为什么unordered_map没有rbegin（）或rend（），但只有begin（）和end（）

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

它是无序的，因此迭代发生的顺序是（或应该）不重要。

Answer 2

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.