Exposing functions, structs, and type aliases to Godot 4 with GDExtension?

I'm messing around with Godot 4 and GDExtension, trying to hook up some C++ code. Right now, I'm a bit stuck because I can't find any good examples on how to expose not just classes, but functions, structs, and type aliases. Here's what I got:

register_types.cpp:

#include "register_types.h"
#include "../godot-cpp/gdextension/gdextension_interface.h"
#include "../godot-cpp/include/godot_cpp/core/class_db.hpp"
#include "../godot-cpp/include/godot_cpp/core/defs.hpp"
#include "../godot-cpp/include/godot_cpp/godot.hpp"
#include "cards.hpp"

void initialize_gdextension_types(godot::ModuleInitializationLevel p_level) {
  if (p_level != godot::MODULE_INITIALIZATION_LEVEL_CORE) {
    return;
  }
  // TODO: register my functions and structs
  // TODO: figure out how to do it
  godot::ObjectDB::register_object<Error>(); // <- no member named 'register_object' :)
}

void uninitialize_gdextension_types(godot::ModuleInitializationLevel p_level) {
  if (p_level != godot::MODULE_INITIALIZATION_LEVEL_CORE) {
    return;
  }
}

extern "C" {
// Initialization
// NOTE: I don't think we need to touch anything here
GDExtensionBool GDE_EXPORT
poker_utils_library_init(GDExtensionInterfaceGetProcAddress p_get_proc_address,
                         GDExtensionClassLibraryPtr p_library,
                         GDExtensionInitialization *r_initialization) {
  godot::GDExtensionBinding::InitObject init_obj(p_get_proc_address, p_library,
                                                 r_initialization);
  init_obj.register_initializer(initialize_gdextension_types);
  init_obj.register_terminator(uninitialize_gdextension_types);
  init_obj.set_minimum_library_initialization_level(
      godot::MODULE_INITIALIZATION_LEVEL_CORE);

  return init_obj.init();
}
}

cards.hpp:

#ifndef CARDS_H_
#define CARDS_H_

#include <string>
#include <variant>

struct Error {
  std::string message;
};

using CardIndexResult = std::variant<int, Error>;

CardIndexResult getCardIndex(const std::string &card);

#endif // CARDS_H_

I tried the godot::ObjectDB::register_object<Error>(); thing, but no dice. So, how do I:

Get the Error struct into Godot?
Make getCardIndex callable from Godot scripts?
Deal with the CardIndexResult type alias?

For future reference

I was able to compile the following:

// register_types.cpp
#include "register_types.h"
#include "../godot-cpp/gdextension/gdextension_interface.h"
#include "../godot-cpp/include/godot_cpp/core/class_db.hpp"
#include "../godot-cpp/include/godot_cpp/core/defs.hpp"
#include "../godot-cpp/include/godot_cpp/godot.hpp"

#include "deck.hpp"

void initialize_gdextension_types(godot::ModuleInitializationLevel p_level) {
  if (p_level != godot::MODULE_INITIALIZATION_LEVEL_CORE) {
    return;
  }
  godot::ClassDB::bind_static_method("@GlobalScope",
                                     godot::D_METHOD("get_deck"), &getDeck);
}

void uninitialize_gdextension_types(godot::ModuleInitializationLevel p_level) {
  if (p_level != godot::MODULE_INITIALIZATION_LEVEL_CORE) {
    return;
  }
}

extern "C" {
// Initialization
// NOTE: I don't think we need to touch anything here
GDExtensionBool GDE_EXPORT
poker_utils_library_init(GDExtensionInterfaceGetProcAddress p_get_proc_address,
                         GDExtensionClassLibraryPtr p_library,
                         GDExtensionInitialization *r_initialization) {
  godot::GDExtensionBinding::InitObject init_obj(p_get_proc_address, p_library,
                                                 r_initialization);
  init_obj.register_initializer(initialize_gdextension_types);
  init_obj.register_terminator(uninitialize_gdextension_types);
  init_obj.set_minimum_library_initialization_level(
      godot::MODULE_INITIALIZATION_LEVEL_CORE);

  return init_obj.init();
}
}

And this is the defintion of the deck.hpp file:

#ifndef GODOT_PLUGIN_DECK_H_
#define GODOT_PLUGIN_DECK_H_

#include "../godot-cpp/include/godot_cpp/godot.hpp"
#include "../godot-cpp/include/godot_cpp/templates/vector.hpp"
#include "../godot-cpp/include/godot_cpp/variant/variant.hpp"

godot::PackedStringArray getDeck();

#endif // GODOT_PLUGIN_DECK_H_

That function is a encapsulation of my original function, see it here:

// deck.cpp
#include "deck.hpp"
#include "../src/deck.hpp"

godot::PackedStringArray getDeck() {
  godot::PackedStringArray cards;
  auto deck = poker::getDeck();

  for (const auto &card : deck) {
    cards.push_back(godot::String(card.c_str()));
  }

  return cards;
};

However, `get_deck` isn't showing at `@GlobalScope`

The extension is being detected by Godot and loaded, after restarting the editor I went right into the documentation of @GlobalScope but get_deck isn't showing. IDK what I'm doing wrong at this point.

I saw the mistake, did some corrections and now things work

So I ended making a class to encapsulate my functions, on register_types.cpp at the initialize_gdextension_types function I put godot::ClassDB::register_class<Poker>(). The class itself takes care of registering its methods:

// poker.cpp
void Poker::_bind_methods() {
  godot::ClassDB::bind_static_method("Poker", godot::D_METHOD("get_deck"),
                                     &Poker::getDeck);
  godot::ClassDB::bind_static_method(
      "Poker", godot::D_METHOD("get_deck_shuffled"), &Poker::getDeckShuffled);
}

Now Godot can see the class and use its methods!

Solution

You're on the right track, but for your custom class or struct to be handled by Godot, it has to inherit from Godot's Object type (or any other class), since you'll need a few specific member functions for the interaction with the engine (including scripting).

To do this, you have to use inheritance and an extra macro inserting the additional members:

class Error: public godot::Object {
  GDCLASS(Error, godot::Object);
}

This should allow you to register the class in your initialization function and then pass it around, and create/destroy instances of it:

godot::ClassDB::register_class<Error>();

If your custom class needs additional members, you'd add a protected member static void _bind_methods(); to your Error class, which is then able to use similar ClassDB methods to add any callbacks or properties you want to expose.

To bind a global function that's not a member, you can use ``, something like this:

godot::ClassDB::bind_static_method("ParentClassName", "getCardIndex", &getCardIndex, godot::String());

I'm not sure how to do it with global scope, I think you can pass "@GlobalScope" or so, but I've never done it before and can only type from memory right now, so take this note with a grain of salt.

Exposing functions, structs, and type aliases to Godot 4 with GDExtension?

For future reference

However, get_deck isn't showing at @GlobalScope

I saw the mistake, did some corrections and now things work

However, `get_deck` isn't showing at `@GlobalScope`