如何取消提升asio io_service帖子

Question

如何取消已发布的回调:

getIoService()->post(boost::bind(&MyClass::myCallback, this));

并保持其他发布的回调不受影响？

问题是我有一些从不同线程接收事件的对象,我将它们发布到ioservice以处理主线程中的事件.如果在某些时候我想要删除我的对象 - ioservice将尝试在已销毁的对象中执行已发布的回调.在这种情况下,我不能在对象中存储任何标志,因为它将被删除.

有一种可能的解决方案使用enable_shared_from_this和shared_from_this(),但不知道是否另一种解决办法还是不行.

谢谢

Answer 1

正如Sam回答的那样,无法有选择地取消已发布的处理程序.

如果目标是阻止在生命周期已过期的对象上调用成员函数,那么使用enable_shared_from_this是惯用的解决方案.这种方法的一个结果是对象的生命周期至少延伸到处理程序的生命周期.如果可以延迟对象的析构函数,则考虑将对象绑定到处理程序shared_from_this().

另一方面,如果需要立即进行破坏,那么考虑编写一个与实例弱绑定的仿函数. 这个问题讨论了与a的绑定weak_ptr,并提供了一些研究/讨论链接.这是一个与对象弱绑定的仿函数的简化完整示例:

#include <iostream>
#include <boost/asio.hpp>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/make_shared.hpp>
#include <boost/shared_ptr.hpp>

/// @brief Mocked up type.
class MyClass:
  public boost::enable_shared_from_this<MyClass>
{
public:
  MyClass()     { std::cout << "MyClass()"         << std::endl; }
  ~MyClass()    { std::cout << "~MyClass()"        << std::endl; }
  void action() { std::cout << "MyClass::action()" << std::endl; }
};

/// @brief weak_binder is a functor that binds a member function
///        to a weakly managed object instance.  I.e. this
///        functor will not extend the life of the instance to
///        which it has been bound.
template <typename Fn,
          typename C>
struct weak_binder
{
private:
  typedef typename C::element_type element_type;
public:

  /// @brief Constructor.
  weak_binder(Fn& fn, C& c) : fn_(fn), c_(c)
  {}

  /// @brief Conditional invoke Fn if C still exists.
  void operator()()
  {
    std::cout << "weak_binder::operator()" << std::endl;
    // Create a shared pointer from the weak pointer.  If
    // succesful, then the object is still alive.
    if (boost::shared_ptr<element_type> ptr = c_.lock())
    {
      // Invoke the function on the object.
      (*ptr.*fn_)();
    }
  }
private:
  Fn fn_;
  boost::weak_ptr<element_type> c_;
};

/// @brief Helper function to create a functor that weakly
///        binds to a shared object.
template <typename Fn,
          typename C>
weak_binder<Fn, C> weak_bind(Fn fn, C c)
{
  return weak_binder<Fn, C>(fn, c);
}

int main()
{
  boost::asio::io_service io_service;
  boost::shared_ptr<MyClass> my_class = boost::make_shared<MyClass>();

  // my_class will remain alive for this handler because a shared_ptr
  // is bound to handler B, and handler B will only be destroyed after
  // handler A has been destroyed.
  io_service.post(weak_bind(&MyClass::action,
                            my_class->shared_from_this())); // A

  // my_class will remain alive for this handler because it is bound
  // via a shared_ptr.
  io_service.post(boost::bind(&MyClass::action,
                              my_class->shared_from_this())); // B

  // my_class will not be alive for this handler, because B will have
  // been destroyed, and the my_class is reset before invoking the
  // io_service.
  io_service.post(weak_bind(&MyClass::action,
                            my_class->shared_from_this())); // C

  // Reset the shared_ptr, resulting in the only remaining shared_ptr
  // instance for my_class residing within handler B.
  my_class.reset();
  io_service.run();
}

结果输出:

MyClass()
weak_binder::operator()
MyClass::action()
MyClass::action()
~MyClass()
weak_binder::operator()

可以看出,MyClass::action()只调用了两次:一次weak_binder是在实例处于活动状态时(处理程序A),一次boost::bind是通过shared_ptr(处理程序B)维护实例的位置.调用处理程序C,但weak_binder::operator()检测到实例已被销毁,从而导致静默无操作.