STL(9)之distance函數組源碼

1.一些說明：

<span style="font-size:18px;"> /**
   *  @brief A generalization of pointer arithmetic.
   *  @param  __first  An input iterator.
   *  @param  __last  An input iterator.
   *  @return  The distance between them.
   *
   *  Returns @c n such that __first + n == __last.  This requires
   *  that @p __last must be reachable from @p __first.  Note that @c
   *  n may be negative.
   *
   *  For random access iterators, this uses their @c + and @c - operations
   *  and are constant time.  For other %iterator classes they are linear time.
  */</span>

 

2.輸入迭代器的版本：

template<typename _InputIterator>
    inline typename iterator_traits<_InputIterator>::difference_type
    __distance(_InputIterator __first, _InputIterator __last,
               input_iterator_tag)
    {
      // concept requirements
      __glibcxx_function_requires(_InputIteratorConcept<_InputIterator>)

      typename iterator_traits<_InputIterator>::difference_type __n = 0;
      while (__first != __last)
	{
	  ++__first;
	  ++__n;
	}
      return __n;
    }

 

3.For RandomAccessIterator:

template<typename _RandomAccessIterator>
    inline typename iterator_traits<_RandomAccessIterator>::difference_type
    __distance(_RandomAccessIterator __first, _RandomAccessIterator __last,
               random_access_iterator_tag)
    {
      // concept requirements
      __glibcxx_function_requires(_RandomAccessIteratorConcept<
				  _RandomAccessIterator>)
      return __last - __first;
    }

4.其他迭代器：

  template<typename _InputIterator>
    inline typename iterator_traits<_InputIterator>::difference_type
    distance(_InputIterator __first, _InputIterator __last)
    {
      // concept requirements -- taken care of in __distance
      return std::__distance(__first, __last,
        std::__iterator_category(__first));
    }