On Sat, 2010-08-07 at 00:45 +0200, Anders Logg wrote:
> On Fri, Aug 06, 2010 at 09:02:00PM +0100, Garth N. Wells wrote:
> > I have trouble following the logic in the DOLFIN XML io code, which I
> > suspect is because of the pointers to callback functions being passed to
> > libxml2. Has anyone looked at libxml++
> > (http://library.gnome.org/devel/libxml++-tutorial/stable/) which
> > provides C++ bindings to libxml2?
>
> No, and I'm not sure if it's worth the trouble.
>
> I think the libxml2 interface is not much of a problem, at least not
> since we realized how to only set the callbacks we are interested in:
>
The problem I have is that the few times that I've looked at it to fix
or extend something I have trouble following how it works. A part of
this is the lack of comments in the code.
> XMLFile::XMLFile(std::ostream& s)
> : GenericFile(""), sax(0), outstream(&s)
> {
> // Set up the sax handler.
> sax = new xmlSAXHandler();
>
> // Set up handlers for parser events
> sax->startDocument = sax_start_document;
> sax->endDocument = sax_end_document;
> sax->startElement = sax_start_element;
> sax->endElement = sax_end_element;
> sax->warning = sax_warning;
> sax->error = sax_error;
> sax->fatalError = sax_fatal_error;
> }
>
> Ola did a very good job at redesigning the parsing to allow reuse of
> code. For example, we can reuse the parsing of MeshFunction data
> inside the <data> tag inside the <mesh> tag. Previously, every top
> level tag had its own implementation which prevented reuse of code for
> parsing nested data.
>
> This is what makes the implementation difficult to follow, but it's
> essentially a stack of parsers where we push and pop the handler
> currently responsible for accepting the callbacks from libxml2 when it
> reads data.
>
> I don't think it would be easier if we used the C++ interface. It
> would probably be less transparent. Right now, our interaction with
> libxml2 is very minimal: setting the callbacks and receiving the data
> parsed by libxml2.
>
My first impression is that it would be more transparent and simpler -
we just pass the file name to libxml++ and implement the functions
'on_start_element' and 'on_end_element'.
Garth
> --
> Anders
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