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From: Ville Voutilainen <ville.voutilainen@gmail.com>
Date: Wed, 22 Sep 2021 20:53:17 +0300
Message-ID: <CAFk2RUYZ4Tykcqg=eKJc=-cnmDhOT=o2BOKJKRDgH5HCDc6KmQ@mail.gmail.com>
Subject: Re: [PATCH] assert that deleting by pointer to base in unique_ptr
 does not cause UB
To: Antony Polukhin <antoshkka@gmail.com>
Cc: "libstdc++" <libstdc++@gcc.gnu.org>,
 gcc-patches List <gcc-patches@gcc.gnu.org>
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On Wed, 22 Sept 2021 at 20:49, Antony Polukhin <antoshkka@gmail.com> wrote:
>
> =D1=81=D1=80, 22 =D1=81=D0=B5=D0=BD=D1=82. 2021 =D0=B3. =D0=B2 20:23, Vil=
le 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 insid=
e
> > > std::unique_ptr.
> >
> > I don't understand the sizeof(_Tp) =3D=3D 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_=C2=ADptr<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.

Right, but the delete-expression on a non-polymorphic type where the
static type and the dynamic
type are different is UB regardless of whether the derived type adds member=
s.