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### 2256. On `vector` iterator invalidation

Section: 24.3.11.5 [vector.modifiers] Status: NAD Submitter: Howard Hinnant Opened: 2013-04-29 Last modified: 2020-05-08

Priority: 3

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Discussion:

24.3.11.5 [vector.modifiers]/p3 says:

>     iterator erase(const_iterator position);
>     iterator erase(const_iterator first, const_iterator last);

> > Effects: Invalidates iterators and references at or after the point of the erase.

Consider this example:

>     #include <vector>
>     #include <cassert>

>     int main()
>     {
>       typedef std::vector<int> C;
>       C c = {1, 2, 3, 4};
>       C::iterator i = c.begin() + 1;
>       C::iterator j = c.end() - 1;
>       assert(*i == 2);
>       assert(*j == 4);
>       c.erase(c.begin());
>       assert(*i == 3); // Why is this not perfectly fine?!
>     }

Why has the iterator `i` been invalidated? It still refers to a perfectly reasonable, fully constructed object. If `vector::iterator` were to be implemented as a pointer (which is legal), it is not possible for that last line to do anything but run fine.

The iterator `j` on the other hand now points at end, and any iterators that may now point beyond `end()`, into uninitialized memory, are clearly invalid.

But why do we say that an iterator that must point to a valid object is invalid? This looks to me like we simply got sloppy in our specification.

[2016-05 Issues Telecon]

This is related to 2698

[2017-03-04, Kona]

NAD. "Works as designed" Also, the example does not work in today's world of `launder`.

Proposed resolution: