First I want to say, publicly, thank you for all your help. David Abrahams wrote:

"Neal D. Becker" <nbecker@hns.com> writes:

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I was struggling with const correctness and iterator adaptors. At first I was using iterator_facade. I found it difficult to get dereference to work correctly.

What were you trying to do and what was the problem?

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After changing to iterator_adaptor I got things working. I'm wondering if the approach I used is "correct".

I include the whole class here, because it isn't really very big.

I was using a Ring class which contains an iterator, which is cycle_iterator<std::vector<Complex>::iterator>.

If a Ring const& is passed

Sorry, I forget the details now, it was something like this. Initially I tried to use iterator_facade. Let me try to give a shortened version. Suppose template<typename T> struct Ring { cycle_iterator<std::vector<T>::iterator> it; }; Then template<typename cont_t> F (cont_t const& r) { In F we now use the values in r, which will try to dereference the iterator it, and I got (cryptic) errors. Then I tried to use iterator_adaptor instead (after all, this really is an iterator adaptor), and had better luck. [...]

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reference operator[] (difference_type n) { return *(base + PositiveMod (position + n)); }

What's wrong with the operator[] supplied by iterator_adaptor? [...]

I believe in my case there was a difference in efficiency, which is critical in my app. Computing a new iterator required recalculating "wrap", while just using operator[] does not require the extra step. I have made all the changes you suggested and everything seems to be working fine. The updated version is enclosed. ,----[ /disk1/nbecker/shannon2/src/cycle_iterator.hpp ] | #ifndef cycle_iterator_H | #define cycle_iterator_H | | #include <boost/iterator/iterator_adaptor.hpp> | #include <iterator> | | namespace boost { | | //! This is a cycle iterator that does keep track of wraparound. | template<typename BaseIterator, typename offset_t> | class cycle_iterator : public | boost::iterator_adaptor<cycle_iterator<BaseIterator, offset_t>, | BaseIterator | > | { | public: | typedef typename boost::iterator_adaptor<cycle_iterator<BaseIterator, | offset_t>, | BaseIterator | > super_t; | | typedef typename super_t::difference_type difference_type; | typedef typename super_t::reference reference; | | explicit cycle_iterator() {} | | explicit cycle_iterator (BaseIterator const& _b, BaseIterator const& | _e, offset_t offset=0) : | base(_b), size (std::distance (_b, _e)) { | SetPos (offset); | } | | template <typename OtherBase> | cycle_iterator (cycle_iterator<OtherBase,offset_t> const& other, | typename enable_if_convertible<OtherBase, BaseIterator>::type* = 0) : | base (other.base), | size (other.size), | position (other.position), | wrap (other.wrap) | {} | | private: | friend class boost::iterator_core_access; | | void increment () { | ++position; | if (position >= size) { | ++wrap; | position -= size; | } | } | | void decrement () { | --position; | if (position < 0) { | --wrap; | position += size; | } | } | | void SetPos (offset_t newpos) { | position = newpos % size; | wrap = newpos / size; | if (position < 0) { | --wrap; | position += size; | } | } | | void advance (difference_type n) { | offset_t newpos = realposition() + n; | SetPos (newpos); | } | | template<typename OtherBase> | difference_type | distance_to (cycle_iterator<OtherBase, offset_t> const& y) const { | if (size == 0) | return 0; | | else { | offset_t pos1 = realposition(); | offset_t pos2 = y.realposition(); | return -(pos1 - pos2); | } | } | | template<typename OtherBase> | bool equal (cycle_iterator<OtherBase, offset_t> const& y) const { | return distance_to (y) == 0; | } | | reference dereference() const { return *(base + position); } | | offset_t PositiveMod (offset_t x) const { | offset_t y = x % size; | if (y < 0) | y += size; | return y; | } | | public: | | | reference operator[] (difference_type n) const { return *(base + | PositiveMod (position + n)); } | | offset_t offset() const { return position; } | | offset_t realposition () const { | return position + wrap * size; | } | | | // private: | | BaseIterator base; | offset_t size; | offset_t position; | offset_t wrap; | }; | | template<typename offset_t, typename BaseIterator> | cycle_iterator<BaseIterator, offset_t> make_cycle_iterator(BaseIterator | b, BaseIterator e, offset_t offset=0) { | return cycle_iterator<BaseIterator, offset_t> (b, e, offset); | } | | template<typename BaseIterator> | cycle_iterator<BaseIterator, int> make_cycle_iterator(BaseIterator b, | BaseIterator e, int offset=0) { | return cycle_iterator<BaseIterator, int> (b, e, offset); | } | | } //namespace boost | | #endif `----