类型萃取,标准库学到的

```cpp
  template<typename _Iterator>
    struct __iterator_traits<_Iterator,
			     __void_t<typename _Iterator::iterator_category,
				      typename _Iterator::value_type,
				      typename _Iterator::difference_type,
				      typename _Iterator::pointer,
				      typename _Iterator::reference>>
    {
      typedef typename _Iterator::iterator_category iterator_category;
      typedef typename _Iterator::value_type        value_type;
      typedef typename _Iterator::difference_type   difference_type;
      typedef typename _Iterator::pointer           pointer;
      typedef typename _Iterator::reference         reference;
    };