为什么我不能增加std :: unordered_map迭代器？

Question

std::unordered_map<int, int> _cache;

std::vector<std::unordered_map<int, int>::iterator> _lruList;

this works

std::rotate(_lruList.begin(), _lruList.begin() + 1, _lruList.end());

but this doesn't

std::rotate(_cache.begin(), _cache.begin() + 1, _cache.end()); // error occurs on _cache.begin() + 1 saying "error type"

Which doesn't really make sense to me because they are both iterators except one is for a vector and one is for a unordered_map

Then I also tried this std::rotate(_cache.begin(), _cache.begin() ++, _cache.end());

but I got the following errors: _Left: you can't assign to a variable that is const _Right: you can't assign to a variable that is const

Answer 1

unordered_map iterators are forward iterators. That means they can only move one step at a time, only forward, and getting from one position to another requires traversing all of the intervening positions. As such, forward iterators do not support operator+ , because it would be an O(n) operation. The authors of the standard library felt that when people see a + b , they expect that to be O(1), and so if an iterator type can't meet that requirement, then the operator should not be supported.

vector iterators are random access, which means they do support operator+ , because it can be implemented as O(1). You can do this instead:

std::rotate(_cache.begin(), std::next(_cache.begin()), _cache.end());

Except that also won't work, because std::rotate is a modifying operation. And you can't modify the keys of elements in unordered_map .