Re: [boost] [property tree][review]

From: Ryan Gallagher (ryan.gallagher_at_[hidden])
Date: 2006-04-25 02:00:25

Matias Capeletto <matias.capeletto <at> gmail.com> writes:
> This is where the path concept will allow some magic. IMO the 'most'
> important property of the having a path class is the orthogonality
> that it introduce. Iterate with a path in a ptree and constructing a
> path are now two clearly separated process.
> 

I have to agree with this statement.  I've been thinking that path should be 
a separate concept.  Instances of the concept could even be worked on 
separately.

I really see building a path much as you would build an expression template 
to be evaluated when invoked by the getter.  I've been thinking something like:

template<typename Path>
ptree ptree::get(Path const& p)
{
   return eval(p, *this);
}

One good concretization of this concept would be an xpath-like implementation. 
Some of the additions for this would be quite similar to what you mentioned in 
a previous thread with your overload of operator%.  I would instead be 
interested in seeing operator[] implemented in this way since this is more
xpath-like and IMHO easier to read.  (Disadvantage is precedence.)  That is 

xpath_expression& xpath_expression::operator[](where_expression const&).

where_expression would be very similar to lambda.  Based on your example
from the other thread:

data.get((p / "log")[_1 / "text" == "error1"] / "checked" + "id");

Following along the xpath and xslt I'd also like to see grouping.  In my mind
there should be an accessor to get the group resulting from evaluating the
xpath expression.  A group would be modeled by a view of ptree nodes.
For:
<p>
  <log><text>error1</text><checked id=0>foo</checked></log>
  <log><text>warning</text><checked id=1>foo</checked></log>
  <log><text>error1</text><checked id=2>foo</checked></log>
  <log><text>error1</text><unchecked id=3>foo</unchecked></log>
</p>

group result_group 
  = tree.get((xpath("p") / "log")[_1 / "text" == "error1"] / "checked" + "id");

BOOST_FOREACH(ptree& result, result_group)
{
  cout << result.data() << " ";
}

would output: "0 2".

Just my 2c,

-Ryan