myclass.cr

class MyClass
  def initialize(val)
    @my_val = val
  end

  def print_val
    puts "My value is #{@my_val}"
  end

  def my_val
    @my_val
  end
end

fun myclass_new(v : Int32) : Void*
  inst = Pointer(MyClass).malloc(1)
  inst[0] = MyClass.new v

  inst as Pointer(Void)
end

fun myclass_print(instance : Void*)
  inst = instance as Pointer(MyClass)
  inst[0].print_val
end

myclass.lua

local ffi = require("ffi")

ffi.cdef[[
  void* myclass_new(int val);
  void myclass_print(void* instance);
]]

local MyClass = {}

MyClass.new = function(val)
  local inst = {}

  inst.__native_instance = ffi.C.myclass_new(val)

  inst.print_val = function(self)
    ffi.C.myclass_print(self.__native_instance)
  end

  return inst
end

return MyClass