Why does std::iterator not define const_reference and const_pointer?

Question

So, I made my own "standard compliant" container. I defined nested classes called iterator and const_iterator which derive from

std::iterator<std::bidirectional_iterator_tag, value_type>

where value_type is a typedef inside my new container class. I derive from std::iterator so that I can easily do

typedef typename std::iterator<std::bidirectional_iterator_tag,value_type>::value_type value_type;
typedef typename std::iterator<std::bidirectional_iterator_tag,value_type>::difference_type difference_type;
// ... etcetera

inside my nested iterator classes. However, cppreference.com says std::iterator does not have typedefs for a const_reference nor for a const_pointer. I can typedef them myself, but I am puzzled as to why these typedefs are omitted.

Answer 1

I've just reviewed the spec. const_pointer is only defined as part of the allocator types, std::allocator, std::allocator_traits, and container types, and has nothing whatsoever to do with iterators. Therefore, it makes sense that std::iterator does not define a const_pointer typedef.

This makes sense when you think about it, because the constness of the value pointed at is inferred by the type of the iterator itself. In the containers, iterator always refers to a mutable value, and const_iterator always refers to a const value, so neither has any need for a const_pointer typedef.

Since the concept of a const_iterator is slightly confusing, lets quickly review const. The key thing to take away is that a const iterator and a const_iterator are very different things, and yes, there are const const_iterators.

 a regular iterator models these two concepts
 int*; //changable pointer, to a changable int.  Modeled by iterator.
 int* const; //constant pointer, to a changable int.  Modeled by const iterator.
 //note that iterator doesn't ever refer to `const int`.

 a const_iterator models these two concepts
 const int*; //changable pointer to constant int. Modeled by const_iterator
 const int* const; //constant pointer to constnat int. Modeled by const const_iterator.
 //note that const_iterator ALWAYS refers to `const int`.