traits help

From: Rex_chaos (rex_chaos_at_21cn.com)
Date: 10/23/03 

Date: 23 Oct 2003 13:22:26 -0700

Hi there,
  I am learning template programming and there is a problem about
traits. Now consider a container and an iterator. Here is the code

// tag for const iterator and non-const iterator
struct non_const_iterator_tag {};
struct const_iterator_tag {};

// traits
template <typename T,
          typename tag=non_const_iterator_tag>
struct Iter_traits
{
  typedef random_access_iterator_tag iterator_category;
  typedef T value_type;
  typedef ptrdiff_t difference_type;
  typedef T* pointer;
  typedef T& reference;
};

// For const iterator ...
template <typename T>
struct Iter_traits<T, const_iterator_tag>
{
  typedef random_access_iterator_tag iterator_category;
  typedef T value_type;
  typedef ptrdiff_t difference_type;
  typedef const T* pointer;
  typedef const T& reference;
};

//Iter is an iterator

template <typename T, typename tag=non_const_iterator_tag>
class Iter
{
  typedef typename Iter_traits<T, tag>::value_type value_type;
  typedef typename Iter_traits<T, tag>::reference reference;
  public:
    Iter(T *c) :cont(c) {...}

    // copy constructor to shift the non-const iterator to const
iterator
    Iter(const Iter<T, non_const_iterator_tag>& it) :cont(it.c)
    {...}

  private:
    T *cont;
};

// Container is a container.

template <typename T>
class Container
{
  ...
  public:
    typedef Iter<T> iterator;
    typedef Iter<T, const_iterator_tag> const_iterator;

    iterator begin() {...};
    const_iterator begin() const {...};
    iterator end() {...};
    const_iterator end() const {...};
  private:
    T *data;
 ...
};

int main(void)
{
  Container<int> c;

  // copy constructor should be actived here
  Container<int>::const_iterator cit=c.begin();
  return 0;
}

An error occurs while compliation.

`int* Iter<int, non_const_iterator_tag>::cont' is private

I don't know why. It seems that it's due to the copy constructor.
Please help!

BTW, if I define begin() outside the class. e.g.

template <typename T>
Container<T>::iterator Container<T>::begin(void)
{
 ...
}

The compiler will give a warning:
`typename Container<T>::iterator' is implicitly a typename
mplicit typename is deprecated, please see the documentation for
details

Is that anything wrong? Should I do something to prevent that warning?

Thanks in advance.

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