Nsryjdtyk

In trying to code an iterator, I have the following structure:

template <typename T> class mything {

  typedef unsigned key;

public:

  template <typename subT> class myiter {

    key pos;

  public:

    ...

  }

  using iterator = myiter<T>;

  using const_iterator = myiter<const T>;

  using reverse_iterator = std::reverse_iterator<iterator>;

  using const_reverse_iterator = std::reverse_iterator<const_iterator>;



  iterator begin() { return { 0 }; }

  ...

};

and would like to add the traits by declaring std::iterator_traits<...>. I tried

template<typename T,typename subT> struct std::iterator_traits<typename mything<T>::template myiter<subT>> { ... }

however, this results in a compiler error: "template parameters not deducible in partial specialization:" I tried just

template<typename T> struct std::iterator_traits<typename mything<T>::template myiter<T>> { ... }

which at least is accepted by the compiler, but the trait is not defined, so later in std::reverse_iterator I get the error: "no type named 'iterator_category' in 'struct std::iterator_traits::myiter >'
class reverse_iterator".

The only thing that compiles, for now, is to explicity declare

template<> struct std::iterator_traits<mything<int>::myiter<int>> { ... }

which obviously I don't want to do for all the classes.

Here is a minimal working example: changing #if 0 to #if 1 produces the errors.

#include <iterator>



template <typename T> T myobj = 0;



template <typename T> class mything {

  typedef unsigned key;

public:

  template <typename subT> class myiter {

    key pos;

  public:

    myiter(key _pos) : pos(_pos) { }

    subT &operator *() const { return myobj<subT>; }

    myiter operator ++() { return myiter(pos+1); }

    myiter operator --() { return myiter(pos-1); }

    std::ptrdiff_t operator-(const myiter &that) const { return pos-that.pos; }

  };



    using iterator = myiter<T>;

    using const_iterator = myiter<const T>;

    using reverse_iterator = std::reverse_iterator<iterator>;

    using const_reverse_iterator = std::reverse_iterator<const_iterator>;



    iterator begin() { return { 0 }; }

    iterator end() { return { 10 }; }

    const_iterator begin() const { return { 0 }; }

    const_iterator end() const { return { 10 }; }

    const_iterator cbegin() const { return begin(); }

    const_iterator cend() const { return end(); }



    reverse_iterator rbegin() { return reverse_iterator{end()}; }

    reverse_iterator rend() { return reverse_iterator{begin()}; }

    const_reverse_iterator rbegin() const { return const_reverse_iterator{end()}; }

    const_reverse_iterator rend() const { return const_reverse_iterator{begin()}; }

    const_reverse_iterator crbegin() const { return rbegin(); }

    const_reverse_iterator crend() const { return rend(); }

};



#if 0

template<typename T,typename subT> struct std::iterator_traits<typename mything<T>::template myiter<subT>> {

  using difference_type = std::ptrdiff_t;

  using size_type = std::size_t;

  using value_type = subT;

  using pointer = subT*;

  using reference = subT&;

  using iterator_category = std::random_access_iterator_tag;

};



#else

template <> struct std::iterator_traits<mything<int>::myiter<int>> {

  using difference_type = std::ptrdiff_t;

  using size_type = std::size_t;

  using value_type = int;

  using pointer = int*;

  using reference = int&;

  using iterator_category = std::random_access_iterator_tag;

};



template <> struct std::iterator_traits<mything<int>::myiter<const int>> {

  using difference_type = std::ptrdiff_t;

  using size_type = std::size_t;

  using value_type = const int;

  using pointer = const int*;

  using reference = const int&;

  using iterator_category = std::random_access_iterator_tag;

};

#endif



#include <cassert>

#include <iostream>

#include <sstream>

int main(void) {

  mything<int> thing;



  std::ostringstream s;

  std::copy(thing.cbegin(), thing.cend(), std::ostream_iterator<const int>(s, " "));

  assert(s.str() == "0 0 0 0 0 0 0 0 0 0 ");



  s.str("");

  std::copy(thing.rbegin(), thing.rend(), std::ostream_iterator<const int>(s, " "));

  assert(s.str() == "0 0 0 0 0 0 0 0 0 0 ");



  auto const& ro_thing = thing;

  s.str("");

  std::copy(ro_thing.cbegin(), ro_thing.cend(), std::ostream_iterator<const int>(s, " "));

  assert(s.str() == "0 0 0 0 0 0 0 0 0 0 ");

  s.str("");

  std::copy(ro_thing.rbegin(), ro_thing.rend(), std::ostream_iterator<const int>(s, " "));

  assert(s.str() == "0 0 0 0 0 0 0 0 0 0 ");



  return 0;

}