incrementing iterator of std::unordered_map

Question

Why are lines 7 and 8 bad?? Why it's bad to in-/decrement some iterator?

#include <unordered_map>

int main()
{
    std::unordered_multimap<int,int> myumm({{1,3},{3,2},{5,5},{0,9}});
    auto first = myumm.begin();
    auto second = first+1;      // bad
    auto third = --myumm.end(); // bad too
    auto fourth = myumm.end();
}

Answer 1

std::unordered_multimap offers Forward Iterators. These are iterators which you can assign, dereference, compare, and increment.

To be able to decrement an iterator ( --it ), you need at least a Bidirectional Iterator (such as offered by std::multimap ).

To be able to add (an arbitrary number) to an iterator ( it + 1 ), you need a Random Access Iterator (such as offered by std::vector ). To advance a weaker iterator by more than one place, use std::advance(it, 42) (for advancing it in place), or std::next(it, 42) (which returns the incremented copy and does not modify it ).