dolfin team mailing list archive

["Garth N. Wells" <gnw20@xxxxxxxxx>]

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