Updation of child object isn't reflected in parent Object

[traw]

I've following snippet, where I read and modify child obj attributes with at, But it doesn't get reflected in parent Obj. At the end of following code _root is written to file, But with previoius values of _child["array"].

void fun(std::vector<int> &vec) {
        std::ifstream in(file);
        _root = Json::parse(in);
        in.close();
        if (_root.find("child") != _root.end()) {
            _child = _root.at("child");
            _child["array"] = vec;
          std::ofstream writer(file);
          writer << root;    
        }
}

For example Previous values for _child["array"] were [1, 2] and vec contains [99, 98]. So at the end it should be 99, 98.

Previous Json values:

{
  "child" : {
     "array" : [1, 2]
  }
} 

Expected Json Values:

{
  "child" : {
     "array" : [99, 98]
  }
} 

Am I doing it right?

[nlohmann]

I do not know the type of _child, but if it is json &, then it should work.

This is a small example:

#include "json.hpp"
#include <iostream>

using json = nlohmann::json;

json _root;

void fun(std::vector<int> &vec) {
    if (_root.find("child") != _root.end()) {
        json& _child = _root.at("child");
        _child["array"] = vec;
        std::cout << _root << std::endl;
    }
}

int main(int argc, char** argv)
{
    _root = R"({
        "child" : {
            "array" : [1, 2]
        }
    })"_json;

    std::vector<int> vec = {98, 99};
    fun(vec);
}

Output: {"child":{"array":[98,99]}}