c++ - Why does std::iterator not define const_reference and const_pointer? -

so, made own "standard compliant" container. defined nested classes called iterator , const_iterator derive from

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

where value_type typedef inside new container class. derive std::iterator can 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 nested iterator classes. however, cppreference.com says std::iterator not have typedefs const_reference nor const_pointer. can typedef them myself, puzzled why these typedefs omitted.

i've reviewed spec. const_pointer defined part of allocator types, std::allocator, std::allocator_traits, , container types, , has nothing whatsoever iterators. therefore, makes sense std::iterator not define const_pointer typedef.

this makes sense when think it, because constness of value pointed @ inferred type of iterator itself. in containers, iterator refers mutable value, , const_iterator refers const value, neither has need const_pointer typedef.

since concept of const_iterator confusing, lets review const. key thing take away const iterator , const_iterator very different things, , yes, there const const_iterators.

 regular iterator models these 2 concepts  int*; //changable pointer, changable int.  modeled iterator.  int* const; //constant pointer, changable int.  modeled const iterator.  //note iterator doesn't ever refer `const int`.   const_iterator models these 2 concepts  const int*; //changable pointer constant int. modeled const_iterator  const int* const; //constant pointer constnat int. modeled const const_iterator.  //note const_iterator refers `const int`.