Access to specific index in ptree array - c++

Question

I am using boost library to manipulate a JSON file and I would like to access to a specific index of an array in this JSON.
boost::property_tree::ptree& jsonfile;
const boost::property_tree::ptree& array =
jsonfile.get_child("my_array");
What I would like to do is accessing to the value stored at index :
// This code does not compile
int value = array[index].get < int > ("property");

Answer 1

Just code it using the iterators:
template <typename T = std::string>
T element_at(ptree const& pt, std::string name, size_t n) {
return std::next(pt.get_child(name).find(""), n)->second.get_value<T>();
}
If you want to have the index checked for bounds:
template <typename T = std::string>
T element_at_checked(ptree const& pt, std::string name, size_t n) {
auto r = pt.get_child(name).equal_range("");
for (; r.first != r.second && n; --n) ++r.first;
if (n || r.first==r.second)
throw std::range_error("index out of bounds");
return r.first->second.get_value<T>();
}
Demo
Live On Coliru
#include <boost/property_tree/json_parser.hpp>
#include <iostream>
using boost::property_tree::ptree;
template <typename T = std::string>
T element_at(ptree const& pt, std::string name, size_t n) {
return std::next(pt.get_child(name).find(""), n)->second.get_value<T>();
}
template <typename T = std::string>
T element_at_checked(ptree const& pt, std::string name, size_t n) {
auto r = pt.get_child(name).equal_range("");
for (; r.first != r.second && n; --n) ++r.first;
if (n || r.first==r.second)
throw std::range_error("index out of bounds");
return r.first->second.get_value<T>();
}
int main() {
ptree pt;
{
std::istringstream iss("{\"a\":[1, 2, 3, 4, 5, 6]}");
read_json(iss, pt);
}
write_json(std::cout, pt, false);
// get the 4th element:
std::cout << element_at_checked(pt, "a", 3) << "\n";
// get it as int
std::cout << element_at_checked<int>(pt, "a", 3) << "\n";
// get non-existent array:
try { std::cout << element_at_checked<int>(pt, "b", 0) << "\n"; } catch(std::exception const& e) { std::cout << e.what() << "\n"; }
try { std::cout << element_at_checked<int>(pt, "a", 6) << "\n"; } catch(std::exception const& e) { std::cout << e.what() << "\n"; }
}
Prints
{"a":["1","2","3","4","5","6"]}
4
4
No such node (b)
index out of bounds