Can't get_to vector of object inside itself with Nlohmann's JSON

Question

I've got the following code (simplified):

namespace nlohmann {
// https://github.com/nlohmann/json/issues/1749#issuecomment-772996219
template <class T>
void to_json(nlohmann::json &j, const std::optional<T> &v) {
    if (v.has_value())
        j = *v;
    else
        j = nullptr;
}

template <class T>
void from_json(const nlohmann::json &j, std::optional<T> &v) {
    if (j.is_null())
        v = std::nullopt;
    else
        v = j.get<T>(); /* ERROR LOCATION */
}
} // namespace nlohmann

namespace discordpp{
class Component {
  public:
    Component(const std::optional<std::vector<Component>> &components)
        : components(components) {}

    std::optional<std::vector<Component>> components;

    // (Resolved) NLOHMANN_DEFINE_TYPE_INTRUSIVE(Component, components)
    friend void to_json(nlohmann::json &nlohmann_json_j,
                        const Component &nlohmann_json_t) {
        nlohmann_json_j["components"] = nlohmann_json_t.components;
    }
    friend void from_json(const nlohmann::json &nlohmann_json_j,
                          Component &nlohmann_json_t) {
        nlohmann_json_j.at("components").get_to(nlohmann_json_t.components); /* ERROR LOCATION */
    }
};
}// namespace discordpp

Compiling returns the error error: no matching function for call to ‘nlohmann::basic_json<>::get<std::vector<discordpp::Component, std::allocator<discordpp::Component> > >() const’ Is there maybe something I could predeclare to solve this?

Tried setting up an adl_serializer like this, same issue

namespace nlohmann {
template<> struct adl_serializer<discordpp::Component> {
    static void to_json(json &nlohmann_json_j, const discordpp::Component &nlohmann_json_t) {
        NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(
            NLOHMANN_JSON_TO, type, custom_id, disabled, style, label, emoji,
            url, options, placeholder, min_values, max_values, components))
    }

    static void from_json(const json &nlohmann_json_j, discordpp::Component &nlohmann_json_t) {
        NLOHMANN_JSON_EXPAND(NLOHMANN_JSON_PASTE(
            NLOHMANN_JSON_FROM, type, custom_id, disabled, style, label, emoji,
            url, options, placeholder, min_values, max_values, components))
    }
};
} // namespace nlohmann

I changed it to a vector of shared pointers and now I get error: no matching function for call to ‘nlohmann::basic_json<>::get<discordpp::Component>() const’ in the adl_serializer I wrote to handle it

template <typename T> struct adl_serializer<std::vector<std::shared_ptr<T>>> {
    static void to_json(json &j, const std::vector<std::shared_ptr<T>> &v) {
        j = json::array();
        for (auto t : v) {
            j.push_back(*t);
        }
    }

    static void from_json(const json &j, std::vector<std::shared_ptr<T>> &v) {
        if (!j.is_array()) {
            throw json::type_error::create(317, "j must be an array", j);
        }
        for (auto t : j) {
            v.push_back(std::make_shared<T>(j.get<T>())); /* Error is here */
        }
    }
};

Mirrored at https://github.com/nlohmann/json/discussions/3047

Edit:

Full source (do a recursive clone): https://github.com/DiscordPP/echo-bot/tree/dev-objects

Failed build: https://github.com/DiscordPP/echo-bot/runs/3799394029?check_suite_focus=true

Edit 2: It's fine when I do std::vector<std::shared_ptr<Component>> components;, it just doesn't seem to be able handle 2 third party parsers, maybe?

Answer 1

I was doing some thinking on @Niels's answer and I realized that friend void from_json is taking an already-constructed Component& but Component doesn't have a default constructor. I added Component() : components(std::nullopt) {} and we're off to the races!