C++11 clock

一 简介

1. C++11 在chrono库中提供了三种时钟 clock: system_clock，steady_clock 和 high_resolution_clock。
2. 头文件 < chrono >。

二 system_clock

1. system_clock 来自系统范围实时时钟的壁钟时间(wall clock)。

2. system_clock是唯一有能力映射其时间点到 C 风格时间的 C++ 时钟。

   It is the only C++ clock that has the ability to map its time points to C-style time.

3. system_clock可以不单调：大多数系统上，系统时间可以在任何时候被调节。

   It may not be monotonic: on most systems, the system time can be adjusted at any moment.
   The system_clock’s time value can be internally adjusted at any time by the operating system.

4. 例子

   // 是否为稳定时钟
   cout << "system_clock::is_steady: " << std::boolalpha << system_clock::is_steady << endl;
   {
     auto start = system_clock::now();
   
     int i = 0;
     while (i < 10000)
       i++;
   
     duration<double> time = system_clock::now() - start;
     cout << "time: " << time.count() << endl;
   }
   {
     auto t = system_clock::to_time_t(system_clock::now());  // to_time_t
     cout << asctime(gmtime(&t)) << endl;
     cout << asctime(localtime(&t)) << endl;
   
     duration<double> time =
         system_clock::now() - system_clock::from_time_t(t); // from_time_t
     cout << "time: " << time.count() << endl;
   }
   

• 输出

  system_clock::is_steady: false
  time: 1.61e-05
  Tue Sep 29 03:55:00 2020
  Tue Sep 29 11:55:00 2020
  time: 0.506771
  

三 steady_clock

1. steady_clock是单调时钟。

   Class std::chrono::steady_clock represents a monotonic clock.

2. 常用于耗时计算 。(stopwatch)

3. 例子

   cout << "steady_clock::is_steady: " << std::boolalpha << steady_clock::is_steady << endl;
   {
     auto start = steady_clock::now();
   
     int i = 0;
     while (i < 10000)
       i++;
   
     duration<double> time = steady_clock::now() - start;
     cout << "time: " << time.count() << endl;
   }
   

• 输出

  steady_clock::is_steady: true
  time: 1.6078e-05
  

四 high_resolution_clock

1. high_resolution_clock是当前系统能够提供的最高精度的时钟。

2. high_resolution_clock在不同标准库实现之间实现不一致，应该避免使用它。通常它只是 std::chrono::steady_clock 或 std::chrono::system_clock 的别名，但实际是哪个取决于库或配置。

   It may be an alias of std::chrono::system_clock or std::chrono::steady_clock, or a third, independent clock.

3. 例子

   cout << "high_resolution_clock::is_steady: " << std::boolalpha
        << high_resolution_clock::is_steady << endl;
   {
     auto start = high_resolution_clock::now();
   
     int i = 0;
     while (i < 10000)
       i++;
   
     duration<double> time = high_resolution_clock::now() - start;
     cout << "time: " << time.count() << endl;
   }
   

• 输出

  high_resolution_clock::is_steady: true
  time: 1.6077e-05
  

五 参考

• cppreference