如何取消boost asio io_service帖子

正如Sam所回答的，不可能选择性地取消已发布的处理程序。

如果目标是防止对生存期已过期的对象调用成员函数，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，对象的生存期至少延长到处理程序的生存期。如果对象的析构函数可以延迟，那么可以考虑通过shared_from_this()将对象绑定到处理程序。

另一方面，如果销毁需要立即进行，那么考虑编写一个弱绑定到实例的函子。这个问题讨论了与weak_ptr的绑定，并提供了一些研究/讨论链接。以下是弱绑定到对象的函子的简化完整示例：

#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:：运算符（）MyClass:：action（）MyClass:：action（）~MyClass（）weak_binder:：operator（）

可以看出，MyClass::action()只被调用两次：一次是在实例处于活动状态时通过weak_binder调用（处理程序A），另一次是通过boost::bind调用，其中实例通过shared_ptr维护（处理程序B）。处理程序C被调用，但weak_binder::operator()检测到实例已被破坏，导致静默的no-op。

您不能通过io_service以这种方式选择性地取消回调。一种选择是将逻辑移动到更高的级别，例如MyClass内部。示例实现可能是：

class MyClass : public boost::enable_shared_from_this<MyClass>
{
public:
    typedef boost::shared_ptr<MyClas> Ptr;
    static Ptr create( boost::asio::io_service& io_service ) { 
        const Ptr result( new MyClass );
        io_service.post( boost::bind(&MyClass::myCallback, result) );
        return result;
    }
    void myCallback() { 
        if ( _canceled ) return;
    }
    void cancel() { _canceled = true; }
private:
    MyClass() : _canceled(false) { }
private:
    bool _canceled;
};

该类使用boost::shared_ptr来强制执行共享所有权语义。这样做可以确保只要回调在调度之前保留在io_service队列中，对象的生存期就会持续。