QXmlStreamReader Class Reference
[QtCore module]

Detailed Description

The QXmlStreamReader class provides a fast parser for reading well-formed XML via a simple streaming API.

QXmlStreamReader is a faster and more convenient replacement for Qt's own SAX parser (see QXmlSimpleReader). In some cases it might also be a faster and more convenient alternative for use in applications that would otherwise use a DOM tree (see QDomDocument). QXmlStreamReader reads data either from a QIODevice (see setDevice()), or from a raw QByteArray (see addData()).

Qt provides QXmlStreamWriter for writing XML.

The basic concept of a stream reader is to report an XML document as a stream of tokens, similar to SAX. The main difference between QXmlStreamReader and SAX is how these XML tokens are reported. With SAX, the application must provide handlers (callback functions) that receive so-called XML events from the parser at the parser's convenience. With QXmlStreamReader, the application code itself drives the loop and pulls tokens from the reader, one after another, as it needs them. This is done by calling readNext(), where the reader reads from the input stream until it completes the next token, at which point it returns the tokenType(). A set of convenient functions including isStartElement() and text() can then be used to examine the token to obtain information about what has been read. The big advantage of this pulling approach is the possibility to build recursive descent parsers with it, meaning you can split your XML parsing code easily into different methods or classes. This makes it easy to keep track of the application's own state when parsing XML.

A typical loop with QXmlStreamReader looks like this:

   QXmlStreamReader xml;
   ...
   while (!xml.atEnd()) {
         xml.readNext();
         ... // do processing
   }
   if (xml.hasError()) {
         ... // do error handling
   }

QXmlStreamReader is a well-formed XML 1.0 parser that does not include external parsed entities. As long as no error occurs, the application code can thus be assured that the data provided by the stream reader satisfies the W3C's criteria for well-formed XML. For example, you can be certain that all tags are indeed nested and closed properly, that references to internal entities have been replaced with the correct replacement text, and that attributes have been normalized or added according to the internal subset of the DTD.

If an error occurs while parsing, atEnd() and hasError() return true, and error() returns the error that occurred. The functions errorString(), lineNumber(), columnNumber(), and characterOffset() are for constructing an appropriate error or warning message. To simplify application code, QXmlStreamReader contains a raiseError() mechanism that lets you raise custom errors that trigger the same error handling described.

The QXmlStream Bookmarks Example illustrates how to use the recursive descent technique to read an XML bookmark file (XBEL) with a stream reader.