This tutorial demonstrates the use of the boost::asio::strand class to synchronise callback handlers in a multithreaded program.

 本示例程序示范了使用boost::asio::strand 类来创建多线程程序中的同步回调句柄。

The previous four tutorials avoided the issue of handler synchronisation by calling the boost::asio::io_service::run() function from one thread only. As you already know, the asio library provides a guarantee that callback handlers will only be called from threads that are currently calling boost::asio::io_service::run(). Consequently, calling boost::asio::io_service::run() from only one thread ensures that callback handlers cannot run concurrently.

 前四个例程只是在单线程下使用 boost::asio::io_service::run() 函数来避免处理函同步。如你所见，Asio库保证回调句柄仅能被当前正在调用boost::asio::io_service::run()函数的线程调用。因此，在单线程中调用boost::asio::io_service::run()能确保回调句柄不被并发运行。

The single threaded approach is usually the best place to start when developing applications using asio. The downside is the limitations it places on programs, particularly servers, including:

 单线程通常是使用Asio开发应用程序最好的方式。下面是Asio在程序中的局限性，尤其是服务器方面，包括：

操作需要较长时间处理才能完成时弱响应  

在大规模的多处理机系统中表现不佳 

If you find yourself running into these limitations, an alternative approach is to have a pool of threads calling boost::asio::io_service::run(). However, as this allows handlers to execute concurrently, we need a method of synchronisation when handlers might be accessing a shared, thread-unsafe resource.

如果你发现自己陷入这些局限时，一个可供选择的方法是创建一个每个线程都调用boost::asio::io_service::run()的线程池。不过，因为这允许并发操作，当访问一个共享、非线程安全的资源时，我们需要一个同步方式。

#include <iostream>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

We start by defining a class called printer, similar to the class in the previous tutorial. This class will extend the previous tutorial by running two timers in parallel.

让我们从定义一个名为printer的类开始，这与前一个示例中的类很相似。这个类是上一个例子的扩展，这里我们使用两个并行的定时器。

class printer
{
public:

In addition to initialising a pair of boost::asio::deadline_timer members, the constructor initialises the strand_ member, an object of type boost::asio::strand.

An boost::asio::strand guarantees that, for those handlers that are dispatched through it, an executing handler will be allowed to complete before the next one is started. This is guaranteed irrespective of the number of threads that are calling boost::asio::io_service::run(). Of course, the handlers may still execute concurrently with other handlers that were not dispatched through an boost::asio::strand, or were dispatched through a different boost::asio::strand object.

 除了初始化一对boost::asio::deadline_timer 成员变量外，构造函数还初始化一个boost::asio::strand类型的strand_成员变量。

boost::asio::strand 对象保证：对于通过它来分派执行的众操作中，只有一个操作执行完成之后才允许进入下一个操作。这种保证与多少个线程调用boost::asio::io_service::run()无关。当然，如果不是通过一个boost::asio::strand对象分派，或者通过其它不同的boost::asio::strand对象分派，这些操作仍旧可能是并发的。

  printer(boost::asio::io_service& io)
    : strand_(io),
      timer1_(io, boost::posix_time::seconds(1)),
      timer2_(io, boost::posix_time::seconds(1)),
      count_(0)
  {

When initiating the asynchronous operations, each callback handler is "wrapped" using the boost::asio::strand object. The boost::asio::strand::wrap() function returns a new handler that automatically dispatches its contained handler through the boost::asio::strand object. By wrapping the handlers using the same boost::asio::strand, we are ensuring that they cannot execute concurrently.

 当开始同步操作时，每一个回调句柄都使用boost::asio::strand对象进行“包装”。boost::asio::strand::wrap() 函数返回一个新的通过boost::asio::strand对象自动分派的内部句柄。通过同一boost::asio::strand对象对句柄进行“包装”，我们可以保证操作不会并发执行。

    timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
    timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
  }

  ~printer()
  {
    std::cout << "Final count is " << count_ << " ";
  }

In a multithreaded program, the handlers for asynchronous operations should be synchronised if they access shared resources. In this tutorial, the shared resources used by the handlers (print1 and print2) are std::cout and the count_ data member.

在一个多线程程序中，当访问同一共享资源时，异步操作必须是同步的。在本例中，print1和print2函数使用的共享资源为std::cout 和count_数据成员。

  void print1()
  {
    if (count_ < 10)
    {
      std::cout << "Timer 1: " << count_ << " ";
      ++count_;

      timer1_.expires_at(timer1_.expires_at() + boost::posix_time::seconds(1));
      timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
    }
  }

  void print2()
  {
    if (count_ < 10)
    {
      std::cout << "Timer 2: " << count_ << " ";
      ++count_;

      timer2_.expires_at(timer2_.expires_at() + boost::posix_time::seconds(1));
      timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
    }
  }

private:
  boost::asio::strand strand_;
  boost::asio::deadline_timer timer1_;
  boost::asio::deadline_timer timer2_;
  int count_;
};

The main function now causes boost::asio::io_service::run() to be called from two threads: the main thread and one additional thread. This is accomplished using an boost::thread object.

Just as it would with a call from a single thread, concurrent calls to boost::asio::io_service::run() will continue to execute while there is "work" left to do. The background thread will not exit until all asynchronous operations have completed.

 在主函数中，boost::asio::io_service::run() 现在被两个线程调用：主线程和一个附加线程。这一切依赖于boost::thread对象来完成。

正如它被一个单线程调用一样，boost::asio::io_service::run() 的并发调用会一直持续到无任何“工作”可做。后台线程直到所有异步操作都完成后才会退出。

int main()
{
  boost::asio::io_service io;
  printer p(io);
  boost::thread t(boost::bind(&boost::asio::io_service::run, &io));
  io.run();
  t.join();

  return 0;
}

See the full source listing

 查看本例的全部源码：

//
// timer.cpp
// ~~~~~~~~~
//
// Copyright (c) 2003-2008 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 <iostream>
#include <boost/asio.hpp>
#include <boost/thread.hpp>
#include <boost/bind.hpp>
#include <boost/date_time/posix_time/posix_time.hpp>

class printer
{
public:
  printer(boost::asio::io_service& io)
    : strand_(io),
      timer1_(io, boost::posix_time::seconds(1)),
      timer2_(io, boost::posix_time::seconds(1)),
      count_(0)
  {
    timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
    timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
  }

  ~printer()
  {
    std::cout << "Final count is " << count_ << " ";
  }

  void print1()
  {
    if (count_ < 10)
    {
      std::cout << "Timer 1: " << count_ << " ";
      ++count_;

      timer1_.expires_at(timer1_.expires_at() + boost::posix_time::seconds(1));
      timer1_.async_wait(strand_.wrap(boost::bind(&printer::print1, this)));
    }
  }

  void print2()
  {
    if (count_ < 10)
    {
      std::cout << "Timer 2: " << count_ << " ";
      ++count_;

      timer2_.expires_at(timer2_.expires_at() + boost::posix_time::seconds(1));
      timer2_.async_wait(strand_.wrap(boost::bind(&printer::print2, this)));
    }
  }

private:
  boost::asio::strand strand_;
  boost::asio::deadline_timer timer1_;
  boost::asio::deadline_timer timer2_;
  int count_;
};

int main()
{
  boost::asio::io_service io;
  printer p(io);
  boost::thread t(boost::bind(&boost::asio::io_service::run, &io));
  io.run();
  t.join();

  return 0;
}