Re: Lua 5 and tags

[Mark Hamburg <mhamburg@...>]

Warning untested (and uncompiled) code:

LUALIB_API int luaL_newmetatable_ud (lua_State *L, void *tident) {
  lua_pushlightuserdata(L, tident);
  lua_rawget(L, LUA_REGISTRYINDEX);  /* get registry[ tident ] */
  if (!lua_isnil(L, -1))  /* tident already in use? */
    return 0;  /* leave previous value on top, but return 0 */
  lua_pop(L, 1);
  lua_newtable(L);  /* create metatable */
  lua_pushlightuserdata(L, tident);
  lua_pushvalue(L, -2);
  lua_rawset(L, LUA_REGISTRYINDEX);  /* registry[ tident ] = metatable */
  lua_pushvalue(L, -1);
  lua_pushlightuserdata(L, tident);
  lua_rawset(L, LUA_REGISTRYINDEX);  /* registry[metatable] = tident */
  return 1;
}


LUALIB_API void  luaL_getmetatable_ud (lua_State *L, void *tident) {
  lua_pushlightuserdata(L, tident);
  lua_rawget(L, LUA_REGISTRYINDEX);
}


LUALIB_API void *luaL_checkudata_ud (lua_State *L, int ud, void *tident) {
  if (!lua_getmetatable(L, ud)) return NULL;  /* no metatable? */
  lua_rawget(L, LUA_REGISTRYINDEX);  /* get registry[metatable] */
  if (lua_touserdata(L, -1) == tident) {
    lua_pop(L, 1);
    return lua_touserdata(L, ud);
  }
  else {
    lua_pop(L, 1);
    return NULL;
  }
}

(Interesting. luaL_checkudata doesn't throw on errors. It should probably be
named luaL_toudata. If the check routine did throw, one might then want a
name either at creation or check time but just for constructing the error
message.)

Mark