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
     
              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 
     
      #include 
      
       #include 
       
        #include 
        
         #include 
         
          #include 
          
           #include 
           
            #include 
            
             namespace boost{ namespace signals2 { namespace detail { class tracked_objects_visitor; class trackable_pointee; typedef boost::variant
             
              
               , boost::weak_ptr
               
                , detail::foreign_void_weak_ptr > void_weak_ptr_variant; typedef boost::variant
                
                 
                  , detail::foreign_void_shared_ptr > void_shared_ptr_variant; class lock_weak_ptr_visitor { public: typedef void_shared_ptr_variant result_type; template
                  
                    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
                   
                     &) const { return boost::shared_ptr
                    
                     (); } }; class expired_weak_ptr_visitor { public: typedef bool result_type; template
                     
                       bool operator()(const WeakPtr &wp) const { return wp.expired(); } }; } class slot_base { public: typedef std::vector
                      
                        tracked_container_type; typedef std::vector
                       
                         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