using std::function to provide signal handler

function_signal_handler.cpp

#include <functional>
#include <iostream>

namespace {
std::function<void(int)> shutdown_handler;
void signal_handler(int signal) { shutdown_handler(signal); }
} // namespace

int main(int argc, char *argv[]) {
  std::signal(SIGINT, signal_handler);
  MyTCPServer server;
  shutdown_handler = [&](int signal) {
    std::cout << "Server shutdown...\n";
    server.shutdown();
  };
  server.do_work_for_ever();
}