[luabind] How to pass value from C++ to lua function by reference?

[1 1 <roa...@gm...>]

When programming on C++, you can do the following:

void byReference(int &y){
    y = 5;}
int main(){
   int x = 2;      // x = 2
   byReference(x); // x = 5}

How to do the same using luabind?

Luabind docs says
<http://www.rasterbar.com/products/luabind/docs.html#calling-lua-functions>:

If you want to pass a parameter as a reference, you have to wrap it with
the Boost.Ref.

Like this:

int ret = call_function(L, "fun", boost::ref(val));

 But when I'm trying to do this:

#include <iostream>#include <conio.h>
extern "C" {
    #include "lua.h"
    #include "lualib.h"
    #include "lauxlib.h"}
#include <luabind\luabind.hpp>
using namespace std;using namespace luabind;
int main() {
  lua_State *myLuaState = luaL_newstate();
  open(myLuaState);

  int x = 2;

  do
  {
    luaL_dofile(myLuaState, "script.lua");

    cout<<"x before = "<< x <<endl;

    try
    {
        call_function<void>(myLuaState, "test", boost::ref(x));
    }
    catch(const std::exception &TheError)
    {
        cerr << TheError.what() << endl;
    }

    cout<<"x after = "<< x <<endl;

   } while(_getch() != 27);

  lua_close(myLuaState);}

script.lua:

function test(x)
    x = 7end

It gives me a "Trying to use unregistered class" error.

So, how to pass value from C++ to lua function by reference, so I can
change it inside the script and it will be changed in c++ program too? I'm
using boost 1.55.0, lua 5.1, luabind 0.9.1

Thanks.
P.S:
I posted the same question on StackOverflow
<http://stackoverflow.com/questions/25099437/luabind-how-to-pass-value-from-c-to-lua-function-by-reference>,
but seems like nobody knows what to do.