On Thu, Jul 10, 2025 at 6:55 PM Ell <ell_se_at_[hidden]> wrote:

>
>
> On 7/11/25 01:28, Brian Bi wrote:
> >
> >
> > On Thu, Jul 10, 2025 at 5:57 PM Ell via Std-Discussion <std-
> > discussion_at_[hidden] <mailto:std-discussion_at_[hidden]>>
> wrote:
> >
> > If an array provides storage for an object, does destroying the
> object
> > "invalidate" the contents of the array?
> >
> > void f () {
> > unsigned char a[1];
> >
> > using T = unsigned char;
> >
> > T* p = new (a) T (123);
> > unsigned char x = a[0];
> > p->~T ();
> >
> > unsigned char y = a[0];
> >
> > assert (x == y);
> > }
> >
> >
> > In this case, we have to answer the question of whether the newly
> > created object is an element of the array, or whether the original
> > element is still there and the array is providing storage for the new
> > object.
> >
> > [intro.object]/2 <http://eel.is/c++draft/intro.object#2> states that the
> > new object in this particular case is a subobject of the array `a`.
> > Presumably an array cannot have two elements with index 0, so we can
> > infer that the original `a[0]`'s lifetime ends as in [basic.life]/2.5
> > <http://eel.is/c++draft/basic.life#2.5>. The array doesn't provide
> > storage for the new `unsigned char` object. When the new object is later
> > destroyed, `a[0]` is out-of-lifetime and attempting to access its value
> > results in UB.
> >
> > If you had created an object of some other type, say, `char`, in the
> > storage of `a`, the standard is unclear as to what happens to the value
> > of `a[0]` in that case (either during or after the `char` object's
> > lifetime).
>
> Huh, I didn't even consider this distinction. I didn't specifically mean
> for the types to be the same. Though, the way I'm reading
> [intro.object], paragraphs 2 and 3 aren't mutually exclusive, and the
> code fits both (whether or not this makes sense).
>

No, because [intro.object]/3 starts with "If a complete object is created
[...]"

If the conditions of [intro.object]/2 have been met, then the created
object is a subobject, not a complete object.

-- 
*Brian Bi*