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Re: Footnote in section on Address-Of Operator

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JoJoModding <jojohostert@gmail.com> writes:

> in the paragraph on address and indirection operators (6.5.3.2 in the
> C23 draft N3047), there is a footnote (footnote 117 in that draft),
> which says that
>
>> &*E is equivalent to E (even if E is a null pointer)

This seems to be a case where a footnote might add confusion rather than
clarity.  The normative text makes it clear that &*E can't be equivalent
to E in every way because &*E is not an lvalue.  And &*E has type
constraints that E does not have.

> This seems to imply that sizeof(&*E) == sizeof(E), which is unexpected
> if E is an array.

There are much simpler examples if the apparent non-equivalence.  If p
is a pointer object, p can be assigned to by &*p can't be.  And due to
the clause about constraints.  &*(void *)0 is a constraint violation,
but (void *)0 is obviously fine.

I say "apparent" because equivalence is a slippery term.  It does not
mean "exactly the same as" but something much less specific so it may
have been chosen for this very reason.

> Further, we have that
>> If an invalid value has been assigned to the pointer, the behavior of
>> the unary * operator is undefined.
>
> However the footnote says that &*E is equivalent to E, so if E is an
> invalid pointer value, *E would be undefined behavior, but &*E is not?

Yes &*p is fine even if p is an invalid pointer because undefined
behaviour only exists if *p is evaluated, and nether the * nor the & are
evaluated in &*p.

> ... &* does still remove UB even though "the constraints on the
> operators still apply"?

It removes some but not all.  &*0 is a constraint violation (and hence
UB), but &*(int *)0 is not.  De-referencing a null pointer is not a
constraint violation.

> How is one to read this footnote, and the paragraph in general? Why
> does it try to say that things are "equivalent" that sometimes are
> not?

Well, I think it means equivalent in some ways and not in others.  But
I'm not sure it adds any clarity to the normative wording that has been
around for many years.

-- 
Ben.

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Footnote in section on Address-Of Operator JoJoModding <jojohostert@gmail.com> - 2022-11-23 14:40 -0800
  Re: Footnote in section on Address-Of Operator Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-24 00:05 +0000
    Re: Footnote in section on Address-Of Operator Tim Rentsch <tr.17687@z991.linuxsc.com> - 2022-11-23 18:48 -0800
      Re: Footnote in section on Address-Of Operator Ben Bacarisse <ben.usenet@bsb.me.uk> - 2022-11-24 13:26 +0000

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