Obtaining const_iterator from iterator

Question

Is there a metafunction f that maps an iterator to its corresponding const_iterator?

I.e. f::ite

Answer 1

I can think of something for a reverse_iterator: using the base member function the decltype, one could extract the return type to get back to the iterator.

However there is no such function for iterator / const_iterator, so it's hard to see how this could be achieved, short of providing an inner typedef or requiring explicit specialization.