How to provide function json& to_json() which is similar as 'void to_json(json&j, const CObject& obj)' ?

[akeyliu]

• Describe what you want to achieve.
  I've deifned functions to_json for user defined classes， i.e.
  class foo
  {
  friend inline void to_json(json& j, const foo& obj) {...}.
  }
  These code work well.
  But I wonder if we can define the function like 'json& to_json(void) {...}' as a memeber function in the class foo?
  The purpose for this function is that I want to use virtual function of basic class, and all derived classes can automatic define their reload funtion such as 'cout << to_json() << endl';
  If can, how to define this function?

• Describe what you tried.

• Describe which system (OS, compiler) you are using.
  Ubuntu 18.04 LTS, g++ 7.3;

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

[nlohmann]

This works:

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

using json = nlohmann::json;

class Base
{
  public:
    virtual json as_json() const = 0;
};

class A : public Base
{
  public:
    json as_json() const override { return json("A"); }
};

class B : public Base
{
  public:
    json as_json() const override { return json("B"); }
};

void to_json(json& j, const Base& obj)
{
    j = obj.as_json();
}

int main()
{
    A a;
    B b;
    std::cout << json(a) << " " << json(b) << std::endl;
}

[akeyliu]

Thank a lot, and I confirmed it's OK for me.