Question

It seems weird to iterate through an unordered_map, with begin(), end() and a forward iterator.

If so, why does it not have rbegin(), rend() and bi-direction iterators too?
is there any technical reason?

Answer 1

Cite from The C++ Standard Library

Reverse iterators let algorithms operate backward by switching the
call of an increment operator internally into a call of the decrement
operator, and vice versa. All containers with bidirectional iterators
or random-access iterators (all sequence containers except
forward_list and all associative containers) can create reverse
iterators via their member functions rbegin() and rend(). Since C++11,
the corresponding member functions returning read-only iterators,
crbegin() and crend(), are also provided.

For forward_lists and unordered containers, no backward-iteration
interface (rbegin(), rend(), etc.) is provided. The reason is that the
implementation requires only singly linked lists to go through the
elements.