ср, 22 сент. 2021 г. в 20:23, Ville Voutilainen <ville.voutilai...@gmail.com>: > > On Wed, 22 Sept 2021 at 20:09, Antony Polukhin via Libstdc++ > <libstd...@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
