sol2/examples/source/shared_ptr_inheritance.cpp

#define SOL_ALL_SAFETIES_ON 1
#include <sol/sol.hpp>

#include <memory>
#include <iostream>

struct Shape {
	virtual ~Shape() = default;
};

struct Box : Shape { };

SOL_BASE_CLASSES(Box, Shape);
SOL_DERIVED_CLASSES(Shape, Box);

int main() {
	sol::state lua;
	lua.open_libraries(sol::lib::base);

	lua.new_usertype<Shape>("Shape", sol::no_constructor);

	lua.new_usertype<Box>("Box", sol::factories([&]() {
		auto b = std::make_shared<Box>();
		std::cout << "create Box@" << std::hex << b.get() << '\n';
		return b;
	}));

	lua.set_function("inspect_shape_table", [](const sol::table& args) {
		std::shared_ptr<Shape> defbox = nullptr;
		// check if there's a field with the name "shape"
		auto s = args.get<sol::optional<std::shared_ptr<Shape>>>("shape");
		std::cout << "has   : " << std::boolalpha << s.has_value() << '\n';

		// get the field named "shape" or use the default value
		std::cout << "get_or: " << std::hex << args.get_or<std::shared_ptr<Shape>>("shape", defbox).get() << '\n';

		// this works but I can't test for existence beforehand...
		std::cout << "get   : " << std::hex << args.get<std::shared_ptr<Shape>>("shape").get() << '\n';
	});

	sol::protected_function_result result = lua.safe_script("inspect_shape_table({shape=Box.new()})");
	sol_c_assert(result.valid());

	return 0;
}