如何做到这一点清楚所有张贴的任务，这在股已经排队？ [英] How do it clear all posted tasks which already queued in a strand?

问题描述

如何明确其已在排队的所有张贴任务的 io_service对象::股？我看到升压文件没有类似的方法。

How do it clear all posted tasks which already queued in a io_service::strand? I see no similar method from boost document.

推荐答案

我还没有找到需要它，因为它能够正确合理设计异步调用链来解决。一般地，Boost.Asio的API以这样一种方式是经过精心设计，它prevents复杂的应用程序，从在异步流变得复杂。

I have yet to find a need for it, as it can be resolved correctly with properly designing the asynchronous call chains. Generally, the Boost.Asio API is carefully designed in such a way that it prevents complex applications from becoming complicated in the asynchronous flow.

如果您已检查调用链，并且是绝对肯定的是，努力重新设计他们比引入清除链的复杂性更大的当前和未来的风险，那么有一种方法来完成它。然而，它确实有删除在链，及其相关的 io_service对象所有未调用处理程序的主要副作用。

If you have examined the call chains, and are absolutely certain that the effort to redesign them is a greater current and future risk than introducing the complication of clearing a strand, then there is a way to accomplish it. However, it does have the major side effect of deleting all uninvoked handlers within the strand, and its associated io_service.

在链被破坏，它的<一个href=\"http://www.boost.org/doc/libs/1_53_0/doc/html/boost_asio/reference/io_service__strand/_strand.html\"相对=nofollow>析构函数时间表未调用的延期调用处理程序 io_service对象维护非并发的保证。在 io_service对象 析构函数指出，定于递延调用了未调用处理程序对象被销毁。因此，通过控制寿命的链和 io_service对象，可以清除一个链中的处理程序。

When a strand is destroyed, its destructor schedules uninvoked handlers for deferred invocation on the io_service maintaining the guarantee of non-concurrency. The io_service destructor states that uninvoked handler objects that were scheduled for deferred invocation are destroyed. Thus, by controlling the lifetime of a strand and io_service, one can clear the handlers in a strand.

下面是一个辅助类，一个过于简单的例子 clearable_strand 。

Here is an overly simplified example with a helper class clearable_strand.

#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/optional.hpp>
#include <boost/utility/in_place_factory.hpp>

class clearable_strand
{
public:
  clearable_strand(boost::asio::io_service& main_io_service)
    : main_io_service_(main_io_service)
  {
    clear();
  }

public:
  template <typename Handler>
  void post(const Handler& handler)
  {
    // Post handler into the local strand.
    local_strand_->post(handler);

    // Local service now has work, so post its run handler to the
    // main service.
    main_io_service_.post(boost::bind(
      &boost::asio::io_service::run_one, boost::ref(local_io_service_)));
  }

  void clear()
  {
    // Destroy previous (if any).
    local_strand_     = boost::none;
    local_io_service_ = boost::none;
    // Reconstruct.
    local_io_service_ = boost::in_place();
    local_strand_     = boost::in_place(boost::ref(local_io_service_.get()));
  }

private:
  boost::asio::io_service&                 main_io_service_;
  boost::optional<boost::asio::io_service> local_io_service_;
  boost::optional<boost::asio::strand>     local_strand_;
};

要尽量减少影响，所以，只有的处理程序被破坏，该类使用内部 io_service对象 链而不是安装在链主 io_service对象。工作时发布到链，处理程序，然后发布到主 io_service对象将菊花链方式和维修当地 io_service对象。

To minimize the effect so that only the strand's handlers are destroyed, the class uses an internal io_service rather than attaching the strand to main io_service. When work is posted to the strand, a handler is then posted to the main io_service that will daisy chain and service the local io_service.

和其用法：

void print(unsigned int x)
{
  std::cout << x << std::endl;
}

int main()
{
  boost::asio::io_service io_service;
  io_service.post(boost::bind(&print, 1));

  clearable_strand strand(io_service);
  strand.post(boost::bind(&print, 2));
  strand.post(boost::bind(&print, 3));
  strand.clear(); // Handler 2 and 3 deleted.

  strand.post(boost::bind(&print, 4));
  io_service.run();
}

运行程序将输出 1 和 4 。我想强调，这是一个过于简单的例子，在一个非复杂的方式提供线程安全，以匹配的提高:: ASIO ::股可是一个挑战。

Running the program will output 1 and 4. I would like to stress that it is an overly simplified example, and providing thread-safety in a non-complicated manner to match that of boost::asio::strand can be a challenge.

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