>
> That pair holds the "first" data member with type const T1. That's very
> much a const object.
>
Hm, I thought that because it's inside a struct, it's actually a
"subobject", not an "object". I always understood const_cast rule in a way
that you can change const-ness as long as the underlying memory is not a
read-only and I don't understand how it's possible to have a struct which
has some members of it in a read-only memory and others in a read-write one.

On Mon, May 22, 2023 at 1:42 PM Lénárd Szolnoki via Std-Proposals <
std-proposals_at_[hidden]> wrote:

>
>
> On 22 May 2023 11:29:33 BST, Oleksandr Koval via Std-Proposals <
> std-proposals_at_[hidden]> wrote:
> >>
> >> The element type is pair<K const, V> and the data member layout of pair
> is
> >> fully prescribed.
> >
> >I think we were talking about std::set here, not about std::map.
> >Anyway, are you sure about the `pair<const T1, T2>` case? const_cast says:
> >
> >> const_cast makes it possible to form a reference or pointer to non-const
> >> type that is actually referring to a const object
> >> <https://en.cppreference.com/w/cpp/language/cv>
> >>
> >Const object says:
> >
> >> *const object* - an object whose type is const-qualified, or a
> >> non-mutable subobject of a const object.
> >>
> >To me it doesn't look like `pair<const T1, T2>` satisfies any of the
> above.
>
> That pair holds the "first" data member with type const T1. That's very
> much a const object.
>
> It's an other question that the standard library has to do some magic to
> make "extract" work.
>
> >
> >On Mon, May 22, 2023 at 1:07 PM Edward Catmur <ecatmur_at_[hidden]>
> >wrote:
> >
> >>
> >>
> >> On Mon, May 22, 2023, 04:44 Oleksandr Koval via Std-Proposals <
> >> std-proposals_at_[hidden]> wrote:
> >>
> >>> I don't think it's UB since the underlying memory is not read-only.
> >>>
> >>
> >> The underlying memory very much is read only. The element type is pair<K
> >> const, V> and the data member layout of pair is fully prescribed.
> >>
> >> Arthur's solution can be improved using hints but I think that would
> still
> >>> require checking node(s) around the hint to verify that position is
> correct.
> >>> Btw, boost::container::set works in the way you need.
> >>>
> >>> On Mon, May 22, 2023 at 9:32 AM Thiago Macieira via Std-Proposals <
> >>> std-proposals_at_[hidden]> wrote:
> >>>
> >>>> On Sunday, 21 May 2023 14:09:30 PDT Frederick Virchanza Gotham via
> Std-
> >>>> Proposals wrote:
> >>>> > I wrote the following code today; I have a struct for storing an IP
> >>>> > address as well as two booleans (one for a FTP server and one for a
> >>>> > Samba file server):
> >>>> >
> >>>> > struct IPFileShare {
> >>>> > std::uint32_t ip;
> >>>>
> >>>> <rant>IP addresses aren't 32-bit and haven't been since the late
> 1990s.
> >>>> Is
> >>>> your code stuck in the 80s?</rant>
> >>>>
> >>>> Let's not ever write or even suggest code that fails to work with
> IPv6.
> >>>>
> >>>> > I think strictly speaking, the Standard says that this is undefined
> >>>> > behaviour. How about we change the Standard to say that this
> >>>> > const_cast is allowed so long as it has no effect on the sorting?
> >>>>
> >>>> In addition to what Arthur said, std::set is not just about sorting,
> >>>> it's
> >>>> about identity. The fact that your type seems to carry extra
> information
> >>>> that
> >>>> is not part of the identity does not mean it's the case for all other
> >>>> types.
> >>>>
> >>>> Let's not break the requirements without compelling reason. I didn't
> see
> >>>> a
> >>>> compelling reason why it has to be this way. What other alternatives
> >>>> have you
> >>>> explored and what were their drawbacks?
> >>>>
> >>>> --
> >>>> Thiago Macieira - thiago (AT) macieira.info - thiago (AT) kde.org
> >>>> Software Architect - Intel DCAI Cloud Engineering
> >>>>
> >>>>
> >>>>
> >>>> --
> >>>> Std-Proposals mailing list
> >>>> Std-Proposals_at_[hidden]
> >>>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
> >>>>
> >>>
> >>>
> >>> --
> >>> Regards,
> >>> Oleksandr Koval.
> >>> --
> >>> Std-Proposals mailing list
> >>> Std-Proposals_at_[hidden]
> >>> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
> >>>
> >>
> >
> --
> Std-Proposals mailing list
> Std-Proposals_at_[hidden]
> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals
>

-- 
Regards,
Oleksandr Koval.