SWIG Callback Example for mapping std::function (C++) to Lua functions

callback.cpp

#include <iostream>

#include "lua5.3/lua.hpp"

#include "callback.hpp"

extern "C" {
int luaopen_Callback(lua_State* L);
}

Example::Example(Example::Callback const& callback)
{
  std::cout << "Calling Lua callback" << std::endl;
  if (callback) callback();
}

int main([[maybe_unused]] int argc, char ** argv)
{
  // Create a new lua state
  lua_State *L = luaL_newstate();

  // Open all libraries
  luaL_openlibs(L);

  luaopen_Callback(L);

  // Load and run the Lua file
  luaL_loadfile(L, argv[1]);
  lua_pcall(L, 0, 1, 0);

  lua_close(L);
}

callback.hpp

#include <functional>

class Example {
public:
  using Callback = std::function<void(void)>;

  Example(Example::Callback const& callback = {});
};

callback.i

%module Callback

%include <lua_fnptr.i>

%{
#include "callback.hpp"
%}

%typemap(typecheck) Example::Callback & {
  $1 = lua_isfunction(L, $input);
}

%typemap(in) Example::Callback & (Example::Callback temp) {
  // Create a reference to the Lua callback
  SWIGLUA_REF fn;
  swiglua_ref_set(&fn, L, $input);

  temp = [&fn]() {
    swiglua_ref_get(&fn);

    lua_pcall(fn.L, 0, 0, 0);
  };

  $1 = &temp;
}

%include "callback.hpp"

callback.lua

local function callback()
  print("Lua callback called")
end

Callback.Example(callback)

This has been tested using SWIG 4.0, Clang++ 9 and Lua 5.3.5 on Linux Mint 19. Example compilation lines are:

swig -Wall -Werror -Wallkw -c++ -lua callback.i
clang++-9 -std=c++17 -I /usr/include/lua5.3 callback.cpp callback_wrap.cxx -llua5.3 -o callback

Then run with:

./callback callback.lua 
Calling Lua callback
Lua callback called