Automatic Conversion to JSON with base struct

The following code is mostly copied from documentation Automatic Conversion to JSON

#include <iostream>
#include <vector>
#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/json.hpp>
#include <boost/utility/string_view.hpp>
namespace NS
{
    struct A
    {
        int x = 0;
        std::string s;
    };
    BOOST_DESCRIBE_STRUCT(A, (), (x, s));

    struct C: A
    {
        int y = 0;
    };
    BOOST_DESCRIBE_STRUCT(C, (A), (y));

    template<class T,
        class D1 = boost::describe::describe_members<T,
        boost::describe::mod_public | boost::describe::mod_protected>,
        class D2 = boost::describe::describe_members<T, boost::describe::mod_private>,
        class En = std::enable_if_t<boost::mp11::mp_empty<D2>::value && !std::is_union<T>::value> >
    void tag_invoke(boost::json::value_from_tag const&, boost::json::value& v, T const& t)
    {
        auto& obj = v.emplace_object();

        boost::mp11::mp_for_each<D1>([&](auto D) {

            obj[D.name] = boost::json::value_from(t.*D.pointer);

            });
    }

    template<class T> void extract(boost::json::object const& obj, char const* name, T& value)
    {
        value = boost::json::value_to<T>(obj.at(name));
    }

    template<class T,
        class D1 = boost::describe::describe_members<T,
        boost::describe::mod_public | boost::describe::mod_protected>,
        class D2 = boost::describe::describe_members<T, boost::describe::mod_private>,
        class En = std::enable_if_t<boost::mp11::mp_empty<D2>::value && !std::is_union<T>::value> >
    T tag_invoke(boost::json::value_to_tag<T> const&, boost::json::value const& v)
    {
        auto const& obj = v.as_object();

        T t{};

        boost::mp11::mp_for_each<D1>([&](auto D) {

            extract(obj, D.name, t.*D.pointer);

            });

        return t;
    }
}

namespace NS1
{
    using namespace NS;

    void fun()
    {
        C c;
        auto jv2 = boost::json::value_from(c);
        jv2.as_object()["x"] = 5;
        auto c1 = boost::json::value_to<C>(jv2);

        std::cout << jv2 << std::endl;
        std::cout<<c1.x << std::endl;
    }
}
int main()
{
    NS1::fun();
}

But it seems value_from and value_to will ignore base class. So manually set "x" to 5 is useless on value c1.

I work out a solution by mimicking example of serialization:

#include <iostream>
#include <vector>
#include <boost/describe.hpp>
#include <boost/mp11.hpp>
#include <boost/json.hpp>
#include <boost/utility/string_view.hpp>
namespace NS
{
    struct A
    {
        int x = 0;
        std::string s;
    };
    BOOST_DESCRIBE_STRUCT(A, (), (x, s));

    struct C : A
    {
        int y = 0;
    };
    BOOST_DESCRIBE_STRUCT(C, (A), (y));

    void merge(boost::json::object& dest, const boost::json::object& src)
    {
        for (auto it = src.begin(); it != src.end(); ++it)
        {
            dest.insert_or_assign(it->key(), it->value());
        }
    }

    template<class T,
        class D1 = boost::describe::describe_bases<T, boost::describe::mod_public>,
        class D2 = boost::describe::describe_bases<T,
        boost::describe::mod_protected | boost::describe::mod_private>,
        class D3 = boost::describe::describe_members<T,
        boost::describe::mod_public | boost::describe::mod_protected>,
        class D4 = boost::describe::describe_members<T, boost::describe::mod_private>,
        class En = std::enable_if_t< boost::mp11::mp_empty<D2>::value&&
        boost::mp11::mp_empty<D4>::value && !std::is_union<T>::value> >
    void tag_invoke(boost::json::value_from_tag const&, boost::json::value& v, T const& t)
    {
        auto& obj = v.emplace_object();

        //base
        boost::mp11::mp_for_each<D1>([&](auto D) {
            using B = typename decltype(D)::type;
            const B& base = t;
            merge(obj, boost::json::value_from(base).as_object());
            });
        //member
        boost::mp11::mp_for_each<D3>([&](auto D) {
            obj[D.name] = boost::json::value_from(t.*D.pointer);
            });
    }

    template<class T> void extract(boost::json::object const& obj, char const* name, T& value)
    {
        value = boost::json::value_to<T>(obj.at(name));
    }

    template<class T,
        class D1 = boost::describe::describe_bases<T, boost::describe::mod_public>,
        class D2 = boost::describe::describe_bases<T,
        boost::describe::mod_protected | boost::describe::mod_private>,
        class D3 = boost::describe::describe_members<T,
        boost::describe::mod_public | boost::describe::mod_protected>,
        class D4 = boost::describe::describe_members<T, boost::describe::mod_private>,
        class En = std::enable_if_t< boost::mp11::mp_empty<D2>::value&&
        boost::mp11::mp_empty<D4>::value && !std::is_union<T>::value> >
    T tag_invoke(boost::json::value_to_tag<T> const&, boost::json::value const& v)
    {
        auto const& obj = v.as_object();

        T t{};

        boost::mp11::mp_for_each<D1>([&](auto D) {
            using B = typename decltype(D)::type;
            B& base = t;
            base = boost::json::value_to<B>(v);
            });

        boost::mp11::mp_for_each<D3>([&](auto D) {
            extract(obj, D.name, t.*D.pointer);
            });

        return t;
    }

}

namespace NS1
{
    using namespace NS;

    void fun()
    {
        C c;
        auto jv = boost::json::value_from(c);
        std::cout << jv << std::endl;

        jv.as_object()["x"] = 99;
        std::cout << jv << std::endl;
        auto c1 = boost::json::value_to<C>(jv);

        auto jv1 = boost::json::value_from(c1);
        std::cout << jv1 << std::endl;
    }
}
int main()
{
    NS1::fun();
}

Thank developers of boost.describe and boost.json.

boostorg / describe

Automatic Conversion to JSON with base struct #42