On Thu, Aug 13, 2020 at 9:58 AM Arthur O'Dwyer via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> To implement `iterator_to` for anything except `vector`, you have to rely
> on a cast like this:
>
> struct Node {
> T t;
> Node *next;
> };
> Node *node_from_t(T *t) {
> return reinterpret_cast<Node *>(t);
> }
> int main() {
> Node n;
> assert(node_from_t(&n.t) == &n);
> }
>
> I'm not sure if this is well-defined. As long as `t` is the first member
> of `Node`, I think it might actually be well-defined; but I wouldn't bet
> money on it.
> Problem is, library implementors tend to be *extremely* loath to rely on
> anything smelling of UB in their implementations.
>
> my $.02,
> –Arthur
>

This is well-defined. A pointer to the first non-static data member of a
struct is pointer-convertible with a pointer to the struct.

Also there's plenty of UB in standard library container implementations.
Very difficult to implement stuff like extract() otherwise.

Barry