I am using the nlohmann JSON C library to read a JSON file with this structure:

{
  "Main": [
    {
      "obj1": "bar"
    },
    {
      "obj2": "foo"
    }
  ]
}

The main object with key known to the user "Main" contains an array of objects with unknown key names.

I want to transfer the JSON object in the following program to a C structure. How could this be done?

#include <nlohmann/json.hpp>
#include <iostream>
#include <fstream>
using json = nlohmann::json;

int main()
{
  std::ifstream ifs("../test/test_2.json");
  json js = json::parse(ifs);

  for (json::iterator it = js.begin(); it != js.end();   it) 
  {
    std::cout << it.key() << " :\n";
    std::cout << it.value() << "\n";
  }

  if (js.contains("Main"))
  {
    json a = js["Main"];
    for (size_t idx = 0; idx < a.size();   idx)
    {
      json o = a.at(idx);
      std::cout << o << "\n";
    }
  }
  return 0;
}

Output:

Main :
[{"obj1":"bar"},{"obj2":"foo"}]
{"obj1":"bar"}
{"obj2":"foo"}

CodePudding user response：

You could parse the vector of maps under Main with:

auto objects{ j.at("Main").get<objects_t>() };

Where:

using object_t = std::map<std::string, std::string>;
using objects_t = std::vector<object_t>;

[Demo]

#include <fmt/ranges.h>
#include <iostream>  // cout
#include <map>
#include <nlohmann/json.hpp>
#include <string>
#include <vector>

using json = nlohmann::json;

using object_t = std::map<std::string, std::string>;
using objects_t = std::vector<object_t>;

int main() {
    std::string str{R"({ "Main": [ { "obj1": "bar" }, { "obj2": "foo" } ] })"};
    json j = json::parse(str);
    auto objects{ j.at("Main").get<objects_t>() };
    fmt::print("{}", objects);
}

// Outputs:
//
//   [{"obj1": "bar"}, {"obj2": "foo"}]

CodePudding user response：

Yes, but the API gives an automatic way to do this by means of NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE; it seems it would be like just defining a vector like this, but this program gives the error

[json.exception.out_of_range.403] key 'key' not found

program

#include <nlohmann/json.hpp>
#include <iostream>
#include <fstream>
using json = nlohmann::json;

struct key_value_t
{
  std::string key;
  std::string value;
};
NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE(key_value_t, key, value);

int main()
{
  try
  {
    std::string str{ R"({ "Main": [ { "obj1": "bar" }, { "obj2": "foo" } ] })" };
    json js = json::parse(str);

    std::vector<key_value_t> kv;
    if (js.contains("Main"))
    {
      js.at("Main").get_to(kv);
    }
  }

  catch (std::exception& e)
  {
    std::cout << e.what() << '\n';
  }
  return 0;
}