C / C++ / MFC

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow

template<typename Iterator>
Iterator advance(Iterator it,size_t n)
{
  for(size_t i=0;i<n;++i){
    ++it;
  }
  return it;
}

// specialization of advance for pointers
template<typename T>
Iterator advance(T* it,size_t n)
{
  return it+n;
}

template<typename Iterator>
Iterator advance(Iterator it,size_t n)
{
  if(Iterator is random-access){
    return it+n;
  }
  else{
    for(size_t i=0;i<n;++i){
      ++it;
    }
    return it;
  }
}

//tag types, whose only serve is to signal the kind of iterator
struct random_access_category{};
struct default_access_category{};
 
template<typename Iterator>
Iterator advance(Iterator it,size_t n)
{
  return advance_helper<typename Iterator::category>(it,n);
}
  
template<typename IteratorCategory,typename Iterator>
Iterator advance_helper(Iterator it,size_t n)
{
  // default impplementation
  for(size_t i=0;i<n;++i){
    ++it;
  }
  return it;
}
  
template<typename Iterator>
Iterator advance_helper<randon_access_category>(Iterator it,size_t n)
{
  // specialization for randon-acess iterators
  return it+n;
}

• An iterator is not guaranteed to have such a category typedef, no matter how bad you need it.
• Pointers cannot have a nested typedef

struct random_access_category{};
struct default_access_category{};
 
// traits class for describing an iterator type
// by default, an iterator is assummed *not* to be random-access
template <typename Iterator>
struct iterator_traits
{
  typedef default_access_category category;
};
 
// but we know pointers are randon-access
template <typename T>
struct iterator_traits<T*>
{
  typedef random_access_category category;
};
 
Iterator advance(Iterator it,size_t n)
{
  return advance_helper<typename iterator_traits<Iterator>::category>(it,n);
}
  
// the rest as before

#include "FastIteratorLib.h" // some third party library

template<typename T>
struct iterator_traits<fast_iterator_lib::fast_iterator<T> >
{
  typedef random_access_category category;
}

"Opinions are neither right nor wrong. I cannot change your opinion. I can, however, change what influences your opinion." - David Crow