Lua bindings, Global Variables

Tags: C++ , Lua (2015-03-31)

Yeah, yeah, I know. Global variables are bad, treacherous, make the program hard to reason about, lead to spaghetti code, and smell funny. However, they are a good first step for passing information back and forth between Lua and C++.

First, the Lua code that we want to run. This is in a separate file named script.lua.

function lua_func()
  x = x + 1
end

Each time, it increments a global variable. Next, the basic C++ code to set up the lua_State and define a global variable.

// Initialize the lua_State
lua_State* L = luaL_newstate();

// Define the global variable 'x'
lua_pushnumber(L, 0);
lua_setglobal("x");

// Load the file
luaL_loadfile(L, "script.lua");
lua_pcall(L, 0, LUA_MULTRET, 0);

// Call the function
lua_getglobal(L, "lua_func");
lua_pcall(L, 0, 0, 0);

// Get the value of 'x'
lua_getglobal(L, "x");
int x = lua_tonumber(L, -1);
lua_pop(L, 1);

assert(x == 1);

// Close the lua_State
lua_close(L);

This works, but isn’t very clean. Everything must be done directly. The state must be manually closed, which could easily be forgotten. Let’s start encapsulating some of this behavior. Here, Push(lua_State* L, T value) and Read<T>(lua_State* L) are templated function which call the appropriate lua_push* function or lua_to* function for the given type.

class LuaState{
public:
  LuaState() {
    L = luaL_newstate();
  }

  ~LuaState() {
    lua_close(L);
  }

  void LoadFile(const char* name){
    luaL_loadfile(L, name);
    lua_pcall(L, 0, LUA_MULTRET, 0);
  }

  template<typename T>
  void SetGlobal(const char* name, T value){
    Push(L, value);
    lua_setglobal(L, name);
  }

  template<typename T>
  T CastGlobal(const char* name){
    lua_getglobal(L, name);
    T output = Read<T>(L, -1);
    lua_pop(L, 1);
    return output;
  }

  lua_State* L
};

Note that the held lua_State* is currently public. As the implementation becomes more complete, this will become a private variable.

Now, with this, the usage becomes much simpler.

LuaState L;
L.SetGlobal("x", 0);
L.LoadFile("script.lua");

// Call the function
lua_getglobal(L.state, "lua_func");
lua_pcall(L.state, 0, 0, 0);

// Get the value of 'x'
int x = L.CastGlobal<int>("x");

assert(x == 1);

That function call is still rather ugly. We’ll be focusing on that in the next post.

Lunderberg

Lua bindings, Global Variables