Re: [boost] [GSoC] Boost.XML

Subject: Re: [boost] [GSoC] Boost.XML
From: Phil Endecott (spam_from_boost_dev_at_[hidden])
Date: 2010-03-21 09:46:41

Jose wrote:
> On Sun, Mar 21, 2010 at 12:00 AM, Phil Endecott
> <spam_from_boost_dev_at_[hidden]> wrote:
>
>> In fact, something "XPath-like" but also more "C++-like" would be the next
>> step to improve the "user" code in my application. ?Currently I have too
>> much verbose iteration looking for the elements that I want. ?It would be
>> great to have a XPath-like DSL for finding these elements. ?(An application
>> for Proto?)
>
> Have you tried pugixml?
>
> http://code.google.com/p/pugixml/

Thanks; I have seen it, but I hadn't noticed that it has an XPath 
implementation.  (In other respects, it is similar in concept to 
RapidXML i.e. it needs O(N) memory.)

 From its examples:

// You can use a sometimes convenient path function
cout << doc.first_element_by_path("bookstore/book/price").child_value() 
<< endl;

// And you can use powerful XPath expressions
cout << doc.select_single_node("/bookstore/book[@title = 
'ShaderX']/price").node().child_value() << endl;

// Compile query that prints total price of all Gems book in store
xpath_query query("sum(/bookstore/book[contains(@title, 'Gems')]/price)");
cout << query.evaluate_number(doc) << endl;

I'm not convinced that a full XPath implementation is really useful 
(e.g. that final example using the XPath sum() function and the 
contains() test; I think it would be better to do those in C++).  But 
some subset, implemented as a DSL, would be good:

element e = doc.first_element_matching("bookstore" / "book" / "price");

That could expand to something that assumed doc modelled an 'XML 
container' concept, as in:

element_iterator i = find(doc.child_elements().begin(),doc.child_elements().end(),"bookstore");
if (i==doc.end()) throw NotFound();
element_iterator j = find(i->child_elements().begin(),i->child_elements().end(),"book");
if (j==i->end()) throw NotFound();
element_iterator k = find(j->child_elements().begin(),j->echild_elements().nd(),"price");
if (k==j->end()) throw NotFound();
element e = *k;

I guess that if there's something I want to propose, it's that we 
consider what this XML container concept should look like.  In 
particular, I think that DOM is not what we want because it's too 
dissimilar to other C++ concepts (i.e. iterators).  (An adaptor for DOM 
would be possible, however.)

Regards,  Phil.