这是什么意思？

C/C++ code

 namespace detail {    // For partial specialization workaround    enum event_visitor_enum    { on_no_event_num,      on_initialize_vertex_num, on_start_vertex_num,      on_discover_vertex_num, on_finish_vertex_num, on_examine_vertex_num,      on_examine_edge_num, on_tree_edge_num, on_non_tree_edge_num,      on_gray_target_num, on_black_target_num,      on_forward_or_cross_edge_num, on_back_edge_num,      on_edge_relaxed_num, on_edge_not_relaxed_num,      on_edge_minimized_num, on_edge_not_minimized_num    };    template<typename Event, typename Visitor>    struct functor_to_visitor : Visitor    {      typedef Event event_filter;      functor_to_visitor(const Visitor& visitor) : Visitor(visitor) {}    };  } // namespace detail

functor_to_visitor(const Visitor& visitor) : Visitor(visitor) {}//一个对象用来初始化一个类型？什么意思啊？

[解决办法]
functor_to_visitor 继承 Visitor
所以调用Visitor来初始化 functor_to_visitor中的Visitor 部分