XPath result - null or zero length?

[Will Sappington <ws...@ndma.us>]

I searched the archives for "xpath result" (unquoted) and didn't get any
useful hits so:

The SimpleXPathAPI example does this after establishing the context
node:

// OK, let's evaluate the expression...
const XObjectPtr	theResult(
				theEvaluator.evaluate(
				theDOMSupport,
				theContextNode,
				XalanDOMString(argv[3]).c_str(),
				thePrefixResolver));

assert(theResult.null() == false);

cout << "The string value of the result is:"
	 << endl
	 << theResult->str()
	 << endl
	 << endl;

It would appear from this that the test for an unsuccessful XPath
evaluation is theResult.null() returning true.  However, if I evaluate
an XPath expression containing an element or attribute that is not in
the file, that test fails - .null() returns false - and after converting
the result to a regular C char string, it is empty, zero length.  If I
give the SimpleXPathAPI sample program an expression that will not
match, it prints "The string value of the result is ", indicating that
it too got a zero length result instead of null.  

The API docs for XPathEvaluator::evaluate() just say that it returns the
result of the search.  To find out exactly what that is, I dug through
the XPathEvaluator::evaluate() code as far as I could but I got lost in
all the cases in XPath::executeMore().  Searched the archive but didn't
find anything.

Q: which test should I use for an unsuccessful match, null or zero
length, or could I get both and they mean different things?

Thanks in advance,

-will

Re: XPath result - null or zero length?

[David Bertoni <db...@apache.org>]

Will Sappington wrote:
> I searched the archives for "xpath result" (unquoted) and didn't get any
> useful hits so:
> 
> The SimpleXPathAPI example does this after establishing the context
> node:
> 
> // OK, let's evaluate the expression...
> const XObjectPtr	theResult(
> 				theEvaluator.evaluate(
> 				theDOMSupport,
> 				theContextNode,
> 				XalanDOMString(argv[3]).c_str(),
> 				thePrefixResolver));
> 
> assert(theResult.null() == false);
> 
> cout << "The string value of the result is:"
> 	 << endl
> 	 << theResult->str()
> 	 << endl
> 	 << endl;
> 
> It would appear from this that the test for an unsuccessful XPath
> evaluation is theResult.null() returning true.  However, if I evaluate
> an XPath expression containing an element or attribute that is not in
> the file, that test fails - .null() returns false - and after converting
> the result to a regular C char string, it is empty, zero length.  If I
> give the SimpleXPathAPI sample program an expression that will not
> match, it prints "The string value of the result is ", indicating that
> it too got a zero length result instead of null.  

XObjectPtr is a smart-pointer class, so the call to the null() member 
function just determines whether or not it's pointing to an XObject 
instance. In this case,

Any calls made using the -> operator are passed on to the underlying 
XObject instance.  The XObject class holds the actual result of the 
evaluation.  It's type depends on the type of the XPath expression you've 
evaluated.

If you get a zero-length string, that just means that the result of the 
expression has that value.  That could be an empty string, or an empty 
node-set.

> 
> The API docs for XPathEvaluator::evaluate() just say that it returns the
> result of the search.  To find out exactly what that is, I dug through
> the XPathEvaluator::evaluate() code as far as I could but I got lost in
> all the cases in XPath::executeMore().  Searched the archive but didn't
> find anything.

The result of an XPath 1.0 expression can have a type of string, number, 
boolean or node-set.  I'm assuming most of your expressions are location 
paths, so those will be of type node-set.  In that case, you can call 
XObject::nodeset(), to get a reference to the underlying NodeRefListBase 
instance.  However, if the expression is not a node-set, then calling that 
function will result in an exception.  If you're worried about the type of 
the resulting XObject instance, you can call XObject::getType()

> 
> Q: which test should I use for an unsuccessful match, null or zero
> length, or could I get both and they mean different things?

You call nodeset() to get a reference to the underlying NodeRefListBase, 
then call getLength() to see how many nodes there are.

Dave