int caller(int (*callback)(void *arg), void * arg = NULL) {
  return callback(arg);
}

int main(int argc, char **argv) {

  const char *str = "world";
  caller([&](void *arg) {
      printf("hello %sn", str);
      return 0;
  }, NULL);

  return 0;
}

printf("hello %sn", str)

int caller(std::function<int(void*)> callback, void * arg = NULL) {
  return callback(arg);
}

caller([](void *arg) {
      printf("hello %sn", (const char*)arg);
      return 0;
  }, str);