Re: [PATCH] assert that deleting by pointer to base in unique_ptr does not cause UB

[Antony Polukhin <antoshkka@gmail.com>]

ср, 22 сент. 2021 г. в 20:23, Ville Voutilainen <ville.voutilainen@gmail.com>:
>
> On Wed, 22 Sept 2021 at 20:09, Antony Polukhin via Libstdc++
> <libstdc++@gcc.gnu.org> wrote:
> >
> > std::unique_ptr allows construction from std::unique_ptr of derived
> > type as per [unique.ptr.single.asgn] and [unique.ptr.single.ctor]. If
> > std::default_delete is used with std::unique_ptr, then after such
> > construction a delete is called on a pointer to base. According to
> > [expr.delete] calling a delete on a non similar object without a
> > virtual destructor is an undefined behavior.
> >
> > This patch turns that undefined behavior into static assertions inside
> > std::unique_ptr.
>
> I don't understand the sizeof(_Tp) == sizeof(_Up) part in the
> static_assert. I fail to see how
> a same-size check suggests that the types are similar enough that a
> delete-expression works.

I used the following logic:
[unique.ptr.single.*] sections have the constraint that
"unique_­ptr<U, E>::pointer is implicitly convertible to pointer".
There's already a static assert that T in unique_ptr<T> is not void,
so U either has to be the same type T, or a type derived from T. If a
derived type adds members, then size changes and types are not similar
as the decompositions won't have the qualification-decompositions with
the same n.

-- 
Best regards,
Antony Polukhin