Function register_usertype

extlibs/sol3/include/sol/sol.hpp:21858–22079

	template <typename T>
	inline int register_usertype(lua_State* L, automagic_enrollments enrollments = {}) {
		using u_traits = usertype_traits<T>;
		using u_const_traits = usertype_traits<const T>;
		using u_unique_traits = usertype_traits<detail::unique_usertype<T>>;
		using u_ref_traits = usertype_traits<T*>;
		using u_const_ref_traits = usertype_traits<T const*>;
		using uts = usertype_storage<T>;

		// always have __new_index point to usertype_storage method
		// have __index always point to regular fast-lookup
		// meta_method table
		// if __new_index is invoked, runtime-swap
		// to slow __index if necessary
		// (no speed penalty because function calls
		// are all read-only -- only depend on __index
		// to retrieve function and then call happens VIA Lua)

		// __type entry:
		// table contains key -> value lookup,
		// where key is entry in metatable
		// and value is type information as a string as
		// best as we can give it

		// name entry:
		// string that contains raw class name,
		// as defined from C++

		// is entry:
		// checks if argument supplied is of type T

		// __storage entry:
		// a light userdata pointing to the storage
		// mostly to enable this new abstraction
		// to not require the type name `T`
		// to get at the C++ usertype storage within

		// we then let typical definitions potentially override these intrinsics
		// it's the user's fault if they override things or screw them up:
		// these names have been reserved and documented since sol3

		// STEP 0: tell the old usertype (if it exists)
		// to fuck off
		delete_usertype_storage<T>(L);

		// STEP 1: Create backing store for usertype storage
		// Pretty much the most important step.
		// STEP 2: Create Lua tables used for fast method indexing.
		// This is done inside of the storage table's constructor
		usertype_storage<T>& storage = create_usertype_storage<T>(L);
		usertype_storage_base& base_storage = storage;
		void* light_storage = static_cast<void*>(&storage);
		void* light_base_storage = static_cast<void*>(&base_storage);

		// STEP 3: set up GC escape hatch table entirely
		storage.gc_names_table.push();
		stack_reference gnt(L, -1);
		stack::set_field(L, submetatable_type::named, &u_traits::gc_table()[0], gnt.stack_index());
		stack::set_field(L, submetatable_type::const_value, &u_const_traits::metatable()[0], gnt.stack_index());