How to Serialize derived class to JSON object?

[heartfly]

What do you want to achieve?

I'm trying to serialize my objects. They look like this (overly simplified):

class A {
 double Aa;
 double Ab;
};

class B : public A{
  int Ba;
  int Bb;
};

What have you tried?

I'm trying to create a JSON object like this:

B = {
 "Aa": 1.0,
 "Ab": 2.0,
 "Ba": 3,
 "Bb": 4
}

Can you provide a small code example?

inline void to_json( nlohmann::json& JSON, const A& a ) {
  JSON = { "Aa", a.Aa, "Ab", a.Ab };
}
inline void to_json( nlohmann::json& JSON, const B& b ) {
 // how to call the base class(A)'s to_json and from_json function?
  JSON = { "Ba", b.Ba, "Bb", b.Bb };
}

Please help me know how to call the base class's to_json and from_json function.

Describe which system (OS, compiler) you are using.
LLVM 3.8.1

Describe which version of the library you are using (release version, develop branch).
Fairly recent from master branch.

[nlohmann]

This should work:

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

using json = nlohmann::json;

struct A {
    double Aa;
    double Ab;
};

struct B : public A{
    int Ba;
    int Bb;
};

void to_json( nlohmann::json& JSON, const A& a ) {
    JSON = { {"Aa", a.Aa}, {"Ab", a.Ab} };
}
void to_json( nlohmann::json& JSON, const B& b ) {
    nlohmann::to_json(JSON, static_cast<A>(b));
    JSON.update({ {"Ba", b.Ba}, {"Bb", b.Bb} });
}

int main() {
    A a;
    a.Aa = 1.11;
    a.Ab = 2.22;
    
    B b;
    b.Aa = 1.1;
    b.Ab = 2.2;
    b.Ba = 3;
    b.Bb = 4;
    
    json ja = a;
    std::cout << ja << std::endl;
    
    json jb = b;
    std::cout << jb << std::endl;
}

Output:

{"Aa":1.11,"Ab":2.22}
{"Aa":1.1,"Ab":2.2,"Ba":3,"Bb":4}

[heartfly]

thx.
but how can I call the super class's from_json?
I try this code:

    void from_json( const nlohmann::json& JSON, A& a ) {
        JSON.at("Aa").get_to(a.Aa);
        JSON.at("Ab").get_to(a.Ab);
    }
    void from_json( const nlohmann::json& JSON, B& b ) {
        nlohmann::from_json(JSON, static_cast<A>(b));
        JSON.at("Ba").get_to(b.Ba);
        JSON.at("Ba").get_to(b.Bb);
    }

but got an error

• No matching function for call to object of type 'const nlohmann::detail::from_json_fn'.

[nlohmann]

The error message gives a hint:

json.hpp:3604:10: Candidate function [with BasicJsonType = nlohmann::basic_json<std::map, std::vector, std::__1::basic_string<char>, bool, long long, unsigned long long, double, std::allocator, adl_serializer, std::__1::vector<unsigned char, std::__1::allocator<unsigned char> > >, T = A] not viable: expects an l-value for 2nd argument

Changing the second argument to a reference works:

void from_json( const nlohmann::json& JSON, B& b ) {
    nlohmann::from_json(JSON, static_cast<A&>(b));
    JSON.at("Ba").get_to(b.Ba);
    JSON.at("Bb").get_to(b.Bb);
}

[heartfly]

It works! thank you @nlohmann