Xerces, xpaths, and XML namespaces - c++

Question

I'm trying to use xerces-c in order to parse a rather massive XML document generated from StarUML in order to change some things, but I'm running into issues getting the xpath query to work because it keeps crashing.
To simplify things I split out part of the file into a smaller XML file for testing, which looks like this:
<?xml version="1.0" encoding="utf-8"?>
<XPD:UNIT xmlns:XPD="http://www.staruml.com" version="1">
<XPD:HEADER>
<XPD:SUBUNITS>
</XPD:SUBUNITS>
</XPD:HEADER>
<XPD:BODY>
<XPD:OBJ name="Attributes[3]" type="UMLAttribute" guid="onMjrHQ0rUaSkyFAWtLzKwAA">
<XPD:ATTR name="StereotypeName" type="string">ConditionInteraction</XPD:ATTR>
</XPD:OBJ>
</XPD:BODY>
</XPD:UNIT>
All I'm trying to do for this example is to find all of the XPD:OBJ elements, of which there is only one. The problem seems to stem from trying to query with the namespace. When I pass a very simple xpath query of XPD:OBJ it will crash, but if I pass just OBJ it won't crash but it won't find the XPD:OBJ element.
I assume there's some important property or setting that I'm missing during initialization that I need to set but I have no idea what it might be. I looked up all of the properties of the parser having to do with namespace and enabled the ones I could but it didn't help at all so I'm completely stuck. The initialization code looks something like this, with lots of things removed obviously:
const tXercesXMLCh tXMLManager::kDOMImplementationFeatures[] =
{
static_cast<tXercesXMLCh>('L'),
static_cast<tXercesXMLCh>('S'),
static_cast<tXercesXMLCh>('\0')
};
// Instantiate the DOM parser.
fImplementation = static_cast<tXercesDOMImplementationLS *>(tXercesDOMImplementationRegistry::getDOMImplementation(kDOMImplementationFeatures));
if (fImplementation != nullptr)
{
fParser = fImplementation->createLSParser(tXercesDOMImplementationLS::MODE_SYNCHRONOUS, nullptr);
fConfig = fParser->getDomConfig();
// Let the validation process do its datatype normalization that is defined in the used schema language.
//fConfig->setParameter(tXercesXMLUni::fgDOMDatatypeNormalization, true);
// Ignore comments and whitespace so we don't get extra nodes to process that just waste time.
fConfig->setParameter(tXercesXMLUni::fgDOMComments, false);
fConfig->setParameter(tXercesXMLUni::fgDOMElementContentWhitespace, false);
// Setup some properties that look like they might be required to get namespaces to work but doesn't seem to help at all.
fConfig->setParameter(tXercesXMLUni::fgXercesUseCachedGrammarInParse, true);
fConfig->setParameter(tXercesXMLUni::fgDOMNamespaces, true);
fConfig->setParameter(tXercesXMLUni::fgDOMNamespaceDeclarations, true);
// Install our custom error handler.
fConfig->setParameter(tXercesXMLUni::fgDOMErrorHandler, &fErrorHandler);
}
Then later on I parse the document, find the root node, and then run the xpath query to find the node I want. I'll leave out the bulk of that and just show you where I'm running the xpath query in case there's something obviously wrong there:
tXercesDOMDocument * doc; // Comes from parsing the file.
tXercesDOMNode * contextNode; // This is the root node retrieved from the document.
tXercesDOMXPathResult * xPathResult;
doc->evaluate("XPD:OBJ", contextNode, nullptr, tXercesDOMXPathResult::ORDERED_NODE_SNAPSHOT_TYPE), xPathResult);
The call to evaluate() is where it crashes somewhere deep inside xerces that I can't see very clearly, but from what I can see there are a lot of things that look deleted or uninitialized so I'm not sure what's causing the crash exactly.
So is there anything here that looks obviously wrong or missing that is required to make xerces work with XML namespaces?

Answer 1

The solution was right in front of my face the whole time. The problem was that you need to create and pass a resolver to the evaluate() call or else it will not be able to figure out any of the namespaces and will throw an exception. The crash seems to be a bug in xerces since it's crashing on trying to throw the exception when it can't resolve the namespace. I had to debug deep into the xerces code to find it, which gave me the solution.
So to fix the problem I changed the call to evaluate() slightly to create a resolver with the root node and now it works perfectly:
tXercesDOMDocument * doc; // Comes from parsing the file.
tXercesDOMNode * contextNode; // This is the root node retrieved from the document.
tXercesDOMXPathResult * xPathResult;
// Create the resolver with the root node, which contains the namespace definition.
tXercesDOMXPathNSResolver * resolver(doc->createNSResolver(contextNode));
doc->evaluate("XPD:OBJ", contextNode, resolver, tXercesDOMXPathResult::ORDERED_NODE_SNAPSHOT_TYPE), xPathResult);
// Make sure to release the resolver since anything created from a `create___()`
// function has to be manually released.
resolver->release();