luabind: cannot access global variable

Question

I have a C++ class that I want to give access to in the lua script through a global variable, however when I try to use it I get the following error:

terminate called after throwing an instance of 'luabind::error'
  what():  lua runtime error
baz.lua:3: attempt to index global 'foo' (a nil value)Aborted (core dumped)

My Lua script (baz.lua) looks like this:

-- baz.lua
frames = 0
bar = foo:createBar()

function baz()
  frames = frames + 1

  bar:setText("frame: " .. frames)
end

And I'm made a simple and short (as well as I could) main.cpp that recreates this problem:

#include <memory>
#include <iostream>

extern "C" {
  #include "lua.h"
  #include "lualib.h"
  #include "lauxlib.h"
}

#include <boost/ref.hpp>
#include <luabind/luabind.hpp>

class bar
{
public:
  static void init(lua_State *L)
  {
    using luabind::module;
    using luabind::class_;

    module(L)
    [
      class_<bar>("bar")
        .def("setText", &bar::setText)
    ];
  }

  void setText(const std::string &text)
  {
    std::cout << text << std::endl;
  }
};

class foo
{
public:
  foo() :
    L(luaL_newstate())
  {
    int ret = luaL_dofile(L, "baz.lua");
    if (ret != 0) {
      std::cout << lua_tostring(L, -1);
    }

    luabind::open(L);

    using luabind::module;
    using luabind::class_;

    module(L)
    [
      class_<foo>("bar")
        .def("createBar", &foo::createBar)
    ];

    bar::init(L);
    luabind::globals(L)["foo"] = boost::ref(*this);
  }

  boost::reference_wrapper<bar> createBar()
  {
    auto b = std::make_shared<bar>();
    bars_.push_back(b);

    return boost::ref(*b.get());
  }

  void baz()
  {
    luabind::call_function<void>(L, "baz");
  }

private:
  lua_State *L;
  std::vector<std::shared_ptr<bar>> bars_;
};

int main()
{
  foo f;

  while (true) {
    f.baz();
  }
}

This is compiled with:

g++ -std=c++11 -llua -lluabind main.cpp

I've discovered that if I put the bar = foo:createBar() into the baz() function then it doesn't error, so I assume I'm not initialising the globals in the global namespace correctly? Am I missing a luabind function that I need to call before I am able to do this? Or is this just not possible at all...

Thanks!

Answer 1

You're running baz.lua before you register any globals. Put the dofile command after registering your bindings.

The sequence is as follows:

• You invoke foo's constructor in C++, which
• creates a Lua state
• runs lua.baz
• registers your bindings
• then in c++ you call f.baz.