When is it allowed to call lua_gc in C ++ when lua_newuserdata is also used

Below is an example to demonstrate the problem / question; in fact I have functions creating and returning many different userdata objects and in between some of them I can call Lua a garbage collector.

Also, I am using Lua version 5.3.3 running on both Windows and Linux. Error handling is not shown for simplicity.

I have a C ++ class exposed by Lua via standard user data, as described for example in Programming in Lua by Roberto Ierasulsky. My problem is that I'm not sure when Lua has a reference to my object, so when am I allowed to call the garbage collector via the C ++ lua_gc function?

I have, for example, a standard C ++ class Foo with a constructor and destructor. And defined a meta-tagged "FOO" with an __gc input function called ReleaseFoo as shown also.

I am instantiating in Lua via the following C ++ code:

static int NewFoo(lua_State* L)
{
  Foo** foo;
  foo  = (Foo**) lua_newuserdata(L, sizeof(Foo**));
  *foo = new Foo();
  luaL_getmetatable(L, "FOO");
  lua_setmetatable(L, -2);

  // QUESTION: Can I call lua_gc(L, LUA_GCCOLLECT, 0) here without
  //           risking my user data object being garbage collected.
  //           As I see it, Lua does not yet have a reference
  //           to my user data object.

  return 1;
}

static int ReleaseFoo(lua_State* L)
{
  Foo* foo = *(Foo**)lua_touserdata(L,1);
  if (foo)
  {
    delete foo;
     foo = NULL;
  }
  return 0;
}

      

        
        
        
      

    

In Lua, using it would look like (so it's only after the C ++ function NewFoo (L) returns that the link is set, but can I call the garbage collector as shown?):

LUA> foo = NewFoo()