How to extract C++ object pointer from Lua

Question

I have a class in C++ called "Point":

class Point
{
public:
  int x, y;

  //constructor
  Point(int x, int y)
  {
    this->x = x;
    this->y = y;
  }
};

My goal is to be able to instantiate a Point object with a Lua script, and extract the pointer to this object from the Lua stack.

Here is my (currently not working) attempt which hopefully clarifies what exactly I am trying to do; note that this code is essentially modified copy/paste from this tutorial and that I am using Lua 5.2:

static int newPoint(lua_State *L) 
{
    int n = lua_gettop(L);
    if (n != 2) 
       return luaL_error(L, "expected 2 args for Point.new()", n); 

    // Allocate memory for a pointer to object
    Point **p = (Point **)lua_newuserdata(L, sizeof(Point *));  

    double x = luaL_checknumber (L, 1);      
    double y = luaL_checknumber (L, 2); 

    //I want to access this pointer in C++ outside this function
    *p = new Point(x, y);

    luaL_getmetatable(L, "Point"); // Use global table 'Point' as metatable
    lua_setmetatable(L, -2); 

  return 1; 
}

static const luaL_Reg pointFuncs[] = {
  {"new", newPoint},
  {NULL, NULL}
};

//register Point to Lua
void registerPoint(lua_State *L)
{
    lua_createtable(L, 0, 0);
    // Register metatable for user data in registry
    luaL_newmetatable(L, "Point");
    luaL_setfuncs(L, pointFuncs, 0);           
    lua_pushvalue(L,-1);
    lua_setfield(L,-2, "__index");
    lua_setglobal(L, "Point");
}

Point* checkPoint(lua_State* L, int index)
{
  void* ud = 0;
  luaL_checktype(L, index, LUA_TTABLE); 
  lua_getfield(L, index, "__index");
  ud = luaL_checkudata(L, index, "Point");;  

  return *((Point**)ud);     
 }

 int main()
 {
    lua_State *L = luaL_newstate();
    luaL_openlibs(L);
        registerPoint(L);
    luaL_dofile(L, "testz.lua");
    lua_getglobal(L, "Point");
    Point *foo = checkPoint(L, lua_gettop(L));
        std::cout << "x: " << foo->x << " y: " << foo->y;
    lua_close(L);
    return 0;
 }

And here is the Lua script:

local point = Point.new(10,20)

Running this code, I get the following error: "bad argument #3 (Point expected, got table)" at the line: ud = luaL_checkudata(L, index, "Point") inside the checkPoint() function.

If any one could steer me in the right direction, it would be much appreciated.

greatwolf · Accepted Answer · 2013-08-15 23:12:49Z

9

There are a couple of issues in your above usage and binding.

Inside your testing script, local point will not be seen by your host c++ program. This is because, well, it's local to that script. If you want it accessible from c++, either return point as a value from the script or make it global and retrieve it using lua_getglobal(L, "point").

The checkPoint function has some unnecessary code. If you look at the other luaL_check* functions provided by the Lua C API, they mainly check if the given value at the stack index is the right type. If it is then convert that type over to something C++ can use.

So in your 'checkPoint' function all you really need to do is:

Point* checkPoint(lua_State* L, int index)
{
  return *((Point **) luaL_checkudata(L, index, "Point"));
}

If you change your script to this, for example:

local point = Point.new(10,20)
return point

you should be able to access point from C++ in the following way:

// ...
luaL_dofile(L, "testz.lua");

Point *foo = checkPoint(L, -1);
std::cout << "x: " << foo->x << " y: " << foo->y;
// ...

Another important point is regarding C++ object lifetime when exposed to lua. Because you're allocating and constructing with the C++ 'new' operator whenever Point.new is called from the script, you are indirectly saying let lua handle this exposed C++ object's lifetime. That means you'll also want to implement a __gc metamethod to act as a 'finalizer' or non-deterministic destructor. Without this you would have no way to 'delete' the point object and reclaim the space when lua garbage collects the corresponding userdata.

To do this you can augment your code as follows:

Write a deletePoint function that gets called by lua on userdata gc.

static int deletePoint(lua_State *L)
{
  Pointer **p = checkPoint(L, 1);
  delete *p;
  *p = NULL;  // probably not needed but to be safe
  return 0;
}

Add that to your pointFuncs so lua knows about it.

static const luaL_Reg pointFuncs[] =
{
  {"new", newPoint},
  {"__gc", deletePoint},
  {NULL, NULL}
};

edited Aug 15, 2013 at 23:12

answered Aug 15, 2013 at 22:12

greatwolf

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3 Comments

greatwolf Over a year ago

@user2687268 I've added a note about handling userdata clean.

Dmitry Ledentsov Over a year ago

that's a good explanation, but it seems to assume that the garbage collector will not run before the point pointer gets the point from the lua state. This might be not the case with this simple example, but in more elaborate scripts you'll not know when gc will delete your point. Hence, the whole fuss of memory lifetime management in binding libraries.

Dmitry Ledentsov Over a year ago

p.s. one of the typical solution is wrapping your object in a reference-counted object, so that the garbage collector call only frees memory when there are no references on the c++ side, and vice-versa, if you delete the wrapper object on the c++ side, if the original object is still alive as userdata, the memory should be freed by the garbage collector.

Dmitry Ledentsov · Accepted Answer · 2013-08-16 05:58:54Z

6

I'd recommend not writing your own binding, but rather use a tested and documented one, such as luabridge or luabind

Your binding would be reduced to:

getGlobalNamespace (L)
    .beginClass<Point>("Point")
     .addConstructor<void (*) (int,int)>()
     .addData("X", &Point::x)
     .addData("Y", &Point::y)
    .endClass()
;

and your lua would look like

local point = Point(10,20)

with luabridge.

For extracting values, see for example LuaRef in luabridge. If you use c++11 there are some other quite beautiful lua binding libraries.

edited Aug 16, 2013 at 5:58

answered Aug 15, 2013 at 21:15

Dmitry Ledentsov

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2 Comments

user2687268 Over a year ago

I will definitely check out luabind. As a learning experience I do wish to get this working however. Unfortunately I still get the same error as before even after I switched to the colon operator in my Lua code and changed the argument check in newPoint() function. Would you happen to know why this is?

Dmitry Ledentsov Over a year ago

updated my answer as id didn't have an actual explanation, and greatwolf did explain

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