sol2/examples/source/optional_with_iteration.cpp

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

#include <string>
#include <memory>
#include <iostream>

struct thing {
	int a = 20;

	thing() = default;
	thing(int a) : a(a) {
	}
};

struct super_thing : thing {
	int b = 40;
};

struct unrelated { };

int main(int, char*[]) {
	std::cout << "=== optional with iteration ===" << std::endl;

	sol::state lua;

	// Comment out the new_usertype call
	// to prevent derived class "super_thing"
	// from being picked up and cast to its base
	// class
	lua.new_usertype<super_thing>(
	     "super_thing", sol::base_classes, sol::bases<thing>());

	// Make a few things
	lua["t1"] = thing {};
	lua["t2"] = super_thing {};
	lua["t3"] = unrelated {};
	// And a table
	lua["container"] = lua.create_table_with(
	     0, thing { 50 }, 1, unrelated {}, 4, super_thing {});


	std::vector<std::reference_wrapper<thing>> things;
	// Our recursive function
	// We use some lambda techniques and pass the function
	// itself itself so we can recurse, but a regular function
	// would work too!
	auto fx = [&things](auto& f, auto& tbl) -> void {
		// You can iterate through a table: it has
		// begin() and end()
		// like standard containers
		for (auto key_value_pair : tbl) {
			// Note that iterators are extremely frail
			// and should not be used outside of
			// well-constructed for loops
			// that use pre-increment ++,
			// or C++ ranged-for loops
			const sol::object& key = key_value_pair.first;
			const sol::object& value = key_value_pair.second;
			sol::type t = value.get_type();
			switch (t) {
			case sol::type::table: {
				sol::table inner = value.as<sol::table>();
				f(f, inner);
			} break;
			case sol::type::userdata: {
				// This allows us to check if a userdata is
				// a specific class type
				sol::optional<thing&> maybe_thing
				     = value.as<sol::optional<thing&>>();
				if (maybe_thing) {
					thing& the_thing = maybe_thing.value();
					if (key.is<std::string>()) {
						std::cout << "key "
						          << key.as<std::string>()
						          << " is a thing -- ";
					}
					else if (key.is<int>()) {
						std::cout << "key "
						          << key.as<int>()
						          << " is a thing -- ";
					}
					std::cout << "thing.a ==" << the_thing.a
					          << std::endl;
					things.push_back(the_thing);
				}
			} break;
			default:
				break;
			}
		}
	};
	fx(fx, lua);

	std::cout << std::endl;

	return 0;
}