>Yes</td>    <td>Yes</td></tr><tr>    <td>FALSE</td>    <td>TRUE</td>    <td>No <font color="#808080">(Unknown)</font></td>    <td>Yes</td>    <td>No <font color="#808080">(Unknown)</font></td>    <td>Yes</td></tr><tr>    <td>FALSE</td>    <td>FALSE</td>    <td colspan=4 align=center><font color="#808080">(Illegal)</font></td></table><p><h2><a name="properties">Properties</a></h2>   <p>Properties are a more general concept. They also have a unique name,represented as an URI, but their value is <code>void*.</code> Thus nearly everythingcan be used as a property value. This concept involves some danger,though: there are no means to ensure type-safety; the user must take carethat he or she passes the correct type. Properties are useful if a reader supportsspecial handler classes.<p><!-- example! --><p>The URIs used for features and properties often look like URLs, e.g. <code>http://xml.org/sax/features/namespace.</code> This does not mean that whatsoeverdata is required at this address. It is simply a way to define unique names.<p>Everybody can define and use new SAX2 properties for his or herreaders. Property support is however notrequired.<p>To set or query properties the following functions are provided:<a href="qxmlreader.html#6970da">QXmlReader::setProperty()</a>, <a href="qxmlreader.html#67bf3a">QXmlReader::property()</a> and <a href="qxmlreader.html#0dfd4d">QXmlReader::hasProperty()</a>.                      <p><h2>Further reading</h2><p>For a practical example on how to use the Qt SAX2 classes see the <a href="xml-sax-walkthrough.html">tagreader walkthrough.</a><p>More information about XML (e.g. <a href="xml.html#namespaces">namespace</a>)can be found in the <a href="xml.html">introduction to the Qt XML module.</a><p><address><hr><div align="center"><table width="100%" cellspacing="0" border="0"><tr><td>Copyright
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