Re: [Boost-bugs] [Boost C++ Libraries] #7824: "++--rng.end() == rng.end()" can fail for strided ranges

Subject: Re: [Boost-bugs] [Boost C++ Libraries] #7824: "++--rng.end() == rng.end()" can fail for strided ranges
From: Boost C++ Libraries (noreply_at_[hidden])
Date: 2012-12-22 08:49:51

#7824: "++--rng.end() == rng.end()" can fail for strided ranges
--------------------------------------+-------------------------------------
  Reporter:  michel                   |       Owner:  neilgroves
      Type:  Bugs                     |      Status:  new       
 Milestone:  To Be Determined         |   Component:  range     
   Version:  Boost Development Trunk  |    Severity:  Problem   
Resolution:                           |    Keywords:            
--------------------------------------+-------------------------------------

Comment (by michel):

 Just for reference, here is a sample implementation of `strided_iterator`
 for `Random Access Range` (without zero-stride support),
 which can pass `strided.cpp` test (other than `strided_test_zero_stride`).
 {{{
 // for Random Access Range (without zero-stride support)
 template <typename BaseIterator>
 class strided_iterator<BaseIterator, boost::random_access_traversal_tag>
   : public iterator_facade<
         strided_iterator<BaseIterator, boost::random_access_traversal_tag>
       , typename std::iterator_traits<BaseIterator>::value_type
       , boost::random_access_traversal_tag
       , typename std::iterator_traits<BaseIterator>::reference
       , typename std::iterator_traits<BaseIterator>::difference_type
     >
 {
 public:
     // types
     typedef typename std::iterator_traits<BaseIterator>::reference
 reference;
     typedef typename std::iterator_traits<BaseIterator>::difference_type
 difference_type;

     // ctors
     strided_iterator() {}
     strided_iterator(
         BaseIterator first
       , BaseIterator it
       , BaseIterator last
       , difference_type stride
     )
       : m_begin(first)
       , m_offset((it - first) % stride == 0 ? (it - first) / stride : (it
 - first) / stride + 1)
       , m_stride(stride)
     {}

 private:
     // iterator_facade core operations
     friend class boost::iterator_core_access;

     reference dereference() const
     { return *(m_begin + m_stride * m_offset); }

     bool equal(strided_iterator const& other) const
     { return m_begin == other.m_begin && m_offset == other.m_offset; }

     void increment() { ++m_offset; }
     void decrement() { --m_offset; }
     void advance(difference_type n) { m_offset += n; }

     difference_type distance_to(strided_iterator const& other) const
     { return other.m_offset - m_offset; }

     // data
     BaseIterator m_begin; // the begin iterator of an input range
     difference_type m_offset; // the number of strides taken by this
 iterator
     difference_type m_stride;
 };
 }}}

-- 
Ticket URL: <https://svn.boost.org/trac/boost/ticket/7824#comment:1>
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