boost::asio和活动对象

Asio可以用来包含活动对象的意图:将方法执行与方法调用解耦。额外的需求需要在更高的级别上处理，但在使用Boost时并不会过于复杂。Asio与其他Boost库一起使用

Scheduler可以使用:

• boost::thread用于线程抽象。
• boost::thread_group管理线程的生存期。
• boost::asio::io_service提供线程池。当没有工作挂起时，可能想要使用boost::asio::io_service::work来保持线程存活。

ActivationList可以实现为:

• 。用于获取最高优先级方法请求的MultiIndex。使用提示位置insert()，将保留具有相同优先级的请求的插入顺序。
• std::multiset或std::multimap可选。然而，在c++ 03中，对于具有相同键(优先级)的请求的顺序没有指定。
• 如果Request不需要保护方法，则可以使用std::priority_queue。

Request可以是未指定的类型:

• boost::function和boost::bind可以用来提供类型擦除，同时绑定到可调用类型而不引入Request层次结构。

Futures可以使用Boost。线程的期货支持。

• future.valid()将返回true，如果Request已经添加到ActivationList。
• future.wait()将阻塞等待结果。
• future.get()将阻塞等待结果。
• 如果呼叫者没有使用future，则呼叫者不会被阻塞。
使用Boost的另一个好处。线程的未来是，从Request内产生的异常将传递给Future。

---

下面是一个利用各种Boost库的完整示例，应该满足要求:

