boost : Just iterate over elements of a ptree - loops

Question

This should be simple (I'm just learning boost so I'm missing something)
I have read in some simple JSON using json_read and now have a ptree. All the examples on the web show using ptree.get("entry_name") to obtain an entry. All I want to do is something like:
ptree pt;
read_json(ss,pt);
BOOST_FOREACH(ptree::value_type &v, pt)
{
std::cout << v.{entry_name} << v.{value}
}
i.e. loop through the ptree and write out each name (i.e. what you put into pt.get()) and it's corresponding value.
Sorry if this is simple
Ross

Answer 1

I was searching the same thing, and couldn't find the answer anywhere. It turned out to be pretty simple indeed:
ptree pt;
/* load/fill pt */
for(iterator iter = pt.begin(); iter != pt.end(); iter++)
{
std::cout << iter->first << "," << iter->second.data() << std::endl;
}
iter->first is the entry name, and iter->second.data() is the entry value of the first level. (You can then re-iterate with iter->second.begin()/end() for deeper levels.)
Further, if one such node in this iteration is not a terminal node and is itself a ptree, you can get that as ptree from this iterator itself :
ptree subPt = iter->second.get_child("nodeName");

Answer 2

I'm having troubles with ptree as well, but perhaps this can help:
Check out boost's ptree quick tutorial
v.{entry_name}
would be
v.first
and
v.{value}
v.second.data()
Would that work?

Answer 3

Here's a great example of how to iterate a ptree using BOOST_FOREACH
http://akrzemi1.wordpress.com/2011/07/13/parsing-xml-with-boost/
for direct access using the normal "get" functions look at the example from boost:
http://www.boost.org/doc/libs/1_51_0/doc/html/boost_propertytree/tutorial.html
the documentation page is located here:
http://www.boost.org/doc/libs/1_51_0/doc/html/boost/property_tree/basic_ptree.html
I know its not very well documented but it is helpful.

Answer 4

Old thread, but here's a C++11 version of mr_georg's answer with range-based for loops:
ptree pt;
/* load/fill pt */
for(auto pair : pt)
{
std::cout << pair.first << "," << pair.second.data() << std::endl;
}
For this json:
{
"key1":"value1",
"key2":"value2"
}
It outputs:
key1,value1
key2,value2

Answer 5

This example iterates over a simple JSON object and puts its values into a vector.
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>
#include <boost/lexical_cast.hpp>
#include <iostream>
#include <string>
#include <vector>
#include <sstream>
int main (void)
{
try
{
std::stringstream ss;
std::string json_obj_str = "{ \"unit_1\": 1, \"unit_2\": 2, \"unit_3\": 3 }";
std::vector <float> fvec;
ss << json_obj_str; // put string into stringstream
boost::property_tree::ptree pt;
boost::property_tree::read_json(ss, pt); // put stringstream into property tree
// iterate over JSON properties
for (boost::property_tree::ptree::iterator iter = pt.begin(); iter != pt.end(); iter++)
{
std::cout << iter->first << ": " << iter->second.data() << std::endl;
fvec.push_back(boost::lexical_cast<float>(iter->second.data()));
}
for (size_t i = 0; i < fvec.size(); i++)
{
std::cout << "fvec.at(" << i << ") = " << fvec.at(i) << std::endl;
}
}
catch (const boost::property_tree::ptree_error &e)
{
std::cerr << "property_tree error = " << e.what() << std::endl;
return -2;
}
catch (std::exception const& e)
{
std::cerr << "exception = " << e.what() << std::endl;
return -1;
}
return 0;
}