How to parse nested arrays inside json using C++

Question

I know how to parse "normal" looking JSON data in C++. Usually, I do this, using boost::property_tree and read_json method. It may look like so:

BOOST_FOREAH(ptree::value_type &v, pt.get_child("rows"){
    vec.push_back(v.second.get<std::string>("key"));
}

and the code above corresponds to this JSON file:

{
    "rows":[{
         "key":"1"
     },{
         "key":"2"
     }]
}

However, the Neo4j result-set that I get, looks like:

{
    "columns":{...},
    "data":[[["object 1"]], [["object 2"]], [["object 3"]]]
}

I'm interested and want to parse "data" node. I tried to do it like so:

BOOST_FOREAH(ptree::value_type &v, pt.get_child("data"){
    vec.push_back(v.second.data());
}

but this does not work. I do not get an error, but my vector vec remains empty, or to be more precise it is populated with empty values. So, that when I iterate through this vec I see a number of elements, but they do not have any value. Whereas, I want to have values "object 1", "object 2", "object 3".

github.com/jeaye/jeayeson, json_spirit, to name just two libraries that will do this. — Richard Hodges
– Richard Hodges, Commented Aug 8, 2015 at 20:34
These libraries look promissing, but are you 100% sure, that they will parse my json file? — Jacobian
– Jacobian, Commented Aug 8, 2015 at 20:45
@Jacobian If library claims that it supports parsing of standard JSON then it will parse standard JSON otherwise it is buggy. Or your question is not clear. — c-smile
– c-smile, Commented Aug 8, 2015 at 21:00
The answer is literally in the ~20 lines of documentation specific to the JSON backend. It's also a very frequently asked question here so a quick search of boost-propertytree + array should give you helpful answers in seconds. I would like to conclude with my caveat that Boost doesn't come with a JSON or XML library. It does have a property tree library. People confusing the two are a very frequent occurrence and Stack Overflow is littered with the wreckage of that. — sehe
– sehe, Commented Aug 8, 2015 at 22:02

Jacobian · Accepted Answer · 2015-08-09 07:14:34Z

2

The solution looks like this:

using boost::property::ptree;
ptree pt;
//... populate ptree pt with data from some source
BOOST_FOREACH(ptree::value_type &v, pt.get_child('data')){
    ptree subtree1 = v.second;
    BOOST_FOREACH(ptree::value_type &vs, subtree1){
        ptree subtree2 = vs.second;
        BOOST_FOREACH(ptree::value_type &vs2, subtree2){
            do_something(vs2.second.data());
        }
    }
}

This code makes it possible to parse such JSON structure:

{
 "data":[[["object 1"]], [["object 2"]], [["object 3"]]]
}

So, contrary to what some people are saying, actually, there is no need to use other third-party libraries. Use just boost and you are done.

answered Aug 9, 2015 at 7:14

Jacobian

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2 Comments

Jacobian Over a year ago

Though, I do not like this code. I wish there was some sort of simplification. Since, copy pasting BOOST_FOREACH looks rather technical.

sehe Over a year ago

Indeed. There's often no need for third party libraries. Although you're just lucky this covers your needs. Good luck if ever you need to format a proper JSON number. This is why is even worse to evangelise Boost Property Tree as a JSON library.

xinthose · Accepted Answer · 2019-03-18 13:25:04Z

This is an example of how I do it. You have to know the JSON structure ahead of time.

#include <boost/lexical_cast.hpp>
#include <boost/property_tree/ptree.hpp>
#include <boost/property_tree/json_parser.hpp>

boost::property_tree::ptree pt, sub_pt;
std::string json_str, key, sub_key;
std::stringstream ss;
int value = 0, bus_num = 0;

json_str = "{\"arduino_1\": {\"bus_1\": 17425,\"bus_2\": 1025,\"bus_3\": 0,\"bus_4\": 0,\"bus_5\": 0,\"bus_6\": 0,\"bus_7\": 0,\"bus_8\": 0}}";


ss << json_str; // put string into stringstream
boost::property_tree::read_json(ss, pt);    // put stringstream into property tree
for (boost::property_tree::ptree::iterator iter = pt.begin(); iter != pt.end(); iter++)
{
    // get data
    key = boost::lexical_cast <std::string>(iter->first.data());
    sub_pt = iter->second;

    // iterate over subtree
    for (boost::property_tree::ptree::iterator sub_iter = sub_pt.begin(); sub_iter != sub_pt.end(); sub_iter++)
    {
        // get data
        sub_key = boost::lexical_cast <std::string>(sub_iter->first.data());
        value = boost::lexical_cast <int>(sub_iter->second.data());
    }
}

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