RE: [libxml++] xpath with namespaces

[<Cumming External comneon com>]

Thanks, Daniel. This is really helpful.

> -----Original Message-----
> From: libxmlplusplus-general-admin lists sourceforge net 
> [mailto:libxmlplusplus-general-admin lists sourceforge net] 
> On Behalf Of Daniel Veillard
> Sent: Monday, December 20, 2004 10:28 PM
> To: libxmlplusplus-general lists sourceforge net
> Subject: Re: [libxml++] xpath with namespaces
> 
> 
> On Mon, Dec 20, 2004 at 10:10:08PM +0100, Murray Cumming wrote:
> > On Mon, 2004-12-20 at 02:51 -0500, Daniel Veillard wrote:
> > > > > > >  If I use "//gjob:Jobs" i get "XPath error : 
> Undefined namespace prefix",
> > > > > > >  and "//Jobs" just returns an empty set.
> > > 
> > >   Perfectly normal, the expression uses the gjob prefix 
> and it was not
> > > registered.
> > 
> > Isn't this what
> > <gjob:Helping xmlns:gjob="http://www.gnome.org/some-location";>
> > does?
> 
>  No. The XPath is compiled without a priori knowledge of the instance.
> And a given prefix can be defined twice differently in a document
> 
> <bad>
>   <gjob:Helping xmlns:gjob="http://www.gnome.org/some-location";>
>   <gjob:Helping xmlns:gjob="http://www.gnome.org/some-other-location";>
> </bad>
> 
>  you must define what "gjob" is bound to before "//gjob:Jobs" 
> makes any sense.
> 
> > >   
> http://xmlsoft.org/html/libxml-xpathInternals.html#xmlXPathRegisterNs
> > >   at the C level, it may not be available in libxml++
> > 
> > Thanks Daniel. At the moment, we hide the xmlXPathContext completely
> > inside our Node::find(xpath) function, so we would have to 
> create some
> > XPathContext object that can be passed around.
> 
>   hum, yes the XPath API seems severely broken without ways 
> to associate
> prefixes bindings. Maybe a new entry point allowing to define a set of
> bindings as well as the XPath expression can be done without 
> refactoring
> everything
>     find(const char *expression, namespaces);
>   where namespaces is a list of couples (prefix/URI)
>   
> > Is that a concept that exists in a standard, or just something that
> > allows libxml users to work around broken documents? Many of the
> 
>    it's standard part of XPath evaluation context
>      http://www.w3.org/TR/xpath#section-Introduction
> it's better to read the spec before designing the API around it :-\
> 
> > functions look quite internal. Are there any higher-level 
> documents that
> > explain what they might be used for?
> 
>   Most of it can be derived from the spec plus simple implementation
> decisions. Most of the libxml2 XPath implementation docs are related 
> to libxslt 
>     http://xmlsoft.org/XSLT/internals.html#XPath
> 
>   The prefix binding is really a core property of XPath evaluation,
> easy to get wrong (//foo cannot selected a default namespaced node),
> and a source of FAQs about XPath
> 
> Daniel
> 
> -- 
> Daniel Veillard      | Red Hat Desktop team http://redhat.com/
> veillard redhat com  | libxml GNOME XML XSLT toolkit  
> http://xmlsoft.org/
> http://veillard.com/ | Rpmfind RPM search engine http://rpmfind.net/
> 
> 
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