boost signal2 slot_base

先看成员_tracked_objects，从字面上讲是被跟踪的对象，再看，相关函数 bool expired() const，这个函数是检查_tracked_objects是否已经expired.只不过是使用一些设计模式上的东西，理解也比较好理解，

if(apply_visitor(detail::expired_weak_ptr_visitor(), *it)) return true; 实质也就是将*it传入detail::expired_weak_ptr_visitor()，其实就是调用仿函数并传入参数

      class expired_weak_ptr_visitor
      {
      public:
        typedef bool result_type;
        template<typename WeakPtr>
        bool operator()(const WeakPtr &wp) const
        {
          return wp.expired();
        }
      };

　　详细源码如下

#ifndef BOOST_SIGNALS2_SLOT_BASE_HPP
#define BOOST_SIGNALS2_SLOT_BASE_HPP

#include <boost/shared_ptr.hpp>
#include <boost/weak_ptr.hpp>
#include <boost/signals2/detail/foreign_ptr.hpp>
#include <boost/signals2/expired_slot.hpp>
#include <boost/signals2/signal_base.hpp>
#include <boost/variant/apply_visitor.hpp>
#include <boost/variant/variant.hpp>
#include <vector>

namespace boost
{
  namespace signals2
  {
    namespace detail
    {
      class tracked_objects_visitor;
      class trackable_pointee;

      typedef boost::variant<boost::weak_ptr<trackable_pointee>, boost::weak_ptr<void>, detail::foreign_void_weak_ptr > void_weak_ptr_variant;
      typedef boost::variant<boost::shared_ptr<void>, detail::foreign_void_shared_ptr > void_shared_ptr_variant;
      class lock_weak_ptr_visitor
      {
      public:
        typedef void_shared_ptr_variant result_type;
        template<typename WeakPtr>
        result_type operator()(const WeakPtr &wp) const
        {
          return wp.lock();
        }
        // overload to prevent incrementing use count of shared_ptr associated
        // with signals2::trackable objects
        result_type operator()(const weak_ptr<trackable_pointee> &) const
        {
          return boost::shared_ptr<void>();
        }
      };
      class expired_weak_ptr_visitor
      {
      public:
        typedef bool result_type;
        template<typename WeakPtr>
        bool operator()(const WeakPtr &wp) const
        {
          return wp.expired();
        }
      };
    }

    class slot_base
    {
    public:
      typedef std::vector<detail::void_weak_ptr_variant> tracked_container_type;
      typedef std::vector<detail::void_shared_ptr_variant> locked_container_type;

      const tracked_container_type& tracked_objects() const {return _tracked_objects;}
      locked_container_type lock() const
      {
        locked_container_type locked_objects;
        tracked_container_type::const_iterator it;
        for(it = tracked_objects().begin(); it != tracked_objects().end(); ++it)
        {
          locked_objects.push_back(apply_visitor(detail::lock_weak_ptr_visitor(), *it));
          if(apply_visitor(detail::expired_weak_ptr_visitor(), *it))
          {
            throw expired_slot();
          }
        }
        return locked_objects;
      }
      bool expired() const
      {
        tracked_container_type::const_iterator it;
        for(it = tracked_objects().begin(); it != tracked_objects().end(); ++it)
        {
          if(apply_visitor(detail::expired_weak_ptr_visitor(), *it)) return true;
        }
        return false;
      }
    protected:
      friend class detail::tracked_objects_visitor;

      void track_signal(const signal_base &signal)
      {
        _tracked_objects.push_back(signal.lock_pimpl());
      }

      tracked_container_type _tracked_objects;
    };
  }
} // end namespace boost

#endif // BOOST_SIGNALS2_SLOT_BASE_HPP