// Standard includes
#include <algorithm> // std::find_if
#include <iostream>
#include <string>
// 3rd party includes
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/function.hpp>
#include <boost/make_shared.hpp>
#include <boost/multi_index_container.hpp>
#include <boost/multi_index/ordered_index.hpp>
#include <boost/multi_index/member.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/thread.hpp>
#include <boost/utility/result_of.hpp>
/// @brief scheduler that provides limits with prioritized jobs.
template <typename Priority,
          typename Compare = std::less<Priority> >
class scheduler
{
public:
  typedef Priority priority_type;
private:
  /// @brief method_request is used to couple the guard and call
  ///        functions for a given method.
  struct method_request
  {
    typedef boost::function<bool()> ready_func_type;
    typedef boost::function<void()> run_func_type;
    template <typename ReadyFunctor,
              typename RunFunctor>
    method_request(ReadyFunctor ready,
                   RunFunctor run)
      : ready(ready),
        run(run)
    {}
    ready_func_type ready;
    run_func_type run;
  };
  /// @brief Pair type used to associate a request with its priority.
  typedef std::pair<priority_type,
                    boost::shared_ptr<method_request> > pair_type;
  static bool is_method_ready(const pair_type& pair)
  {
    return pair.second->ready();
  }
public:
  /// @brief Construct scheduler.
  ///
  /// @param max_threads Maximum amount of concurrent task.
  /// @param max_request Maximum amount of request.  
  scheduler(std::size_t max_threads,
            std::size_t max_request)
    : work_(io_service_),
      max_request_(max_request),
      request_count_(0)
  {
    // Spawn threads, dedicating them to the io_service.
    for (std::size_t i = 0; i < max_threads; ++i)
      threads_.create_thread(
        boost::bind(&boost::asio::io_service::run, &io_service_));
  }
  /// @brief Destructor.
  ~scheduler()
  {
    // Release threads from the io_service.
    io_service_.stop();
    // Cleanup.
    threads_.join_all();
  }
  /// @brief Insert a method request into the scheduler.
  ///
  /// @param priority Priority of job.
  /// @param ready_func Invoked to check if method is ready to run.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename ReadyFunctor,
            typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(priority_type priority, 
         const ReadyFunctor& ready_func,
         const RunFunctor& run_func)
  {
    typedef typename boost::result_of<RunFunctor()>::type result_type;
    typedef boost::unique_future<result_type> future_type;
    boost::unique_lock<mutex_type> lock(mutex_);
    // If max request has been reached, then return an invalid future.
    if (max_request_ &&
        (request_count_ == max_request_))
      return future_type();
    ++request_count_;
    // Use a packaged task to handle populating promise and future.
    typedef boost::packaged_task<result_type> task_type;
    // Bind does not work with rvalue, and packaged_task is only moveable,
    // so allocate a shared pointer.
    boost::shared_ptr<task_type> task = 
      boost::make_shared<task_type>(run_func);
    // Create method request.
    boost::shared_ptr<method_request> request =
      boost::make_shared<method_request>(
        ready_func,
        boost::bind(&task_type::operator(), task));
    // Insert into priority.  Hint to inserting as close to the end as
    // possible to preserve insertion order for request with same priority.
    activation_list_.insert(activation_list_.end(),
                            pair_type(priority, request));
    // There is now an outstanding request, so post to dispatch.
    io_service_.post(boost::bind(&scheduler::dispatch, this));
    return task->get_future();
  }
  /// @brief Insert a method request into the scheduler.
  ///
  /// @param ready_func Invoked to check if method is ready to run.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename ReadyFunctor,
            typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(const ReadyFunctor& ready_func,
         const RunFunctor& run_func)
  {
    return insert(priority_type(), ready_func, run_func);
  }
  /// @brief Insert a method request into the scheduler.
  ///
  /// @param priority Priority of job.
  /// @param run_func Invoked when ready to run.
  ///
  /// @return future associated with the method.
  template <typename RunFunctor>
  boost::unique_future<typename boost::result_of<RunFunctor()>::type>
  insert(priority_type priority, 
         const RunFunctor& run_func)
  {
    return insert(priority, &always_ready, run_func);
  }
  /// @brief Insert a method request with default priority into the
  ///        scheduler.
  ///
  /// @param run_func Invoked when ready to run.
  ///
  /// @param functor Job to run.
  ///
  /// @return future associated with the job.
  template <typename RunFunc>
  boost::unique_future<typename boost::result_of<RunFunc()>::type>
  insert(const RunFunc& run_func)
  {
    return insert(&always_ready, run_func);
  }
  /// @brief Cancel all outstanding request.
  void cancel()
  {
    boost::unique_lock<mutex_type> lock(mutex_);
    activation_list_.clear();
    request_count_ = 0;
  } 
private:
  /// @brief Dispatch a request.
  void dispatch()
  {
