How to use json c++ with my own object?

Question

I am planning to use json c++ at https://github.com/nlohmann/json#examples . After reading its simple examples, I still have no idea how to use it with my own object? For example, I have a class

class Student
{
public:
    Student(int id, string const& name)
       : m_id(id), m_name(name)
    {}   

private:
    int m_id;
    string m_name;
};

How to use json to read and write (deserialize and serialize) a Student object?

AFAIK nlohmann-json is not a serialization library. It converts json string into c++ containers and vice-versa. You may want to checkout ThorsSerializer https://github.com/Loki-Astari/ThorsSerializer — Arunmu, Jul 12 '16 at 18:09
@Arunmu That is very good. Does ThorsSerializer support class polymorphism (base class pointer pointing to a derived class)? — user1899020, Jul 12 '16 at 18:19
Unfortunately, most of the C++ JSON libraries do not support serialization and deserialization to/from classes. — adam, Jul 12 '16 at 19:17
See response http://stackoverflow.com/a/39904141/606515 for JSON to/from C++ object conversion lib. — kola, Oct 07 '16 at 21:12
@user1899020 from the author of ThorsSerializer. Yes it does. — Martin York, Jul 08 '19 at 01:25
@Arunmu Why do you say nlohmann/json is not a serialization library. Because "serialization" always refers to a binary format and JSON is text? Thanks! — Patrick, Jan 11 '21 at 19:42
I would guess because it cannot just take an object and serialize it, you have to write all the plumbing yourself even if you can wrap it up in to_json and from_json methods. In most languages you can just give a serializer and object and it will use some default rules to make the json. — Mant101, Jan 14 '21 at 12:25

ΦXocę 웃 Пepeúpa ツ · Answer 1 · 2020-06-26T06:59:55.230

this is another way to do the conversion from json to custom class, that actually fits the best practices defined in the official repo from nlohmann here

https://github.com/nlohmann/json#arbitrary-types-conversions

h:

#ifndef STUDENT_H
#define STUDENT_H

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

class Student
{
public:
    Student();
    Student(int id, const std::string &name);
    int getId() const;
    void setId(int newId);

    std::string getName() const;
    void setName(const std::string &newName);

private:
    int m_id;
    std::string m_name;
};

//json serialization
inline void to_json(nlohmann::json &j, const Student &s)
{
    j["id"] = s.getId();
    j["name"] = s.getName();
}

inline void from_json(const nlohmann::json &j, Student &s)
{
    s.setId((j.at("id").get<int>()));
    s.setName(j.at("name").get<std::string>());
}
#endif // STUDENT_H

cpp:

#include "Student.h"

#include<string>

Student::Student() : Student(0, "")
{

}

Student::Student(int id, const std::string &name) : m_id{id}, m_name{name}
{

}

int Student::getId() const
{
    return this->m_id;
}

void Student::setId(int newId)
{
    m_id = newId;
}

std::string Student::getName() const
{
    return this->m_name;
}

void Student::setName(const std::string &newName)
{
   this->m_name = newName;
}

example:

Student s{0, "x"};
nlohmann::json studentJson = s;
std::cout << "Student from object: " << s.getId() << std::endl;
std::cout << "Student from json: " << studentJson.at("id").get<int>() << std::endl;

//String
std::string jSt = studentJson.dump();  //{"id":0,"name":"x"}
Student s2 = studentJson;
Student s3 = nlohmann::json::parse(jSt);

This requires implementing a getter/setter for all fields. This is probably not wanted for fields that should not be exposed (i.e., declared as _private_). — Patrick, Jan 11 '21 at 19:48

score 5 · Accepted Answer · answered Jul 12 '16 at 18:11

This library doesnt seems to have any interaction with a class for serialization and deserialization. But you can implement it yourself with a constructor and a getter.

using json = nlohmann::json;

class Student
{
public:
    Student(int id, string const& name)
       : m_id(id), m_name(name)
    {} 
    Student(json data)
       : m_id(data["id"]), m_name(data["name"])
    {}

    json getJson()
    {
        json student;
        student["id"] = m_id;
        student["name"] = m_name;

        return student;
    }

private:
    int m_id;
    string m_name;
};

How to use json c++ with my own object?

2 Answers2