Hi Nikl,

On Mon, Dec 08, 2025 at 07:45:29PM +0500, Nikl Kelbon via Std-Proposals wrote:
> The standard needs to better clarify the section on vector iterator
> invalidation.
>
> *Why i think its important:*
>
> Some implementations added checks like: "oh, we must store all .end()
> iterators into global map under mutex and mark them invalid on each
> push_back, it will be SO useful for our developers!"
> So, minimal example where it breaks completely:

> Subject: Re: [std-proposals] vector::push_back must not invalidate past-the-end iterator

Here's some terminology criticism:

Please don't confuse the 'last' with the 'end'.

The 'last' element is an element that does exist. It's the last one.
Then, there's the 'end' pointer, which points one-past-the-last, which
you can't dereference because it doesn't point to any element. It only
serves as a delimiter.


Have a lovely day!
Alex

>
> std::vector<int> v;
> v.reserve(10);
>
> v.push_back(1);
>
> auto b = v.begin();
> auto e = v.end();
> v.push_back(1);
> ++b;
> REQUIRE(b == e); // assertion failure:
> // _STL_VERIFY(this->_Getcont() == _Right._Getcont(), "vector
> iterators incompatible");
>
>
> Its common pattern when using vector to reserve memory and push values,
> there are no "better way to do it", thats why it must be valid
>
>
> *Now about standard:*
>
> here's a quote from the standard regarding append_range and, apparently,
> push_back (https://eel.is/c++draft/vector#modifiers-2):
>
>
> If no reallocation happens, then references, pointers, and iterators before
> the insertion point remain valid but those at or after the insertion point,
> including the past-the-end iterator, are invalidated
>
>
> It explicitly states that despite there are no relocation happen, the
> past-the-end iterator is invalidated, although there's no reason for a
> vector to be so.
> Yes, a* past-the-end iterator will no longer be past-the-end, but that
> doesn't make it invalid*. In any implementation, even a foolish one, it's
> hard to imagine how, without relocation, this iterator could become
> anything other than just an iterator to the last element of the vector.
>
> I think in this case *standard should separate invalidation and what
> happens here - its not invalidation rly.*

> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals

-- 
<https://www.alejandro-colomar.es>