    // Get the current highest priority request ready to run from the queue.
    boost::unique_lock<mutex_type> lock(mutex_);
    if (activation_list_.empty()) return;
    // Find the highest priority method ready to run.
    typedef typename activation_list_type::iterator iterator;
    iterator end = activation_list_.end();
    iterator result = std::find_if(
      activation_list_.begin(), end, &is_method_ready);
    // If no methods are ready, then post into dispatch, as the
    // method may have become ready.
    if (end == result)
    {
      io_service_.post(boost::bind(&scheduler::dispatch, this));
      return;
    }
    // Take ownership of request.
    boost::shared_ptr<method_request> method = result->second;
    activation_list_.erase(result);
    // Run method without mutex.
    lock.unlock();
    method->run();    
    lock.lock();
    // Perform bookkeeping.
    --request_count_;
  }
  static bool always_ready() { return true; }
private:
  /// @brief List of outstanding request.
  typedef boost::multi_index_container<
    pair_type,
    boost::multi_index::indexed_by<
      boost::multi_index::ordered_non_unique<
        boost::multi_index::member<pair_type,
                                   typename pair_type::first_type,
                                   &pair_type::first>,
        Compare
      >
    >
  > activation_list_type;
  activation_list_type activation_list_;
  /// @brief Thread group managing threads servicing pool.
  boost::thread_group threads_;
  /// @brief io_service used to function as a thread pool.
  boost::asio::io_service io_service_;
  /// @brief Work is used to keep threads servicing io_service.
  boost::asio::io_service::work work_;
  /// @brief Maximum amount of request.
  const std::size_t max_request_;
  /// @brief Count of outstanding request.
  std::size_t request_count_;
  /// @brief Synchronize access to the activation list.
  typedef boost::mutex mutex_type;
  mutex_type mutex_;
};
typedef scheduler<unsigned int, 
                  std::greater<unsigned int> > high_priority_scheduler;
/// @brief adder is a simple proxy that will delegate work to
///        the scheduler.
class adder
{
public:
  adder(high_priority_scheduler& scheduler)
    : scheduler_(scheduler)
  {}
  /// @brief Add a and b with a priority.
  ///
  /// @return Return future result.
  template <typename T>
  boost::unique_future<T> add(
    high_priority_scheduler::priority_type priority,
    const T& a, const T& b)
  {
    // Insert method request
    return scheduler_.insert(
      priority,
      boost::bind(&adder::do_add<T>, a, b));
  }
  /// @brief Add a and b.
  ///
  /// @return Return future result.
  template <typename T>
  boost::unique_future<T> add(const T& a, const T& b)
  {
    return add(high_priority_scheduler::priority_type(), a, b);
  }
private:
  /// @brief Actual add a and b.
  template <typename T>
  static T do_add(const T& a, const T& b)
  {
    std::cout << "Starting addition of '" << a 
              << "' and '" << b << "'" << std::endl;
    // Mimic busy work.
    boost::this_thread::sleep_for(boost::chrono::seconds(2));
    std::cout << "Finished addition" << std::endl;
    return a + b;
  }
private:
  high_priority_scheduler& scheduler_;
};
bool get(bool& value) { return value; }
void guarded_call()
{
  std::cout << "guarded_call" << std::endl; 
}
int main()
{
  const unsigned int max_threads = 1;
  const unsigned int max_request = 4;
  // Sscheduler
  high_priority_scheduler scheduler(max_threads, max_request);
  // Proxy
  adder adder(scheduler);
  // Client
  // Add guarded method to scheduler.
  bool ready = false;
  std::cout << "Add guarded method." << std::endl;
  boost::unique_future<void> future1 = scheduler.insert(
    boost::bind(&get, boost::ref(ready)),
    &guarded_call);
  // Add 1 + 100 with default priority.
  boost::unique_future<int> future2 = adder.add(1, 100);
  // Force sleep to try to get scheduler to run request 2 first.
  boost::this_thread::sleep_for(boost::chrono::seconds(1));
  // Add:
  //   2 + 200 with low priority (5)
  //   "test" + "this" with high priority (99)
  boost::unique_future<int> future3 = adder.add(5, 2, 200);
  boost::unique_future<std::string> future4 = adder.add(99,
    std::string("test"), std::string("this"));
  // Max request should have been reached, so add another.
  boost::unique_future<int> future5 = adder.add(3, 300);
  // Check if request was added.
  std::cout << "future1 is valid: " << future1.valid()
          << "nfuture2 is valid: " << future2.valid()
          << "nfuture3 is valid: " << future3.valid()
          << "nfuture4 is valid: " << future4.valid()
          << "nfuture5 is valid: " << future5.valid()
          << std::endl;
  // Get results for future2 and future3.  Do nothing with future4's results.
  std::cout << "future2 result: " << future2.get()
          << "nfuture3 result: " << future3.get()
          << std::endl;
  std::cout << "Unguarding method." << std::endl;
  ready = true;
  future1.wait();
}

执行使用1个线程池，最多4个请求。

• request1被保护到程序结束，并且应该是最后一个运行。
• request2(1 + 100)以默认优先级插入，应该首先运行。
• request3(2 + 200)插入低优先级，应该在request4之后运行。
• request4 ('test' + 'this')以高优先级插入，应该在request3之前运行。
• request5应该由于最大请求而无法插入，并且应该无效。

输出如下:

添加保护方法。开始添加'1'和'100'Future1有效:1Future2有效:1Future3有效:1Future4有效:1Future5有效:0完成添加开始添加test和this完成添加开始添加'2'和'200'完成添加Future2结果:101Future3 result: 202使无防备的方法。guarded_call