test case to have little containers with extra arguments in some cases

optional_extra_data.cpp

#include <cstddef>
#include <utility>
#include <iostream>

template <typename Obj>
using optional_data_type = decltype(std::declval<Obj>().optional());

template <typename Obj>
struct Serializer {
    public:
        Serializer(const Obj& obj): m_id(obj.id()), m_optional(obj.optional()) {
        }
    private:
        int m_id;
        optional_data_type<Obj> m_optional;
};


template <typename Obj>
auto make_serializer(const Obj& o) { return Serializer<Obj>(o); }

class A{
    public:
    int id() const { return m_id; }
    auto optional() const {return nullptr;}
    auto serialize() const { return make_serializer(*this); }
    private:
        int m_id = 0;
};
class B{
    public:
        struct optional_data {
            public:
                optional_data(int i) { std::cout << "Optional data: " << i << std::endl; }
        };
    int id() const { return m_id; }
    auto optional() const {return optional_data(4);}
    
    auto serialize() const { return make_serializer(*this); }
    private:
        int m_id = 1;
};



int main() {
    A a;
    B b;
    std::cout << "A serialize: " << std::endl;
    auto as = a.serialize();
    std::cout << "B serialize: " << std::endl;
    auto bs = b.serialize();
    return 0;
}