EDIT: Part 2

Long time without posting, busy at work, family, etc.

Today I decided to write a bit on Lua and how to integrate it with C++.

Lua is a scripting language with an emphasis on being easily embeddable. Lua is used as the scripting language in Photoshop CS, and World of Warcraft, for example. So if you are looking into adding scriptability to your C or C++ applications Lua is quite suited for the task.

In order to learn Lua (both the language itself and how to embed it into your app) I recommend you Programming in Lua by one of the Lua authors.

You can find tons of tutorials on how to get started with Lua. So I will focus here on how to integrate with C++. Most of the sources I’ve found about Lua - C++ integrations take the approach of frameworks to automatically wrap your objects/classes to be usable from within Lua. I find this approach confusing, I think it’s better to learn how to do it manually, and that’s what I will do here.

Step 0. Compile Lua itself

Download Lua 5.2.1 and compile it. Usually it enough with make macosx or make linux. That will generate liblua.a, the library that we will link to.

Step 1. Create a simple host app

We need a simple host app. Our host app will simply setup Lua and run a script from it. This script will have access to a std::queue from the host app. This will illustrate how you can share objects with the Lua part. Later we will take a more complex example.

Let’s start with the basic skeleton of a lua environment with some communication with its host:

The Makefile

LUAHOME=$(HOME)/tmp/lua-5.2.1/src
all: sampleluahost
sampleluahost: sampleluahost.cpp
	g++ -g sampleluahost.cpp -llua -L$(LUAHOME) -I$(LUAHOME) -o sampleluahost

It uses LUAHOME that should point to the directory containing both liblua.a and the lua*.h files.

The samplehost application

#include <iostream>
#include <lua.hpp>


extern "C" {
  static int l_cppfunction(lua_State *L) {
    double arg = luaL_checknumber(L,1);
    lua_pushnumber(L, arg * 0.5);
    return 1;
  }
}

using namespace std;
int main()
{
  cout << "** Test Lua embedding" << endl;
  cout << "** Init Lua" << endl;
  lua_State *L;
  L = luaL_newstate();
  cout << "** Load the (optional) standard libraries, to have the print function" << endl;
  luaL_openlibs(L);
  cout << "** Load chunk. without executing it" << endl;
  if (luaL_loadfile(L, "luascript.lua")) {
    cerr << "Something went wrong loading the chunk (syntax error?)" << endl;
    cerr << lua_tostring(L, -1) << endl;
    lua_pop(L,1);
  }

  cout << "** Make a insert a global var into Lua from C++" << endl;
  lua_pushnumber(L, 1.1);
  lua_setglobal(L, "cppvar");

  cout << "** Execute the Lua chunk" << endl;
  if (lua_pcall(L,0, LUA_MULTRET, 0)) {
    cerr << "Something went wrong during execution" << endl;
    cerr << lua_tostring(L, -1) << endl;
    lua_pop(L,1);
  }
  
  cout << "** Read a global var from Lua into C++" << endl;
  lua_getglobal(L, "luavar");
  double luavar = lua_tonumber(L,-1);
  lua_pop(L,1);
  cout << "C++ can read the value set from Lua luavar = " << luavar << endl;

  cout << "** Execute a Lua function from C++" << endl;
  lua_getglobal(L, "myluafunction");
  lua_pushnumber(L, 5);
  lua_pcall(L, 1, 1, 0);
  cout << "The return value of the function was " << lua_tostring(L, -1) << endl;
  lua_pop(L,1);

  cout << "** Execute a C++ function from Lua" << endl;
  cout << "**** First register the function in Lua" << endl;
  lua_pushcfunction(L,l_cppfunction);
  lua_setglobal(L, "cppfunction");

  cout << "**** Call a Lua function that uses the C++ function" << endl;
  lua_getglobal(L, "myfunction");
  lua_pushnumber(L, 5);
  lua_pcall(L, 1, 1, 0);
  cout << "The return value of the function was " << lua_tonumber(L, -1) << endl;
  lua_pop(L,1);

  cout << "** Release the Lua enviroment" << endl;
  lua_close(L);
}

The Lua script

print("Hello from Lua")
print("Lua code is capable of reading the value set from C++", cppvar)
luavar = cppvar * 3

function myluafunction(times)
  return string.rep("(-)", times) 
end

function myfunction(arg)
  return cppfunction(arg)
end

The code explained step by step:

