On 1/18/24 4:54 AM, Frederick Virchanza Gotham via Std-Proposals wrote:
> On Thu, Jan 18, 2024 at 4:04 AM Thiago Macieira via Std-Proposals
> <std-proposals_at_[hidden]> wrote:
>>> CHANGE 1: The type 'std::type_info' needs a new consteval static
>>> member function called 'get_polymorphic_facilitator' which returns a
>>> pointer to the class's vtable. (It returns a nullptr is
>>> !std::is_polymorphic_v<T>). The C++ Standard won't need to mention the
>>> word 'vtable', it can just say "polymorphic facilitator" and leave its
>>> implementation as unspecified.
>>>
>>> CHANGE 2: The standard library needs a new standalone constexpr
>>> function called 'std::devirtualise' which changes a 'virtual' pointer
>>> to a 'direct' pointer, which on GNU g++ would be implemented something
>>> like:
>> Change 1 can be skipped if the devirtualize function takes a pointer to the
>> object whose vtable you want to read.
>
> If the base class is abstract (i.e. has some virtual functions marked
> "= 0"), then you won't be able to give it an object of type Base.

Objects of abstract class type don't exist. Well, sort of. An object
under construction or destruction will temporarily have a type of the
abstract base class. But vtable and typeid info still exists in that
case. https://godbolt.org/z/8qPrW5eM8.

Tom.

>
>
>> That's important because Change 1 is not
>> implementable, as none of the type_info objects today have a link back to the
>> vtable, and such a link cannot be added without an ABI incompatibility.
>
> Ok then maybe create a new function inside the <typeinfo> header:
>
> namespace std {
> void const *get_polymorphic_facilitator(std::type_info const
> *) noexcept;
> }
>
> This function will return a nullptr if the 'type_info' refers to a
> type that is !std::is_polymorphic_v<T>.
>
>
>> The library function will likely be implemented as an intrinsic, because the
>> compiler knows where in the object the vtable pointer is located.
>
> Yeah probably something like:
>
> namespace std {
> inline constexpr void const
> *get_polymorphic_facilitator(std::type_info const *const arg) noexcept
> {
> return __get_polymorphic_facilitator(arg);
> }
> }