When using erase to delete first element of vector during reversely iterating, these is a crash caused by outside range

Question

The crash can be reproduced by the below codes.

#include <vector> #include <iostream> using namespace std; int main() { vector<int> tests; tests.push_back(1); tests.erase(tests.rbegin().base()); return 0; }

If I use the normal iterator, the erase function call will not casue crash. Does anyone know what the mechanism is? I would apprecaite a explanation.

By the way, the reason I try to use reversely iterator is that deleting an element of vector during reversely iterating does not skip iterating the element after deleted element without additional codes.

tests.rbegin().base() == tests.end(). The base() iterator is one position behind where the reverse iterator logically points to. It has to be - there's no position before the first element that rend().base() could point to otherwise. — Igor Tandetnik
– Igor Tandetnik, Commented Mar 17, 2020 at 16:40
this includes a very illustrative picture: en.cppreference.com/w/cpp/iterator/reverse_iterator — 463035818_is_not_an_ai
– 463035818_is_not_an_ai, Commented Mar 17, 2020 at 16:42
Thans everyone for your explanations. I know what the mechanism is. — wangsy
– wangsy, Commented Mar 17, 2020 at 16:55

Remy Lebeau · Accepted Answer · 2020-03-17 17:51:40Z

From https://en.cppreference.com/w/cpp/iterator/reverse_iterator/base:

The base iterator refers to the element that is next (from the std::reverse_iterator::iterator_type perspective) to the element the reverse_iterator is currently pointing to. That is &*(rit.base() - 1) == &*rit.

This means, in my understanding, that you are trying to start at the last element with your reverse iterator and when you get its base you are actually accessing the element AFTER the last element. This one will obviously always be outside of range.

I hope this helps clarify. If I misunderstood something, feel free to correct me.

Thanks a lot. I learn something. I appreciate your explanation.
Thanks you @RemyLebeau for the update on my formatting. Good for the next answer.

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