On 1/28/25 18:16, organicoman via Std-Proposals wrote:
> The resulting pointer is unspecified: means dereferencing it is UB.

My reading of "pointer value is unspecified" is that, quite literally,
the value of the pointer is unspecified (i.e. unknown). Not necessarily
the same or unchanged, as you suggest, but unknown. The consequences of
dereferencing it only follow from that.

And to be clear, while this is ok:

short* p1 = new short();
void* p2 = static_cast< void* >(p1);
short* p3 = static_cast< short* >(p2);
assert(p1 == p3);

this is not:

short* p1 = new short();
void* p2 = static_cast< void* >(p1);
double* p3 = static_cast< double* >(p2);
void* p4 = static_cast< void* >(p3);
short* p5 = static_cast< short* >(p4);
assert(p1 == p5); // allowed to fail

Only the conversion from T* to void* is guaranteed to not change the
pointer value. It is not guaranteed in the other direction. See
[conv.ptr]/2.

> Because the compiler is not allowed to alter the bits of A. It will keep
> it as given.
>
> I can do the following.
>
> double* ptr = reinterpret_cast<double*>(0x7fff934a9d83); // misaligned
> for double
>
> ptr value will stay 0x7fff934a9d83, but it is unusable otherwise:
> *ptr is UB
> ++ptr is UB
> --ptr is UB
> ptr+int the result is UB.
> ...etc
>
>
>
>
> Sent from my Galaxy
>
>
> -------- Original message --------
> From: Sebastian Wittmeier via Std-Proposals <std-
> proposals_at_[hidden]>
> Date: 1/28/25 3:40 PM (GMT+01:00)
> To: std-proposals_at_[hidden]
> Cc: Sebastian Wittmeier <wittmeier_at_[hidden]>
> Subject: Re: [std-proposals] Make all data pointers intercompatible
>
> According to CPP Reference this is currently not okay:
>
> "If expression represents the address <https://en.cppreference.com/w/
> cpp/language/pointer#Pointers> |A| of a byte <https://
> en.cppreference.com/w/cpp/language/memory_model#Byte> in memory but |A|
> does not satisfy the alignment <https://en.cppreference.com/w/cpp/
> language/object#Alignment> requirement of |T|, then the resulting
> pointer value is unspecified."
>
> https://en.cppreference.com/w/cpp/language/static_cast
>
>
> -----Ursprüngliche Nachricht-----
> *Von:* organicoman via Std-Proposals <std-proposals_at_[hidden]>
> No it could not, and it is not allowed. The only thing you will
> MAYBE get is a misaligned pointer to a double memory location, and
> you should not dereference that pointer, because of aliasing
> incompatibility otherwise UB.
>
> Try the following code:
> #include <stdio.h>
>
> int main()
> {
> char arr[8];
> double* ptr = static_cast<double*>(static_cast<void*>(arr+3));
>
>