如果还有异步操作等待basic_waitable_timer被破坏怎么办？

Question

如果还有异步操作等待basic_waitable_timer被破坏怎么办？行为记录在任何地方吗？

Answer 1

当一个IO对象(例如basic_waitable_timer)被销毁时,它的析构函数将调用destroy()IO对象的服务(不要与之混淆io_service),传递IO对象的实现.一个basic_waitable_timer人的服务是waitable_timer_service并满足WaitableTimerService类型要求.WaitableTimerService的要求定义了destroy()取消异步等待操作的后置条件,使它们尽快完成,并且取消操作的处理程序将传递错误代码boost::asio::error::operation_aborted.

service.destroy(impl);→隐式取消异步等待操作,就像通过调用一样service.cancel(impl, e).

service.cancel(impl, e);→导致任何未完成的异步等待操作尽快完成.取消操作的处理程序应传递错误代码error::operation_aborted.设置e表示成功或失败.

请注意,已排队等待调用的操作的处理程序将不会被取消,并且将具有error_code反映操作成功的操作.

以下是演示此行为的完整示例:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/asio/steady_timer.hpp>

void demo_deferred_completion()
{
  std::cout << "[demo deferred completion]" << std::endl;
  boost::asio::io_service io_service;
  auto wait_completed = false;

  // Use scope to force lifetime.  
  {
    // Create the timer and initiate an async_wait operation that
    // is guaranteed to have expired.
    boost::asio::steady_timer timer(io_service);

    // Post a ready-to-run no-op completion handler into the io_service.
    // Although the order is unspecified, the current implementation
    // will use a predictable order.
    io_service.post([]{});

    // Initiate an async_wait operation that will immediately expire.
    timer.expires_at(boost::asio::steady_timer::clock_type::now());
    timer.async_wait(
        [&](const boost::system::error_code& error)
        {
          std::cout << "error: " << error.message() << std::endl;
          assert(error == boost::system::error_code()); // Success.
          wait_completed = true;          
        });

    // While this will only run one handler (the noop), it will
    // execute operations (async_wait), and if they are succesful
    // (time expired), the completion handler will be posted for
    // deferred completion.
    io_service.run_one();
    assert(!wait_completed); // Verify the wait handler was not invoked.
  } // Destroy the timer.

  // Run the handle_wait completion handler.
  io_service.run();
  assert(wait_completed);
}

void demo_cancelled()
{
  std::cout << "[demo cancelled]" << std::endl;
  boost::asio::io_service io_service;

  // Use scope to force lifetime.
  {
    boost::asio::steady_timer timer(io_service);

    // Initiate an async_wait operation that will immediately expire.
    timer.expires_at(boost::asio::steady_timer::clock_type::now());
    timer.async_wait(
        [](const boost::system::error_code& error)
        {
          std::cout << "error: " << error.message() << std::endl;
          assert(error ==
                 make_error_code(boost::asio::error::operation_aborted));
        });
  } // Destroy the timer.

  // Run the handle_wait completion handler.
  io_service.run();
}

int main()
{
  demo_deferred_completion();
  demo_cancelled();
}

输出:

[demo deferred completion]
error: Success
[demo cancelled]
error: Operation canceled