C++20 STL Cookbook/

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Time Events with std::chrono

Time Events with std::chrono

Learn to implement time events with std::chrono.

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The std::chrono library provides tools for measuring and reporting time and intervals. Many of these classes and functions were introduced with C++11. There have been significant changes and updates for C++20, but at the time of writing, many of those updates are not yet implemented.

Using the chrono library, this recipe explores techniques for timing events.

How to do it

The system_clock class is used for reporting the current date and time. The steady_clock and high_resolution_clock classes are used for timing events. Let's look at the differences between these clocks:

  • Because these names can be long and unwieldy, we'll use some type aliases throughout this recipe:

using std::chrono::system_clock;
using std::chrono::steady_clock;
using std::chrono::high_resolution_clock;
using std::chrono::duration;
using seconds = duration<double>;
using milliseconds = duration<double, std::milli>;
using microseconds = duration<double, std::micro>;
using fps24 = duration<unsigned long, std::ratio<1, 24>>;

The duration class represents an interval between two points in time. These aliases are convenient for using different intervals.

  • We can get the current time and date by using the system_clock class:

auto t = system_clock::now();
cout << format("system_clock::now is {:%F %T}\n", t);

The system_clock::now() function returns a time_point object. The <chrono> library includes a format() specialization for time_point that uses strftime() format specifiers.

The output is:

system_clock::now is 2023-02-05 13:52:15

The <iomanip> header includes put_time(), which works like strftime() for ostream:

std::time_t now_t = system_clock::to_time_t(t);
cout << "system_clock::now is " << std::put_time(std::localtime(&now_t), "%F %T") << '\n';

put_time() takes a pointer to a C-style time_t* value. system_clock::to_ time_t converts a time_point object to time_t. This gives the same output as our ...