c++ - Access to specific index in ptree array -


i using boost library manipulate json file , access specific index of array in json.

boost::property_tree::ptree& jsonfile; const boost::property_tree::ptree& array =   jsonfile.get_child("my_array");

what accessing value stored @ index :

// code not compile int value = array[index].get < int > ("property");

just code using 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 want have index checked 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("");      (; 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("");      (; 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);      // 4th element:     std::cout << element_at_checked(pt, "a", 3) << "\n";      // int     std::cout << element_at_checked<int>(pt, "a", 3) << "\n";      // 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

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