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