C++日期与时间 chrono库介绍及使用教程

//define in std::chrono namspace
    /// `chrono::duration` represents a distance between two points in time
    template<typename _Rep, typename _Period = ratio<1>>
      class duration;
			//库中预先定义了常用的时间长度

  std::chrono::seconds d1(100);
  std::chrono::seconds d2(50);
  //d1: 100 d2: 50
  std::cout << "d1: " << d1.count()  << " d2: " << d2.count() << std::endl;
  std::chrono::seconds d3 = d1 - d2; //同单位基本计算
  std::cout << "d3: " << d3.count() << std::endl; //d3: 50
 //std::chrono::duration_cast 使用
   std::chrono::milliseconds m = std::chrono::duration_cast<std::chrono::milliseconds>(d1);
  //100s == 100000ms
  std::cout << "100s == " << m.count() << "ms" << std::endl;
 //其他用法，当前线程休眠 1s sleep_for 需要加 #include<thread>
  std::this_thread::sleep_for(std::chrono::seconds(1));

  using namespace std::chrono_literals;
  //使用字面量 ""y ""d(C++20) 
  auto ymd1 = 2023y / std::chrono::November / 10d; // 2023-11-10
  auto ymd2 = 2023y / std::chrono::September / 10d; // 2023-9-10
  if(ymd1 > ymd2) {
 // ymd1 > ymd2
    std::cout << "ymd1 > ymd2" << std::endl;
  }

  //获取系统时间戳,实例化系统时钟
  std::chrono::system_clock clock;
  std::chrono::system_clock::time_point now = clock.now();
  std::chrono::_V2::system_clock::duration d = now.time_since_epoch();
  //纳秒单位时间戳
  std::cout << "timestamp: " << d.count() << std::endl;
  //转化为秒单位
  std::chrono::seconds t = std::chrono::duration_cast<std::chrono::seconds>(d);
  std::cout << "sec: " << t.count() << std::endl;
//简化写法
//  std::chrono::system_clock clock;
//  auto t = std::chrono::duration_cast<std::chrono::seconds>(
//  			clock.now().time_since_epoch()).count();
//  std::cout << "sec: " << t << std::endl;

  const auto start = std::chrono::high_resolution_clock::now();
  //do something
  std::this_thread::sleep_for(std::chrono::seconds(1));
  const auto end = std::chrono::high_resolution_clock::now();
  auto duration = start - end;
  //1000118000ns 1000ms 1s 
  std::cout << duration.count() << "ns " << 
    std::chrono::duration_cast<std::chrono::milliseconds>(duration).count()<<  "ms " <<
    std::chrono::duration_cast<std::chrono::seconds>(duration).count() << "s " <<
    std::endl;

  auto now = std::chrono::system_clock::now();
  std::time_t now_c = std::chrono::system_clock::to_time_t(now);
  //cur time Thu Nov  9 23:42:38 2023
  std::cout << "cur time " << std::ctime(&now_c) << std::endl;
  //或者转化为当地时间
  //自定义格式输出
  tm * t = std::localtime(&now_c);
	char buffer[80];
	std::strftime(buffer, 80, "%Y-%m-%d %H:%M:%S", t);
   //cur time: 2023-11-09 23:42:38
  std::cout << "cur time: " << buffer << std::endl;

  //使用字面量 c++20 ""y ""d  c++11 支持 ""h ""min ""s ""ms ""us
  using namespace std::chrono_literals;//在此空间定义字面量
  auto s = 60s; // 相当于std::chrno::seconds(60)
  std::cout << "sec: " << s.count() << std::endl;
  auto min = 1min; // 相当于std::chrono::minutes(1):
  std::cout << "min: " << min.count() << std::endl;