Re: [xml] xmlXPathEval(*str,*ctxt)

["Robert G. Brown" <rgb phy duke edu>]

On Thu, 28 Mar 2002, Daniel Veillard wrote:

>   it's not a bug. ht eobject recturned will not be null in either
> case, in both case it will be flagged as a being a node set in 
> the first case obj->nodeset is not NULL and is of zero len and in
> the second case obj->nodeset is NULL.
>   They have the same semantic.
>
>   it simply won't allocate a node set array if not needed.

So from a practical point of view, I need to do something like:

 xp_op = xmlXPathEval(xpath, xp_doc);

 /* no such path in xp_doc */
 if( xp_op == NULL ) {
   if((verbose == DEBUG_ALL) || (verbose == DEBUG_EXTRACT_VALUES)){
     printf("DEBUG_EXTRACT_VALUES: xpath %s not found!  Returning NULL.\n",xpath);
   }
   /* Free the path objects so we don't leak */
   xmlXPathFreeObject(xp_op);
   return numvals;
 }
 /* A second way no path can be returned */
 if( xp_op != NULL && xp_op->nodesetval == NULL) {
   if((verbose == DEBUG_ALL) || (verbose == DEBUG_EXTRACT_VALUES)){
     printf("DEBUG_EXTRACT_VALUES: xpath %s not found!  Returning NULL.\n",xpath);
   }
   xmlXPathFreeObject(xp_op);
   return numvals;
 }

which does seem to work in my code, thanks!

You'll have to forgive my ignorance -- I'm trying to learn to use these
marvelous tools from the documentation (and source) as I haven't found a
whole lot of example xpath code out there, and the inline documentation
for xmlXPathEval(str,ctxt) says it returns:

  "...the xmlXPathObjectPtr resulting from the evaluation or NULL."

which I interpreted to mean that if the path string didn't exist in the
associated context, it would return NULL directly instead of a non-NULL
pointer to an empty object (with no nodeset array), since I could see no
reason to ever need an empty object in association with a context EXCEPT
to test and see that it wasn't there. I tried following the source back
to figure this out but freely admit that the inline magic of:

   ret->comp = xmlXPathNewCompExpr();

defeated me -- looks like it would take a day or two to figure out.

Is there a reason that an empty object is allocated and returned even if
the xpath doesn't exist in the associated context?  What can one do with
with such an empty object?

   rgb

-- 
Robert G. Brown                        http://www.phy.duke.edu/~rgb/
Duke University Dept. of Physics, Box 90305
Durham, N.C. 27708-0305
Phone: 1-919-660-2567  Fax: 919-660-2525     email:rgb phy duke edu