//
// Created by light on 20-2-7.
//
#include <iostream>
using namespace std;

#include <mutex>

class singleton {
private:
  singleton() {}
  static singleton *p;
  static mutex lock_;

public:
  static singleton *instance();
};

singleton *singleton::p = nullptr;

singleton *singleton::instance() {
  lock_guard<mutex> guard(lock_);
  if (p == nullptr)
    p = new singleton();
  return p;
}