Initialization

    lua_State *L;
    L = luaL_newstate();
    luaL_openlibs(L);
    if (luaL_loadfile(L, "luascript.lua")) {
      cerr << "Something went wrong loading the chunk (syntax error?)" << endl;
      cerr << lua_tostring(L, -1) << endl;
      lua_pop(L,1);
    }

That creates a lua_State loads the standard libs in it and also loads the code in luascript.lua.

Adding variables from C++ into Lua

    lua_pushnumber(L, 1.1);
    lua_setglobal(L, "cppvar");
    if (lua_pcall(L,0, LUA_MULTRET, 0)) {
      cerr << "Something went wrong during execution" << endl;
      cerr << lua_tostring(L, -1) << endl;
      lua_pop(L,1);
    }

Then it sets a global variable in Lua from C++ code using lua_setglobal. If you don’t know what are the lua_pushxxxx and the Lua stack, etc. I recoment that you take a look at the Lua Reference Manual and Programming in Lua. More or less Lua and the C++ communicate through the stack that lives in lua_State and there is bunch of function to manipulate that stack. So in order to set a global in Lua from C++ you must push the value into the stack and call lua_setglobal that will pop the value in the stack and assign it to the identifier provided inside the Lua environment.

After setting the global cppvar it executes the loaded chunk of code (that is in the stack) with lua_pcall. The Lua code is able to read and print the value of cppvar. The lua code will also set a new global luavar that we will access from C++.

Reading a Lua variable from C++

    lua_getglobal(L, "luavar");
    double luavar = lua_tonumber(L,-1);
    lua_pop(L,1);
    cout << "C++ can read the value set from Lua luavar = " << luavar << endl;

Calling a Lua function from C++

    cout << "** Execute a Lua function from C++" << endl;
    lua_getglobal(L, "myluafunction");
    lua_pushnumber(L, 5);
    lua_pcall(L, 1, 1, 0);
    cout << "The return value of the function was " << lua_tostring(L, -1) << endl;
    lua_pop(L,1);

The example won’t be complete without function calling so that’s the next step. Calling a Lua function from C++ it’s quite easy. Function in Lua are first class values, so that means that it’s just a like reading a any other value. lua_getglobal will get the value and put it on the stack and then we push the function arguments into the stack and use lua_pcall to call the function (that is the stack). The returned value from the function will be pushed in the stack and that’s were the C++ code will get it, lua_tostring and then it will remove from the stack with lua_pop.

Calling a C++ function from Lua

    lua_pushcfunction(L,l_cppfunction);
    lua_setglobal(L, "cppfunction");

    lua_getglobal(L, "myfunction");
    lua_pushnumber(L, 5);
    lua_pcall(L, 1, 1, 0);
    cout << "The return value of the function was " << lua_tonumber(L, -1) << endl;
    lua_pop(L,1);

The other way around it’s more complex. You can’t just call any function from Lua. It has to has a special signature lua_CFunction, that is, typedef int (*lua_CFunction) (lua_State *L) a function that returns an int and takes a lua_State. This special funciton will communicate with Lua via the lua stack that resides in the lua_State parameter. The return value of the function tell lua how many value the function has pushed into the stack as result values for the function call.

So to make the function accesible from Lua, you create push the function into the stack with lua_pushcfunction and bind it to an identifier in lua with lua_setglobal. Then lua code will be able to invoke this function like any other function. In the example I call the myfunction (which is lua code) and myfunction in turn invokes cppfunction which is “bound” to C++ l_cppfunction. Ah, I almost forgot. l_cppfunction is declared as extern "C" telling the compiler to provide C linkage for this function so it can be called from a C library like Lua is.

Free Lua resources

    lua_close(L);

lua_close will free all resources held by the lua_State L.

Wrap up

I will leave the part on how to wrap C++ class objects in Lua for a later post because I don’t want to make this post too long. Hopefully I’ll post it tomorrow.