How to access lua's object from lua_topointer? - c++

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)
...

Answer 2

You don't want to use lua_topointer for that task. In fact, the only reasonable use of lua_topointer is for debugging purposes (like logging).
As a is a table, you need to use lua_gettable to access one of its fields, or even simpler use lua_getfield. Of course you cannot pass a void* pointer to processObj for that task, but you can use the stack index instead.