On Mon, 22 May 2023 at 05:53, Oleksandr Koval via Std-Proposals <
std-proposals_at_[hidden]> wrote:

> 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.
>

The abstract machine is perfectly capable of doing that. A picky
implementation could place such a class instance across a page boundary, if
it wanted.

Anyway, the point (nowadays) is optimization - the compiler can assume that
opaque code does not modify a const object (or subobject).

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
>> >>>
>> >>
>> >
>> --
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>>
>
>
> --
> Regards,
> Oleksandr Koval.
> --
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