using json spirit to read the name and value of Value objects in C++

Question

I have a text file that's got data written in JSON format. The data looks something like this --

[
   ...
    {
    "volume": 93, 
    "id": "part-30", 
    "value": 19
  }, 
  {
    "volume": 83, 
    "id": "part-31", 
    "value": 19
  }
  ...
]

After referring to this and this

I've gotten to a point where I can read the "name" field of the following data structure. So all other things aside, my code to read this object looks like this --

// read from parts list file to JSON object.
    const char* file_name2( "parts_list.txt" );
    ifstream is2( file_name2 );
    json_spirit::Value value2;
    read( is2, value2 );
    // const Object& addr_array = value.get_obj();
    vector<Value> jsonObj2 = value2.get_array();
    vector<Value>::iterator it;
    vector<RobotParts> final;

    for(it = jsonObj2.begin(); it!=jsonObj2.end(); it++)
    {
        auto valObj = it->get_obj();
        RobotParts rpObj = RobotParts();
        for(auto vo : valObj)
        {
            if(vo.name_=="volume"){
                string s = vo.value_;
            }
        }
        final.push_back(rpObj);
    }

    cout << final.size() << endl;
    return 0;

But this line here --> vo.value_; seems to be creating a lot of problems. I'm unable to figure out what's the data type of this object. So far I've tried :

1. Reading into an integer. I thought since the volume has an integer value

int i = vo.get_value< int >();

Should work. But instead, it says

error: no member named 'get_value' in
  'json_spirit::Pair_impl<json_spirit::Config_vector<std::__1::basic_string<char> > >'

2. Reading into a string so I could print it out.

string s = vo.value_;

This throws the following error:

error: no viable conversion from 'Value_type' (aka
      'Value_impl<json_spirit::Config_vector<std::__1::basic_string<char> > >') to 'string' (aka 'basic_string<char, char_traits<char>,
      allocator<char> >')

I can, however, print out vo.name_ by using cout << vo.name_ << endl within the loop. This outputs :

...
volume
id
value
volume
id
value
... 

so on.

I know this is due to data type incompatibility but I've spent two-three hours now unable to figure out how to access this value.

How do I access these values corresponding to volume, id and value?