On Sunday, 31 March 2024 08:22:05 PDT Avi Kivity via Std-Proposals wrote:
> If it
> reallocates after the push_back(), and the move constructor can throw,
> then we lose the strong exception guarantee.

It's impossible to reallocate after, because the vector must create space to
store the new element. It can't store the element where there's no room.

> The standard says:
> > Remarks: Causes reallocation if the new size is greater than the old
>
> capacity. Reallocation invalidates all the references, pointers, and
> iterators referring to the elements in the sequence, as well as the
> past-the-end iterator.
>
> So, it says nothing about whether a push_back referring to a vector
> element is legal.
>
> Is this undefined behavior? Should it be specified to work? Should it
> be noted that it is dangerous?

Looks pretty clear to me: if the new size is going to be bigger than the
previous capacity, then it invalidates and therefore the reference stored by
binding the parameter to v.back() is dangling. It's clearly UB to dereference
it and therefore not expected to work.

-- 
Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
   Principal Engineer - Intel DCAI Cloud Engineering