On Tue, 9 Dec 2025, 04:19 Nikl Kelbon via Std-Proposals, <
std-proposals_at_[hidden]> wrote:

> Im glad you now asking for a motivation and not asking "what is past the
> end iterator" or smth what was discussed before
>
> In real code i created my container template, so its iterator combines
> several iterators of its parts. std::vector was one of those parts
>
> Then i wrote append(iterator, iterator) and my code sures it is completely
> right, no reallocation etc etc.
>
> But msvc stl implementation find a way to assert it (by storing all .end()
> iterators in global map under mutex and invalidate them on each push_back)
>
> I dont know any other way to code it. Its impossible. THERE ARE NO OTHER
> WAY!
>

This is not motivation for changing it, you're still just saying it's
possible. That's not a reason to do it.



> Code:
>
>
> #include <vector>
>
> struct mytype {
> std::vector<int> vec;
>
> // in real code it is complex iterator
> // where vector iterator only part of it
> using iterator = std::vector<int>::iterator;
>
> void append(iterator b, iterator e) {
> for (; b != e; ++b) { // ASSERTION FAILURE, COMPARING b and e after
> push_back
> // again, in real code it complex operation with inserting in my
> container
> vec.push_back(*b);
> }
> }
>
> iterator begin() {
> return vec.begin();
> }
> iterator end() {
> return vec.end();
> }
> };
>
> int main() {
> mytype x;
> x.vec = {1, 2, 3, 4};
> x.vec.reserve(100);
> x.append(x.begin(), x.end());
> }
>
> https://godbolt.org/z/cjfT9vran
>
> вт, 9 дек. 2025 г. в 07:18, Jason McKesson via Std-Proposals <
> std-proposals_at_[hidden]>:
>
>> On Mon, Dec 8, 2025 at 1:56 PM Nikl Kelbon via Std-Proposals
>> <std-proposals_at_[hidden]> wrote:
>> >
>> > Thats what standard exists for. Standard decides what is ub and what is
>> not ub. How its even may be argument "its ub" in discussion about changing
>> standard?
>>
>> Because your reasoning for why this change is needed is that many
>> users do it regardless of the fact that it's UB, so it makes sense to
>> change it. That is, your proposal is predicated on the idea that this
>> is common code, and therefore if it isn't common code, then it has no
>> motivation.
>>
>> So, what evidence can you provide that people believe that end
>> iterators should work this way? The fact that it can be implemented is
>> not, by itself, sufficient justification for changing the standard.
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