doc/html/boost_asio/example/cpp11/timers/time_t_timer.cpp – 1.79.0


// time_t_timer.cpp
// ~~~~~~~~~~~~~~~~
// Copyright (c) 2003-2022 Christopher M. Kohlhoff (chris at kohlhoff dot com)
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at

#include <boost/asio.hpp>
#include <ctime>
#include <chrono>
#include <iostream>

// A custom implementation of the Clock concept from the standard C++ library.
struct time_t_clock
  // The duration type.
  typedef std::chrono::steady_clock::duration duration;

  // The duration's underlying arithmetic representation.
  typedef duration::rep rep;

  // The ratio representing the duration's tick period.
  typedef duration::period period;

  // An absolute time point represented using the clock.
  typedef std::chrono::time_point<time_t_clock> time_point;

  // The clock is not monotonically increasing.
  static constexpr bool is_steady = false;

  // Get the current time.
  static time_point now() noexcept
    return time_point() + std::chrono::seconds(std::time(0));

// The boost::asio::basic_waitable_timer template accepts an optional WaitTraits
// template parameter. The underlying time_t clock has one-second granularity,
// so these traits may be customised to reduce the latency between the clock
// ticking over and a wait operation's completion. When the timeout is near
// (less than one second away) we poll the clock more frequently to detect the
// time change closer to when it occurs. The user can select the appropriate
// trade off between accuracy and the increased CPU cost of polling. In extreme
// cases, a zero duration may be returned to make the timers as accurate as
// possible, albeit with 100% CPU usage.
struct time_t_wait_traits
  // Determine how long until the clock should be next polled to determine
  // whether the duration has elapsed.
  static time_t_clock::duration to_wait_duration(
      const time_t_clock::duration& d)
    if (d > std::chrono::seconds(1))
      return d - std::chrono::seconds(1);
    else if (d > std::chrono::seconds(0))
      return std::chrono::milliseconds(10);
      return std::chrono::seconds(0);

  // Determine how long until the clock should be next polled to determine
  // whether the absoluate time has been reached.
  static time_t_clock::duration to_wait_duration(
      const time_t_clock::time_point& t)
    return to_wait_duration(t - time_t_clock::now());

typedef boost::asio::basic_waitable_timer<
  time_t_clock, time_t_wait_traits> time_t_timer;

int main()
    boost::asio::io_context io_context;

    time_t_timer timer(io_context);

    std::cout << "Starting synchronous wait\n";
    std::cout << "Finished synchronous wait\n";

    std::cout << "Starting asynchronous wait\n";
        [](const boost::system::error_code& /*error*/)
          std::cout << "timeout\n";
    std::cout << "Finished asynchronous wait\n";
  catch (std::exception& e)
    std::cout << "Exception: " << e.what() << "\n";

  return 0;

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