Question

In Lua Code

Test = {}
function Test:new()
  local obj = {}
  setmetatable(obj, self)
  self.__index = self
  return obj
end
local a = Test:new()
a.ID = "abc123"
callCfunc(a)

In C Code

int callCfunc(lua_State* l)
{
  SetLuaState(l);
  void* lua_obj = lua_topointer(l, 1);            //I hope get lua's a variable
  processObj(lua_obj);
  ...
  return 0;
}

int processObj(void *lua_obj)
{
  lua_State* l = GetLuaState();
  lua_pushlightuserdata(l, lua_obj);              //access lua table obj
  int top = lua_gettop(l);
  lua_getfield(l, top, "ID");                     //ERROR: attempt to index a userdata value
  std::string id = lua_tostring(l, -1);           //I hoe get the value "abc123"
  ...
  return 0;
}

I get the ERROR: attempt to index a userdata value
How to access lua’s object from lua_topointer() ?

Storing a lua object in C, then calling it from C.

Answer 1

You shouldn’t use lua_topointer as you can’t convert it back to lua object, store your object in the registry and pass it’s registry index:

int callCfunc(lua_State* L)
{
    lua_pushvalue(L, 1);//push arg #1 onto the stack
    int r = luaL_ref(L, LUA_REGISTRYINDEX);//stores reference to your object(and pops it from the stask)
    processObj(r);
    luaL_unref(L, LUA_REGISTRYINDEX, r); // removes object reference from the registry
    ...


int processObj(int lua_obj_ref)
{
    lua_State* L = GetLuaState();
    lua_rawgeti(L, LUA_REGISTRYINDEX, lua_obj_ref);//retrieves your object from registry (to the stack top)
    ...