The following piece of code:

bool fileWritten = false;
QString dataBuffer;
QXmlStreamWriter streamWriter(&dataBuffer);
streamWriter.setCodec("ISO-8859-1");
streamWriter.setAutoFormatting(true);

streamWriter.writeStartDocument();
fileWritten = this->write(streamWriter);
streamWriter.writeEndDocument();

generates a QString with a startDocument that does not contain the encoding. Replacing the QString by anything else (QFile, QBuffer, QByteArray) works well. But as these classes are full of memory leaks they cannot be used safely in a critical commercial product.