Extracting child node values with QXmlItem as a QXmlQuery focus

Question

I've been trying to fetch node text values from this XML file:

  <!DOCTYPE structure>
  <data>
   <x>
    <id>1</id>
    <nam>tytuł</nam>
    <tab>21</tab>
    <ind>5</ind>
    <pre>TY</pre>
    <khw>C.TY</khw>
   </x>
   <x>
    <id>2</id>
    <nam>autor</nam>
    <tab>21</tab>
    <ind>5</ind>
    <pre>FO</pre>
    <khw>C.FO</khw>
   </x>
   <x>
    <id>3</id>
    <nam>hasło korporatywne</nam>
    <tab>21</tab>
    <ind>5</ind>
    <pre>FN</pre>
    <khw>C.FN</khw>
   </x>
  </data>

What I want to do is to fetch every node and it's children and convert it to QMap. I have no trouble with fetching single element, but when it comes to fetch children items by setting result of QXmlQuery as a focus, the QString that i evaluate the child node query is empty. I use this piece of code:

QXmlResultItems results;
QFile structure("./structure.xml"); // xml file, as described earlier
structure.open(QFile::ReadOnly);

QXmlQuery query;
query.setFocus(&structure);
query.setQuery("data/x");
query.evaluateTo(&results);

QXmlItem next = results.next();
while(!next.isNull()) {
    qDebug() << next.toNodeModelIndex().stringValue(); // everything's fine. It prints contents of <x>'s child nodes
    QXmlQuery childQuery;
    QString r;
    childQuery.setFocus(next);
    childQuery.setQuery("./nam/text()"); // already tested: "/nam/text()", "/nam/string()", "x/nam/string()", "data/x/nam/string()" etc... still no luck.
    childQuery.evaluateTo(&r);
    qDebug() << r; // prints \n but it should print content of <nam> node.

    next = results.next();
}

Software I use: Qt 4.7.2 SDK directly from Qt website, QtCreator 2.3.1 on Windows and Linux (without any difference in this particular case, results are the same). I want to be sure that's the problem of my lack of knowledge, rather than software bug, please help

Matisse

The funny thing is that it works with "./id/string()" or even with "./name/string()" if you rename the "nam" tags to "name". – Pulmonic

Looks like the same problem described here qt-project.org/forums/viewthread/25725 – Skater

Answer 1

Unfortunately, it is unclear from Qt Documentation that in cases when you want to use QXmlQuery::setFocus(const QXmlItem& item) overload in order to query child nodes, you should create corresponding QXmlQuery objects using QXmlQuery(const QXmlNamePool& np) constructor to make them share the same QXmlNamePool object. Such sharing, simply speaking, relates queries to each other.

Considering this, your example should look like following:

QFile structure("./structure.xml");
structure.open(QFile::ReadOnly);

QXmlQuery query;
query.setFocus(&structure);
query.setQuery("data/x");

QXmlResultItems results;
query.evaluateTo(&results);

QXmlQuery childQuery(query.namePool());
while (!results.next().isNull()) {
    childQuery.setFocus(results.current());
    childQuery.setQuery("nam/text()");
    QString r;
    childQuery.evaluateTo(&r);
    qDebug() << r;
}

Moreover, you can go further and reuse initial QXmlQuery object:

QFile structure("./structure.xml");
structure.open(QFile::ReadOnly);

QXmlQuery query;
query.setFocus(&structure);
query.setQuery("data/x");

QXmlResultItems results;
query.evaluateTo(&results);

while (!results.next().isNull()) {
    query.setFocus(results.current());
    query.setQuery("nam/text()");
    QString r;
    query.evaluateTo(&r);
    qDebug() << r;
}

Strachan

Answer 2

Instead of using evaluateTo( QString * ) use the QStringList version. It should work.

Tisatisane

Now work. Could you please explain, why it should work? – Skater

Answer 3

It should work like this:

QDomDocument doc("structure");
QFile file("structure.xml");
if( !file.open( IO_ReadOnly ) )
  return -1;
if( !doc.setContent( &file ) )
{
  file.close();
  return -2;
}
file.close();

QDomElement root = doc.documentElement();
if( root.tagName() != "data" )
  return -3;

QDomNode n = root.firstChild();
while( !n.isNull() )
{
  QDomElement e = n.toElement();
  if( !e.isNull() )
  {
    if( e.tagName() == "x" )
    {
      QMessageBox::information( 0, "X", e.attribute("id", "")+ "\n" + e.attribute("nam", "" ) + "\n" + e.attribute("tab", ""));
    }
  }

  n = n.nextSibling();
}

The code is doing a message box for every x (don't have qt on this machine, so can't test it right now)

Pilferage

I'm afraid that the attribute() method is for fetching attribute for specific element, not for fetching its children. In that particular case I should use firstChild().nodeValue() on variable e. I know that I could use this approach, but what I wanted to avoid is too much code to fetch particular data from XML tree. – Rutledge

Answer 4

I had the same problem, and the solution was to have the query and childQuery be exactly the same. You could rewrite your code as :

while(!next.isNull()) {
    qDebug() << next.toNodeModelIndex().stringValue();
    QString r;
    query.setFocus(next);
    query.setQuery("./nam/text()");
    query.evaluateTo(&r);
    qDebug() << r;

    next = results.next();
}

if childQuery is to be in another procedure, you have to pass it by reference.

Pasco