Re: [boost] Proposal: XML APIs in boost

From: Stefan Seefeld (seefeld_at_[hidden])
Date: 2005-11-09 21:21:45

Graham Bennett wrote:
> Hi Doug,
> 
> On Tue, Nov 08, 2005 at 09:28:09PM -0500, Douglas Gregor wrote:

>>Readers are important for some things, DOM is important for other
>>things, but there's no reason to tie the two together in one library
>>or predicate one on the other. 
> 
> 
> Well, there is at least one reason - if the DOM is built on top of a
> reader interface then the DOM library doesn't have to know how to parse
> XML, and is not tied to any particular parser.  Even if you don't agree
> with using a reader interface for this separation layer, I'd hope you
> would agree that some separation is at least necessary.

I wish people would stop being so parser-focussed. I reiterate:
the API I suggest is about manipulating a DOM tree. The fact that
you *might* want to construct it from an XML file by means of a parser
is almost coincidental.

Yes indeed, an implementation of such an XML parser will most likely
use either a SAX or an XmlReader layer beneath, and in fact, libxml2
does exactly that and it would be quite natural to expose those APIs
to C++ in a similar way I propose the DOM wrapper.

>>We can have a XML DOM library that  allows reading, traversing,
>>modifying, and writing XML documents,  then later turn the reading
>>part into a full-fledged streaming  interface for those applications.
> 
> 
> Can you elaborate on how you would enable a DOM structure to present a
> streaming interface?

Not the DOM structure, but the parser ! It's exactly what you are saying
above: Each sensible XML parser will use an API underneath that can be
used to build a public SAX or XmlReader (or both) on top of.

But instead of requiring the parser to be built on such a C++ API I
use a C implenentation that already contains multiple APIs, and I wrap
them *separately* into C++ APIs. For a user of the C++ DOM API it is
totally irrelevant whether the implementation is based on the C++ SAX API
or an internal C SAX API, as long as it adhers to the specification.

> Are you talking about lazy tree building or
> something else?  In any case, I would think it's inherantly difficult to
> retrofit a streaming interface.  Much better to build the streaming
> interface from the start, and build the DOM on top of it.  This can only
> be good for both sides - the reader gets to just be a reader, and the
> DOM gets to just be a DOM.

You haven't talked about the DOM yet, only about a parser. You still
need to provide all the other missing bits, such as an XPath lookup
mechanism, XInclude processing, http support for URI lookup, etc., etc.
I can't stress it enough: the parser is really just a tiny bit of it all.

Regards,
                Stefan