c++ - 提升 asio 优先级队列，从处理程序添加异步操作

Question

我对基于 boost asio 优先级队列示例的设计有疑问。如果我从处理程序中添加一个包装的处理程序，它似乎会丢失:

参见 http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/example/cpp03/invocation/prioritised_handlers.cpp例如。

我按原样使用了所有内容，并将 main() 函数替换为以下代码:

//
// based on prioritised_handlers.cpp
// ~~~~~~~~~~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2015 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 http://www.boost.org/LICENSE_1_0.txt)
//

#include <boost/asio.hpp>
#include <boost/function.hpp>
#include <iostream>
#include <queue>

using boost::asio::ip::tcp;

class handler_priority_queue
{
public:
  void add(int priority, boost::function<void()> function)
  {
    handlers_.push(queued_handler(priority, function));
  }

  void execute_all()
  {
    while (!handlers_.empty())
    {
      queued_handler handler = handlers_.top();
      handler.execute();
      handlers_.pop();
    }
  }

  // A generic wrapper class for handlers to allow the invocation to be hooked.
  template <typename Handler>
  class wrapped_handler
  {
  public:
    wrapped_handler(handler_priority_queue& q, int p, Handler h)
      : queue_(q), priority_(p), handler_(h)
    {
    }

    void operator()()
    {
      handler_();
    }

    template <typename Arg1>
    void operator()(Arg1 arg1)
    {
      handler_(arg1);
    }

    template <typename Arg1, typename Arg2>
    void operator()(Arg1 arg1, Arg2 arg2)
    {
      handler_(arg1, arg2);
    }

  //private:
    handler_priority_queue& queue_;
    int priority_;
    Handler handler_;
  };

  template <typename Handler>
  wrapped_handler<Handler> wrap(int priority, Handler handler)
  {
    return wrapped_handler<Handler>(*this, priority, handler);
  }

private:
  class queued_handler
  {
  public:
    queued_handler(int p, boost::function<void()> f)
      : priority_(p), function_(f)
    {
    }

    void execute()
    {
      function_();
    }

    friend bool operator<(const queued_handler& a,
        const queued_handler& b)
    {
      return a.priority_ < b.priority_;
    }

  private:
    int priority_;
    boost::function<void()> function_;
  };

  std::priority_queue<queued_handler> handlers_;
};

// Custom invocation hook for wrapped handlers.
template <typename Function, typename Handler>
void asio_handler_invoke(Function f,
    handler_priority_queue::wrapped_handler<Handler>* h)
{
  h->queue_.add(h->priority_, f);
}
void low_priority_handler()
{
  std::cout << "Low priority handler\n";
}

int main()
{
  //
  // BASED ON prioritised_handlers.cpp
  // ~~~~~~~~~~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2011 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 http://www.boost.org/LICENSE_1_0.txt)
  //



  //----------------------------------------------------------------------
  using boost::asio::ip::tcp;
  boost::asio::io_service io_service;

  handler_priority_queue pri_queue;

  // Post a completion handler to be run immediately.
  io_service.post(pri_queue.wrap(0, low_priority_handler));


  // Set a deadline timer to expire immediately.
  boost::asio::deadline_timer timer1(io_service);
  timer1.expires_at(boost::posix_time::neg_infin);
  timer1.async_wait(pri_queue.wrap(42, [](const boost::system::error_code& )
                                   {
                                     std::cout << "now" << std::endl;
                                   }));
  // Set a deadline timer to expire later.
  boost::asio::deadline_timer timer2(io_service, boost::posix_time::milliseconds(100));
  boost::asio::deadline_timer timer3(io_service, boost::posix_time::milliseconds(200));
  timer2.async_wait(pri_queue.wrap(100, [&pri_queue, &timer3](const boost::system::error_code& )
                                   {
                                     std::cout << "100ms" << std::endl;
                                     timer3.async_wait(pri_queue.wrap(100, [](const boost::system::error_code& )
                                                                      {
                                                                        std::cout << "200ms" << std::endl;
                                                                      }));

                                   }));

  while (io_service.run_one())
    {
      // The custom invocation hook adds the handlers to the priority queue
      // rather than executing them from within the poll_one() call.
      while (io_service.poll_one())
        ;

      pri_queue.execute_all();
    }
}

//g++ -std=c++14 -Wall -Werror  -rdynamic -lboost_system -lboost_thread -lboost_log -lpthread prioritised_handlers.cpp

这打印:

now
Low priority handler
100ms

Timer3 的 200ms 打印输出丢失了。根据我的 printf 调试方法，自定义调用 Hook asio_handler_invoke() 永远不会为“200ms”操作调用。不幸的是，我不明白为什么。

上述方法有什么问题？

在 Technik 的提示后用示例的其余部分更新了代码

Answer 1

除了您保持(或更确切地说不保持)应用程序存活足够长的时间以使最后一个处理程序完成之外，任何代码都没有问题。

您正在运行 io_service，轮询并运行每个作业。所有作业都得到处理，但它们是延迟的异步调用。您指望 io_service 调用使应用程序保持事件状态，但就 io_service 而言，所有工作都已完成，因此它停止阻塞，您的 main() 函数退出并且您永远看不到最终处理程序的输出.

为了向您证明这一点，只需将 io_service 包装在 ::work 对象中，这将防止 io_service 认为这是它没有工作要做，因此总是阻塞。

boost::asio::io_service::work w(io_service);

将此添加到您的 while (io_service.run_one()) 行之前。