C++でnlohmann::json (読み込み)

#include <iostream>

#include <nlohmann/json.hpp>
#include <fstream>

int main()
{
    // json形式の文字列
    std::string jsontext = R"(
{
    "information":{
      "Name":"Tarou",
      "Age":18,
      "weight":60.5
    },
    "data":{
      "dates":[
        "2023-10-01T12:00:00+09:00",
        "2023-10-02T12:00:00+09:00"
      ],
      "countries":[
        {
          "id":1,
          "name":"Russia"
        },
        {
          "id":2,
          "name":"France"
        }
      ]
    }
}
)";


    //std::string filename = R"(C:\test\data\data.json)";
    //std::ifstream file(filename);
    //nlohmann::json jsondata = nlohmann::json::parse(file);

    nlohmann::json jsondata = nlohmann::json::parse(jsontext);

    {
        nlohmann::json information = jsondata["information"];

        // データの取得
        std::string name = information["Name"];
        int age = information["Age"];
        double weight = information["weight"];

        std::cout << "Name: " << name << std::endl;
        std::cout << "Age: " << age << std::endl;
        std::cout << "Weight: " << weight << std::endl;
    }
    {
        nlohmann::json data = jsondata["data"];
        {
            nlohmann::json dates = data["dates"];
            for (const auto& date : dates) {
                std::string date_str = date.get<std::string>();
                std::cout << "Date: " << date_str << std::endl;
            }
        }
        {
            nlohmann::json countries = data["countries"];
            for (const auto& country : countries) {
                int id = country["id"];
                std::string name = country["name"];
                std::cout << "Country ID: " << id << ", Name: " << name << std::endl;
            }
        }
    }    

}

struct Country {
    int id;
    std::string name;
};

// .get関数を使用するための定義
void from_json(const nlohmann::json& j, Country& p) {
    j.at("id").get_to(p.id);
    j.at("name").get_to(p.name);
}

int main()
{
    std::string filename = R"(C:\test\data\data.json)";
    std::ifstream file(filename);
    nlohmann::json jsondata = nlohmann::json::parse(file);

    nlohmann::json data = jsondata["data"];
    {

        std::vector<std::string> vectors = data["dates"];
        for (const auto& date : vectors) {
            std::cout << "Date from vector: " << date << std::endl;
        }
    }

    {
        std::vector<Country> countries = data["countries"].get<std::vector<Country> >();
        for (const auto& country : countries) {
            std::cout << "Country ID: " << country.id << ", Name: " << country.name << std::endl;
        }

    }

}