我的这样写的就可以啊
QFile file(fileName);
if (!file.open(QIODevice::WriteOnly | QIODevice::Truncate))
{
qDebug("cannot open file!");
return -1;
}
QDomDocument doc;
QDomElement emt;
QDomProcessingInstruction dpi;
dpi = doc.createProcessingInstruction("xml", "version=\"1.0\"");
doc.appendChild(dpi);
QTextStream out(&file);
doc.save(out, 4);