accessing elements from nlohmann json

Question

My JSON file resembles this

{
  "active" : false,
  "list1" : ["A", "B", "C"],
  "objList" : [
    {
     "key1" : "value1",
     "key2" : [ 0, 1 ]
    }
   ]
}

Using nlohmann json now, I've managed to store it and when I do a dump jsonRootNode.dump(), the contents are represented properly.

However I can't find a way to access the contents.

I've tried jsonRootNode["active"], jsonRootNode.get() and using the json::iterator but still can't figure out how to retrieve my contents.

I'm trying to retrieve "active", the array from "list1" and object array from "objList"

Answer 1

The following link explains the ways to access elements in the JSON. In case the link goes out of scope here is the code

#include <json.hpp>

 using namespace nlohmann;

 int main()
 {
     // create JSON object
     json object =
     {
         {"the good", "il buono"},
         {"the bad", "il cativo"},
         {"the ugly", "il brutto"}
     };

     // output element with key "the ugly"
     std::cout << object.at("the ugly") << '\n';

     // change element with key "the bad"
     object.at("the bad") = "il cattivo";

     // output changed array
     std::cout << object << '\n';

     // try to write at a nonexisting key
     try
     {
         object.at("the fast") = "il rapido";
     }
     catch (std::out_of_range& e)
     {
         std::cout << "out of range: " << e.what() << '\n';
     }
 }

Answer 2

In case anybody else is still looking for the answer.. You can simply access the contents using the same method as for writing to an nlohmann::json object. For example to get values from json in the question:

{
  "active" : false,
  "list1" : ["A", "B", "C"],
  "objList" : [
    {
      "key1" : "value1",
      "key2" : [ 0, 1 ]
    }
  ]
}

just do:

nlohmann::json jsonData = nlohmann::json::parse(your_json);
std::cout << jsonData["active"] << std::endl;    // returns boolean
std::cout << jsonData["list1"] << std::endl;     // returns array

If the "objList" was just an object, you can retrieve its values just by:

std::cout << jsonData["objList"]["key1"] << std::endl;    // returns string
std::cout << jsonData["objList"]["key2"] << std::endl;    // returns array

But since "objList" is a list of key/value pairs, to access its values use:

for(auto &array : jsonData["objList"]) {
    std::cout << array["key1"] << std::endl;    // returns string
    std::cout << array["key2"] << std::endl;    // returns array
}

The loop runs only once considering "objList" is array of size 1.

Hope it helps someone

Answer 3

I really like to use this in C++:

for (auto& el : object["list1"].items())
{
  std::cout << el.value() << '\n';
}

It will loop over the the array.