Call C++ method from Lua script

#include <iostream>

#include "lua5.1/lua.hpp"

class Computer {
    public:
        Computer();
        ~Computer();
        void run();
        void test(int i);
    private:
        lua_State *luaState;
};

Computer * get_obj_pointer(lua_State *L)
{
    lua_getglobal(L, "computer");
    if (!lua_istable(L, -1))
        luaL_error(L, "Table 'computer' is not found!");

    lua_pushstring(L, "_self");
    lua_gettable(L, -2);
    if (!lua_isuserdata(L, -1))
        luaL_error(L, "Value 'computer._self' is not found!");

    void *p = lua_touserdata(L, -1);
    lua_pop(L, 1);

    return (Computer *) p;
}

int test_wrap(lua_State *L)
{
    // read arguments
    if(!lua_isnumber(L, -1))
        luaL_error(L, "Expected a Number as a first argument!");
    int i = lua_tonumber(L, -1);

    // restore pointer to object
    Computer * pComp = get_obj_pointer(L);

    // call object method
    if (pComp) {
        pComp->test(i);
    }

    return 0;
}

Computer::Computer(void) {
    luaState = luaL_newstate();
    luaL_openlibs( luaState );

    lua_createtable(luaState, 0, 2);

    lua_pushlightuserdata(luaState, this);
    lua_setfield(luaState, -2, "_self");

    lua_pushcfunction(luaState, test_wrap);
    lua_setfield(luaState, -2, "test");

    lua_setglobal(luaState, "computer");
}

Computer::~Computer(void) {
    lua_close(luaState);
}

void Computer::run(void) {
    int err = luaL_dofile(luaState, "script.lua");
    if (err) {
       std::cout << "Error: " << lua_tostring(luaState, -1);
       lua_pop(luaState, 1);
    }
}

void Computer::test(int i) {
    std::cout << "test: " << i;
}

int main()
{
    Computer c;
    c.run();

    return 0;
}