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Subject: [boost] [PropertyTree] Rewrite - Please Review
From: Sebastian Redl (sebastian.redl_at_[hidden])
Date: 2009-06-12 11:37:37

Hi PropertyTree users,

I have completed the main bulk of the property tree rewrite/cleanup. In
the course of this, I have made the code standard-compliant (it
previously used a std::list of an incomplete type, which is not,
strictly speaking, valid), made it follow the claimed complexities
(find() was O(n), although the docs said it's O(log n)), and revised the
customization interface quite thoroughly.

Inevitably, there were breaking changes in this process.

I would ask everyone who has existing code using PropertyTree to look at
these changes and tell me their best guess at how much effort it would
be to update the code to conform with the new interface, and how I could
make this easier.

The development is still in a branch, so you need to check out the SVN
branch to
try out the new version.

The breaking changes are as follows:
1) Probably the most common change need: ptree::find() now returns the
new type assoc_iterator (const_assoc_iterator for the const version)
instead of iterator (const_iterator). This means that you need to change
the type of the variable you capture the result with to assoc_iterator.
It also means you need to compare the result of find() against
ptree::not_found() instead of ptree::end(). You can get an iterator from
an assoc_iterator by passing it to ptree::to_iterator(). The other
projection direction is currently not supported, although it would be
trivial to add.

2) The number and order of template arguments to basic_ptree has been
changed. The old arguments were
* Key comparison predicate
* Key type
* Path type
* Data type
* Translator type

The new arguments are
* Key type
* Data type
* Key comparison predicate (defaults to std::less<Key>)

The old order prevented defaulting and wasn't very useful in general.
The new order is consistent with std::map. The path and translator
arguments have been removed to simplify the interface of basic_ptree.
Equivalent functionality is available (see below).

If you used your own instantiation of basic_ptree before, instead of
simply using the (wi)ptree typedefs, you will have to adapt your code to
the new order. If you used custom path and/or translator types, read on.

3) put and put_child previously had a final parameter 'bool
do_not_replace'. I don't like boolean parameters, especially those whose
name contains a negation. I removed the parameter, and replaced its
functionality by a pair of new functions, add and add_child. If you
previously called put or put_child passing true for do_not_replace, you
must change these calls to add or add_child, respectively. If you ever
passed false explicitly, you have to remove this parameter.

4) Using custom path types has changed. You can no longer create
individual basic_ptree instantiations with custom path types. Instead, a
key type has a fixed path type associated with it. I can bring this
functionality back pretty easily if required, but I want to see use
cases for this first. (Such things are why I make this call for
reviews.) Less template parameters to ptree is good, in my opinion.
To use a key type other than std::basic_string specializations, you have
to specialize boost::property_tree::path_of for this key type. The
path_of struct contains a single type called type, which is the path to
use for this key type.
The way paths are implemented has also changed thoroughly. Previously,
you had to implement a const and a non-const variant of get_child, plus
put_child (with do_not_replace making that effectively two functions
too). These functions contained a lot of logic that really belonged into
ptree, making implementing custom paths tedious and error-prone.
The new interface requires you to implement four functions, but they're
far simpler. The functions are described in ptree_fwd.hpp in C++0x
concept syntax, but here's the gist:
bool path::empty() const -> is the path empty?
bool path::single() const -> does the path contain a single element?
key_type path::reduce() -> chop off the first part of this path and
return it (basically a pop_front that returns the value).
std::string dump() const -> get a readable form of this path for
exception messages; feel free to return "" here if you don't care.

The new interface should be easy to implement, and the tree traversal is
now where it belongs (in the internal implementation functions walk_path
and force_path).

5) Using custom translators has changed. A ptree no longer has a fixed
translator associated with it. Instead, you can supply a custom
translator to every get_value/get/put_value/put operation or use the
default, which depends on the tree's data type and the type you
extract/insert. An example:

ptree pt;
pt.put("k1.k2", 100); // Uses the default translator for std::string+int.
// Note: the explicit template parameter here will go away.
float f = pt.get<float>("k1.k2", my_special_float_string_converter());

The tree's own data type is called the internal type, while the
parameter/return type is called the external type. The default
translator for a internal+external pair is set by specializing the
boost::property_tree::translator_between struct. By default,
PropertyTree supplies an id_translator for when the internal and
external types are the same, and a stream_translator for converting
between a basic_string and anything else. Together, they faithfully
emulate the behavior of the old default translator.

Writing a custom translator is easy. It's a class that provides two
typedefs and two functions:
typename internal_type -> the internal data type
typename external_type -> the external data type
optional<external_type> Tr::get_value(internal_type) -> convert from the
internal to the external type, boost::none on failure
optional<internal_type> Tr::put_value(external_type) -> convert from the
external to the internal type, boost::none on failure

That's it for the changes. The new version of property tree is not quite
complete yet. I still need to polish the documentation a little and
document the new extension mechanisms properly. I also need to clean up
a little. Finally, the INI parser doesn't work at the moment.

However, the main functionality (tested by property_tree_test) plus the
XML, JSON and INFO parsers should all work now.

Looking forward to your feedback.

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